[PATCH v3 0/3] blk-mq: Fix a race between iterating over requests and freeing requests

Hi Jens,

This patch series fixes the race between iterating over requests and
freeing requests that has been reported by multiple different users over
the past two years. Please consider this patch series for kernel v5.13.

Thank you,

Bart.

Changes compared to v2:
- Converted the single v2 patch into a series of three patches.
- Switched from SRCU to a combination of RCU and semaphores.

Changes compared to v1:
- Reformatted patch description.
- Added Tested-by/Reviewed-by tags.
- Changed srcu_barrier() calls into synchronize_srcu() calls.

Bart Van Assche (3):
  blk-mq: Move the elevator_exit() definition
  blk-mq: Introduce atomic variants of the tag iteration functions
  blk-mq: Fix races between iterating over requests and freeing requests

 block/blk-core.c          | 34 ++++++++++++++-
 block/blk-mq-tag.c        | 87 ++++++++++++++++++++++++++++++++++-----
 block/blk-mq-tag.h        |  8 ++--
 block/blk-mq.c            | 31 ++++++++++----
 block/blk-mq.h            |  1 +
 block/blk.h               | 11 +----
 block/elevator.c          |  9 ++++
 drivers/scsi/hosts.c      | 16 +++----
 drivers/scsi/ufs/ufshcd.c |  4 +-
 include/linux/blk-mq.h    |  2 +
 10 files changed, 160 insertions(+), 43 deletions(-)

Since elevator_exit() only has one caller, move its definition from
block/blk.h into block/elevator.c. Remove the inline keyword since modern
compilers are smart enough to decide when to inline functions that occur
in the same compilation unit.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <redacted>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <redacted>
Cc: John Garry <redacted>
Cc: Khazhy Kumykov <redacted>
Cc: Shin'ichiro Kawasaki <redacted>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk.h      | 9 ---------
 block/elevator.c | 8 ++++++++
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/block/blk.h b/block/blk.h
index 3b53e44b967e..e0a4a7577f6c 100644
--- a/block/blk.h
+++ b/block/blk.h

@@ -198,15 +198,6 @@ void __elevator_exit(struct request_queue *, struct elevator_queue *);
 int elv_register_queue(struct request_queue *q, bool uevent);
 void elv_unregister_queue(struct request_queue *q);
 
-static inline void elevator_exit(struct request_queue *q,
-		struct elevator_queue *e)
-{
-	lockdep_assert_held(&q->sysfs_lock);
-
-	blk_mq_sched_free_requests(q);
-	__elevator_exit(q, e);
-}
-
 ssize_t part_size_show(struct device *dev, struct device_attribute *attr,
 		char *buf);
 ssize_t part_stat_show(struct device *dev, struct device_attribute *attr,

diff --git a/block/elevator.c b/block/elevator.c
index 293c5c81397a..4b20d1ab29cc 100644
--- a/block/elevator.c
+++ b/block/elevator.c

@@ -197,6 +197,14 @@ void __elevator_exit(struct request_queue *q, struct elevator_queue *e)
 	kobject_put(&e->kobj);
 }
 
+static void elevator_exit(struct request_queue *q, struct elevator_queue *e)
+{
+	lockdep_assert_held(&q->sysfs_lock);
+
+	blk_mq_sched_free_requests(q);
+	__elevator_exit(q, e);
+}
+
 static inline void __elv_rqhash_del(struct request *rq)
 {
 	hash_del(&rq->hash);

Since in the next patch knowledge is required of whether or not it is
allowed to sleep inside the tag iteration functions, pass this context
information to the tag iteration functions. I have reviewed all callers of
tag iteration functions to verify these annotations by starting from the
output of the following grep command:

    git grep -nHE 'blk_mq_(all_tag|tagset_busy|queue_tag_busy)_iter'

My conclusions from that analysis are as follows:
- The callback functions passed to blk_mq_queue_tag_busy_iter() from the
  block layer core do not sleep except the block driver timeout handler.
- Sleeping is allowed in the blk-mq-debugfs code that iterates over tags.
- Since the blk_mq_tagset_busy_iter() calls in the mtip32xx driver are
  preceded by a function that sleeps (blk_mq_quiesce_queue()), sleeping is
  safe in the context of the blk_mq_tagset_busy_iter() calls.
- The same reasoning also applies to the nbd driver.
- All blk_mq_tagset_busy_iter() calls in the NVMe drivers are followed by a
  call to a function that sleeps so sleeping inside blk_mq_tagset_busy_iter()
  when called from the NVMe driver is fine.
- scsi_host_busy(), scsi_host_complete_all_commands() and
  scsi_host_busy_iter() are used by multiple SCSI LLDs so analyzing whether
  or not these functions may sleep is hard. Instead of performing that
  analysis, make it safe to call these functions from atomic context.

Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <redacted>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <redacted>
Cc: John Garry <redacted>
Cc: Khazhy Kumykov <redacted>
Cc: Shin'ichiro Kawasaki <redacted>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-mq-tag.c        | 44 ++++++++++++++++++++++++++++++++-------
 block/blk-mq-tag.h        |  4 ++--
 block/blk-mq.c            | 10 ++++-----
 drivers/scsi/hosts.c      | 16 +++++++-------
 drivers/scsi/ufs/ufshcd.c |  4 ++--
 include/linux/blk-mq.h    |  2 ++
 6 files changed, 56 insertions(+), 24 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index e5bfecf2940d..975626f755c2 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c

@@ -322,18 +322,19 @@ static void __blk_mq_all_tag_iter(struct blk_mq_tags *tags,
 }
 
 /**
- * blk_mq_all_tag_iter - iterate over all requests in a tag map
+ * blk_mq_all_tag_iter_atomic - iterate over all requests in a tag map
  * @tags:	Tag map to iterate over.
  * @fn:		Pointer to the function that will be called for each
  *		request. @fn will be called as follows: @fn(rq, @priv,
  *		reserved) where rq is a pointer to a request. 'reserved'
  *		indicates whether or not @rq is a reserved request. Return
- *		true to continue iterating tags, false to stop.
+ *		true to continue iterating tags, false to stop. Must not
+ *		sleep.
  * @priv:	Will be passed as second argument to @fn.
  *
- * Caller has to pass the tag map from which requests are allocated.
+ * Does not sleep.
  */
-void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
+void blk_mq_all_tag_iter_atomic(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
 		void *priv)
 {
 	__blk_mq_all_tag_iter(tags, fn, priv, BT_TAG_ITER_STATIC_RQS);

@@ -348,6 +349,8 @@ void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
  *		indicates whether or not @rq is a reserved request. Return
  *		true to continue iterating tags, false to stop.
  * @priv:	Will be passed as second argument to @fn.
+ *
+ * May sleep.
  */
 void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 		busy_tag_iter_fn *fn, void *priv)

@@ -362,6 +365,31 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 }
 EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
 
+/**
+ * blk_mq_tagset_busy_iter - iterate over all started requests in a tag set
+ * @tagset:	Tag set to iterate over.
+ * @fn:		Pointer to the function that will be called for each started
+ *		request. @fn will be called as follows: @fn(rq, @priv,
+ *		reserved) where rq is a pointer to a request. 'reserved'
+ *		indicates whether or not @rq is a reserved request. Return
+ *		true to continue iterating tags, false to stop. Must not sleep.
+ * @priv:	Will be passed as second argument to @fn.
+ *
+ * Does not sleep.
+ */
+void blk_mq_tagset_busy_iter_atomic(struct blk_mq_tag_set *tagset,
+		busy_tag_iter_fn *fn, void *priv)
+{
+	int i;
+
+	for (i = 0; i < tagset->nr_hw_queues; i++) {
+		if (tagset->tags && tagset->tags[i])
+			__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
+					      BT_TAG_ITER_STARTED);
+	}
+}
+EXPORT_SYMBOL(blk_mq_tagset_busy_iter_atomic);
+
 static bool blk_mq_tagset_count_completed_rqs(struct request *rq,
 		void *data, bool reserved)
 {

@@ -384,7 +412,7 @@ void blk_mq_tagset_wait_completed_request(struct blk_mq_tag_set *tagset)
 	while (true) {
 		unsigned count = 0;
 
-		blk_mq_tagset_busy_iter(tagset,
+		blk_mq_tagset_busy_iter_atomic(tagset,
 				blk_mq_tagset_count_completed_rqs, &count);
 		if (!count)
 			break;

@@ -400,15 +428,17 @@ EXPORT_SYMBOL(blk_mq_tagset_wait_completed_request);
  *		on @q. @fn will be called as follows: @fn(hctx, rq, @priv,
  *		reserved) where rq is a pointer to a request and hctx points
  *		to the hardware queue associated with the request. 'reserved'
- *		indicates whether or not @rq is a reserved request.
+ *		indicates whether or not @rq is a reserved request. Must not
+ *		sleep if @in_atomic_context is %true.
  * @priv:	Will be passed as third argument to @fn.
+ * @in_atomic_context: If true, do not sleep.
  *
  * Note: if @q->tag_set is shared with other request queues then @fn will be
  * called for all requests on all queues that share that tag set and not only
  * for requests associated with @q.
  */
 void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
-		void *priv)
+		void *priv, bool in_atomic_context)
 {
 	struct blk_mq_hw_ctx *hctx;
 	int i;

diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 7d3e6b333a4a..b01c806e4c2d 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h

@@ -42,8 +42,8 @@ extern void blk_mq_tag_resize_shared_sbitmap(struct blk_mq_tag_set *set,
 
 extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
 void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
-		void *priv);
-void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
+		void *priv, bool in_atomic_context);
+void blk_mq_all_tag_iter_atomic(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
 		void *priv);
 
 static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt,

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d4d7c1caa439..203caa14c51a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -117,7 +117,7 @@ unsigned int blk_mq_in_flight(struct request_queue *q,
 {
 	struct mq_inflight mi = { .part = part };
 
-	blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
+	blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi, true);
 
 	return mi.inflight[0] + mi.inflight[1];
 }

@@ -127,7 +127,7 @@ void blk_mq_in_flight_rw(struct request_queue *q, struct block_device *part,
 {
 	struct mq_inflight mi = { .part = part };
 
-	blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi);
+	blk_mq_queue_tag_busy_iter(q, blk_mq_check_inflight, &mi, true);
 	inflight[0] = mi.inflight[0];
 	inflight[1] = mi.inflight[1];
 }

@@ -868,7 +868,7 @@ bool blk_mq_queue_inflight(struct request_queue *q)
 {
 	bool busy = false;
 
-	blk_mq_queue_tag_busy_iter(q, blk_mq_rq_inflight, &busy);
+	blk_mq_queue_tag_busy_iter(q, blk_mq_rq_inflight, &busy, true);
 	return busy;
 }
 EXPORT_SYMBOL_GPL(blk_mq_queue_inflight);

@@ -973,7 +973,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
 	if (!percpu_ref_tryget(&q->q_usage_counter))
 		return;
 
-	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
+	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next, false);
 
 	if (next != 0) {
 		mod_timer(&q->timeout, next);

@@ -2483,7 +2483,7 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
 		.hctx	= hctx,
 	};
 
-	blk_mq_all_tag_iter(tags, blk_mq_has_request, &data);
+	blk_mq_all_tag_iter_atomic(tags, blk_mq_has_request, &data);
 	return data.has_rq;
 }
 

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 2f162603876f..f09e1520a241 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c

@@ -578,8 +578,8 @@ int scsi_host_busy(struct Scsi_Host *shost)
 {
 	int cnt = 0;
 
-	blk_mq_tagset_busy_iter(&shost->tag_set,
-				scsi_host_check_in_flight, &cnt);
+	blk_mq_tagset_busy_iter_atomic(&shost->tag_set,
+				       scsi_host_check_in_flight, &cnt);
 	return cnt;
 }
 EXPORT_SYMBOL(scsi_host_busy);

@@ -677,8 +677,8 @@ static bool complete_all_cmds_iter(struct request *rq, void *data, bool rsvd)
  */
 void scsi_host_complete_all_commands(struct Scsi_Host *shost, int status)
 {
-	blk_mq_tagset_busy_iter(&shost->tag_set, complete_all_cmds_iter,
-				&status);
+	blk_mq_tagset_busy_iter_atomic(&shost->tag_set, complete_all_cmds_iter,
+				       &status);
 }
 EXPORT_SYMBOL_GPL(scsi_host_complete_all_commands);
 

@@ -699,11 +699,11 @@ static bool __scsi_host_busy_iter_fn(struct request *req, void *priv,
 /**
  * scsi_host_busy_iter - Iterate over all busy commands
  * @shost:	Pointer to Scsi_Host.
- * @fn:		Function to call on each busy command
+ * @fn:		Function to call on each busy command. Must not sleep.
  * @priv:	Data pointer passed to @fn
  *
  * If locking against concurrent command completions is required
- * ithas to be provided by the caller
+ * it has to be provided by the caller.
  **/
 void scsi_host_busy_iter(struct Scsi_Host *shost,
 			 bool (*fn)(struct scsi_cmnd *, void *, bool),

@@ -714,7 +714,7 @@ void scsi_host_busy_iter(struct Scsi_Host *shost,
 		.priv = priv,
 	};
 
-	blk_mq_tagset_busy_iter(&shost->tag_set, __scsi_host_busy_iter_fn,
-				&iter_data);
+	blk_mq_tagset_busy_iter_atomic(&shost->tag_set,
+				       __scsi_host_busy_iter_fn, &iter_data);
 }
 EXPORT_SYMBOL_GPL(scsi_host_busy_iter);

diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
index c86760788c72..6d2f8f18e2a3 100644
--- a/drivers/scsi/ufs/ufshcd.c
+++ b/drivers/scsi/ufs/ufshcd.c

@@ -1380,7 +1380,7 @@ static bool ufshcd_any_tag_in_use(struct ufs_hba *hba)
 	struct request_queue *q = hba->cmd_queue;
 	int busy = 0;
 
-	blk_mq_tagset_busy_iter(q->tag_set, ufshcd_is_busy, &busy);
+	blk_mq_tagset_busy_iter_atomic(q->tag_set, ufshcd_is_busy, &busy);
 	return busy;
 }
 

@@ -6269,7 +6269,7 @@ static irqreturn_t ufshcd_tmc_handler(struct ufs_hba *hba)
 		.pending = ufshcd_readl(hba, REG_UTP_TASK_REQ_DOOR_BELL),
 	};
 
-	blk_mq_tagset_busy_iter(q->tag_set, ufshcd_compl_tm, &ci);
+	blk_mq_tagset_busy_iter_atomic(q->tag_set, ufshcd_compl_tm, &ci);
 	return ci.ncpl ? IRQ_HANDLED : IRQ_NONE;
 }
 

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 2c473c9b8990..dfa0114a49fd 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h

@@ -526,6 +526,8 @@ void blk_mq_run_hw_queues(struct request_queue *q, bool async);
 void blk_mq_delay_run_hw_queues(struct request_queue *q, unsigned long msecs);
 void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 		busy_tag_iter_fn *fn, void *priv);
+void blk_mq_tagset_busy_iter_atomic(struct blk_mq_tag_set *tagset,
+		busy_tag_iter_fn *fn, void *priv);
 void blk_mq_tagset_wait_completed_request(struct blk_mq_tag_set *tagset);
 void blk_mq_freeze_queue(struct request_queue *q);
 void blk_mq_unfreeze_queue(struct request_queue *q);

When multiple request queues share a tag set and when switching the I/O
scheduler for one of the request queues associated with that tag set, the
following race can happen:
* blk_mq_tagset_busy_iter() calls bt_tags_iter() and bt_tags_iter() assigns
  a pointer to a scheduler request to the local variable 'rq'.
* blk_mq_sched_free_requests() is called to free hctx->sched_tags.
* blk_mq_tagset_busy_iter() dereferences 'rq' and triggers a use-after-free.

Fix this race as follows:
* Use rcu_assign_pointer() and rcu_dereference() to access hctx->tags->rqs[].
  The performance impact of the assignments added to the hot path is minimal
  (about 1% according to one particular test).
* Protect hctx->tags->rqs[] reads with an RCU read-side lock or with a
  semaphore. Which mechanism is used depends on whether or not it is allowed
  to sleep and also on whether or not the callback function may sleep.
* Wait for all preexisting readers to finish before freeing scheduler
  requests.

Multiple users have reported use-after-free complaints similar to the
following (from https://lore.kernel.org/linux-block/1545261885.185366.488.camel@acm.org/):

BUG: KASAN: use-after-free in bt_iter+0x86/0xf0
Read of size 8 at addr ffff88803b335240 by task fio/21412

CPU: 0 PID: 21412 Comm: fio Tainted: G        W         4.20.0-rc6-dbg+ #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
 dump_stack+0x86/0xca
 print_address_description+0x71/0x239
 kasan_report.cold.5+0x242/0x301
 __asan_load8+0x54/0x90
 bt_iter+0x86/0xf0
 blk_mq_queue_tag_busy_iter+0x373/0x5e0
 blk_mq_in_flight+0x96/0xb0
 part_in_flight+0x40/0x140
 part_round_stats+0x18e/0x370
 blk_account_io_start+0x3d7/0x670
 blk_mq_bio_to_request+0x19c/0x3a0
 blk_mq_make_request+0x7a9/0xcb0
 generic_make_request+0x41d/0x960
 submit_bio+0x9b/0x250
 do_blockdev_direct_IO+0x435c/0x4c70
 __blockdev_direct_IO+0x79/0x88
 ext4_direct_IO+0x46c/0xc00
 generic_file_direct_write+0x119/0x210
 __generic_file_write_iter+0x11c/0x280
 ext4_file_write_iter+0x1b8/0x6f0
 aio_write+0x204/0x310
 io_submit_one+0x9d3/0xe80
 __x64_sys_io_submit+0x115/0x340
 do_syscall_64+0x71/0x210

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <redacted>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <redacted>
Cc: John Garry <redacted>
Cc: Khazhy Kumykov <redacted>
Cc: Shin'ichiro Kawasaki <redacted>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-core.c   | 34 +++++++++++++++++++++++++++++++++-
 block/blk-mq-tag.c | 43 +++++++++++++++++++++++++++++++++++++++----
 block/blk-mq-tag.h |  4 +++-
 block/blk-mq.c     | 21 +++++++++++++++++----
 block/blk-mq.h     |  1 +
 block/blk.h        |  2 ++
 block/elevator.c   |  1 +
 7 files changed, 96 insertions(+), 10 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index fc60ff208497..fabb45ecd216 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -279,6 +279,36 @@ void blk_dump_rq_flags(struct request *rq, char *msg)
 }
 EXPORT_SYMBOL(blk_dump_rq_flags);
 
+/**
+ * blk_mq_wait_for_tag_readers - wait for preexisting tag readers to finish
+ * @set: Tag set to wait on.
+ *
+ * Waits for preexisting __blk_mq_all_tag_iter() calls to finish accessing
+ * hctx->tags->rqs[]. New readers may start while this function is in progress
+ * or after this function has returned. Use this function to make sure that
+ * hctx->tags->rqs[] changes have become globally visible.
+ *
+ * Accesses of hctx->tags->rqs[] by blk_mq_queue_tag_busy_iter() calls are out
+ * of scope for this function. The caller can pause blk_mq_queue_tag_busy_iter()
+ * calls for a request queue by freezing that request queue.
+ */
+void blk_mq_wait_for_tag_readers(struct blk_mq_tag_set *set)
+{
+	struct blk_mq_tags *tags;
+	int i;
+
+	if (set->tags) {
+		for (i = 0; i < set->nr_hw_queues; i++) {
+			tags = set->tags[i];
+			if (!tags)
+				continue;
+			down_write(&tags->iter_rwsem);
+			up_write(&tags->iter_rwsem);
+		}
+	}
+	synchronize_rcu();
+}
+
 /**
  * blk_sync_queue - cancel any pending callbacks on a queue
  * @q: the queue

@@ -412,8 +442,10 @@ void blk_cleanup_queue(struct request_queue *q)
 	 * it is safe to free requests now.
 	 */
 	mutex_lock(&q->sysfs_lock);
-	if (q->elevator)
+	if (q->elevator) {
+		blk_mq_wait_for_tag_readers(q->tag_set);
 		blk_mq_sched_free_requests(q);
+	}
 	mutex_unlock(&q->sysfs_lock);
 
 	percpu_ref_exit(&q->q_usage_counter);

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 975626f755c2..c8722ce7c35c 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c

@@ -209,7 +209,12 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 
 	if (!reserved)
 		bitnr += tags->nr_reserved_tags;
-	rq = tags->rqs[bitnr];
+	/*
+	 * See also the percpu_ref_tryget() and blk_queue_exit() calls in
+	 * blk_mq_queue_tag_busy_iter().
+	 */
+	rq = rcu_dereference_check(tags->rqs[bitnr],
+			!percpu_ref_is_zero(&hctx->queue->q_usage_counter));
 
 	/*
 	 * We can hit rq == NULL here, because the tagging functions

@@ -254,11 +259,17 @@ struct bt_tags_iter_data {
 	unsigned int flags;
 };
 
+/* Include reserved tags. */
 #define BT_TAG_ITER_RESERVED		(1 << 0)
+/* Only include started requests. */
 #define BT_TAG_ITER_STARTED		(1 << 1)
+/* Iterate over tags->static_rqs[] instead of tags->rqs[]. */
 #define BT_TAG_ITER_STATIC_RQS		(1 << 2)
+/* The callback function may sleep. */
+#define BT_TAG_ITER_MAY_SLEEP		(1 << 3)
 
-static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
+static bool __bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr,
+			   void *data)
 {
 	struct bt_tags_iter_data *iter_data = data;
 	struct blk_mq_tags *tags = iter_data->tags;

@@ -275,7 +286,7 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 	if (iter_data->flags & BT_TAG_ITER_STATIC_RQS)
 		rq = tags->static_rqs[bitnr];
 	else
-		rq = tags->rqs[bitnr];
+		rq = rcu_dereference_check(tags->rqs[bitnr], true);
 	if (!rq)
 		return true;
 	if ((iter_data->flags & BT_TAG_ITER_STARTED) &&

@@ -284,6 +295,25 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 	return iter_data->fn(rq, iter_data->data, reserved);
 }
 
+static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
+{
+	struct bt_tags_iter_data *iter_data = data;
+	struct blk_mq_tags *tags = iter_data->tags;
+	bool res;
+
+	if (iter_data->flags & BT_TAG_ITER_MAY_SLEEP) {
+		down_read(&tags->iter_rwsem);
+		res = __bt_tags_iter(bitmap, bitnr, data);
+		up_read(&tags->iter_rwsem);
+	} else {
+		rcu_read_lock();
+		res = __bt_tags_iter(bitmap, bitnr, data);
+		rcu_read_unlock();
+	}
+
+	return res;
+}
+
 /**
  * bt_tags_for_each - iterate over the requests in a tag map
  * @tags:	Tag map to iterate over.

@@ -357,10 +387,12 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 {
 	int i;
 
+	might_sleep();
+
 	for (i = 0; i < tagset->nr_hw_queues; i++) {
 		if (tagset->tags && tagset->tags[i])
 			__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
-					      BT_TAG_ITER_STARTED);
+				BT_TAG_ITER_STARTED | BT_TAG_ITER_MAY_SLEEP);
 	}
 }
 EXPORT_SYMBOL(blk_mq_tagset_busy_iter);

@@ -554,6 +586,9 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
 		kfree(tags);
 		return NULL;
 	}
+
+	init_rwsem(&tags->iter_rwsem);
+
 	return tags;
 }
 

diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index b01c806e4c2d..534377385456 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h

@@ -17,9 +17,11 @@ struct blk_mq_tags {
 	struct sbitmap_queue __bitmap_tags;
 	struct sbitmap_queue __breserved_tags;
 
-	struct request **rqs;
+	struct request __rcu **rqs;
 	struct request **static_rqs;
 	struct list_head page_list;
+
+	struct rw_semaphore iter_rwsem;
 };
 
 extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags,

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 203caa14c51a..e66357da6c6b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -495,8 +495,10 @@ static void __blk_mq_free_request(struct request *rq)
 	blk_crypto_free_request(rq);
 	blk_pm_mark_last_busy(rq);
 	rq->mq_hctx = NULL;
-	if (rq->tag != BLK_MQ_NO_TAG)
+	if (rq->tag != BLK_MQ_NO_TAG) {
 		blk_mq_put_tag(hctx->tags, ctx, rq->tag);
+		rcu_assign_pointer(hctx->tags->rqs[rq->tag], NULL);
+	}
 	if (sched_tag != BLK_MQ_NO_TAG)
 		blk_mq_put_tag(hctx->sched_tags, ctx, sched_tag);
 	blk_mq_sched_restart(hctx);

@@ -838,9 +840,20 @@ EXPORT_SYMBOL(blk_mq_delay_kick_requeue_list);
 
 struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
 {
+	struct request *rq;
+
 	if (tag < tags->nr_tags) {
-		prefetch(tags->rqs[tag]);
-		return tags->rqs[tag];
+		/*
+		 * Freeing tags happens with the request queue frozen so the
+		 * srcu dereference below is protected by the request queue
+		 * usage count. We can only verify that usage count after
+		 * having read the request pointer.
+		 */
+		rq = rcu_dereference_check(tags->rqs[tag], true);
+		WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && rq &&
+			     percpu_ref_is_zero(&rq->q->q_usage_counter));
+		prefetch(rq);
+		return rq;
 	}
 
 	return NULL;

@@ -1111,7 +1124,7 @@ static bool blk_mq_get_driver_tag(struct request *rq)
 		rq->rq_flags |= RQF_MQ_INFLIGHT;
 		__blk_mq_inc_active_requests(hctx);
 	}
-	hctx->tags->rqs[rq->tag] = rq;
+	rcu_assign_pointer(hctx->tags->rqs[rq->tag], rq);
 	return true;
 }
 

diff --git a/block/blk-mq.h b/block/blk-mq.h
index 3616453ca28c..9ccb1818303b 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h

@@ -226,6 +226,7 @@ static inline void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
 					   struct request *rq)
 {
 	blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag);
+	rcu_assign_pointer(hctx->tags->rqs[rq->tag], NULL);
 	rq->tag = BLK_MQ_NO_TAG;
 
 	if (rq->rq_flags & RQF_MQ_INFLIGHT) {

diff --git a/block/blk.h b/block/blk.h
index e0a4a7577f6c..825ae70c7c0b 100644
--- a/block/blk.h
+++ b/block/blk.h

@@ -184,6 +184,8 @@ bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
 void blk_account_io_start(struct request *req);
 void blk_account_io_done(struct request *req, u64 now);
 
+void blk_mq_wait_for_tag_readers(struct blk_mq_tag_set *set);
+
 /*
  * Internal elevator interface
  */

diff --git a/block/elevator.c b/block/elevator.c
index 4b20d1ab29cc..28edcb3f7ceb 100644
--- a/block/elevator.c
+++ b/block/elevator.c

@@ -201,6 +201,7 @@ static void elevator_exit(struct request_queue *q, struct elevator_queue *e)
 {
 	lockdep_assert_held(&q->sysfs_lock);
 
+	blk_mq_wait_for_tag_readers(q->tag_set);
 	blk_mq_sched_free_requests(q);
 	__elevator_exit(q, e);
 }

On Mar 25, 2021 / 19:29, Bart Van Assche wrote:

Hi Bart, thank you for your effort on this problem.

I applied this series on v5.12-rc4 and ran blktests block/005 with a HDD behind
SAS-HBA, and I observed kernel INFO and WARNING [1]. It looks like that
tags->iter_rwsem is not initialized. I found blk_mq_init_tags() has two paths
to "return tags". I think when blk_mq_init_tags() returns at the first path to
"return tags", tags->iter_rwsem misses the initialization. To confirm it, I
moved the init_rwsem() before the first "return tags", then saw the kernel
messages disappeared (use-after-free disappeared also).

Could you relook the hunk?

[1]

[   80.079549] run blktests block/005 at 2021-03-26 17:21:49
[   80.183881] INFO: trying to register non-static key.
[   80.189545] the code is fine but needs lockdep annotation.
[   80.195725] turning off the locking correctness validator.
[   80.201906] CPU: 3 PID: 1229 Comm: check Not tainted 5.12.0-rc4+ #8
[   80.208863] Hardware name: Supermicro Super Server/X10SRL-F, BIOS 2.0 12/17/2015
[   80.216949] Call Trace:
[   80.220091]  dump_stack+0x93/0xc2
[   80.224111]  register_lock_class+0x1b58/0x1b60
[   80.229248]  ? lock_chain_count+0x20/0x20
[   80.233953]  ? __kasan_slab_free+0xe5/0x110
[   80.238833]  ? slab_free_freelist_hook+0x4b/0x140
[   80.244231]  ? kmem_cache_free+0xf4/0x5b0
[   80.248937]  ? is_dynamic_key+0x1b0/0x1b0
[   80.253643]  ? mark_lock+0xe4/0x2fd0
[   80.257918]  ? queue_attr_store+0x8b/0xd0
[   80.262625]  ? kernfs_fop_write_iter+0x2cb/0x460
[   80.267937]  ? new_sync_write+0x355/0x5e0
[   80.272642]  ? vfs_write+0x5b2/0x860
[   80.276917]  ? ksys_write+0xe9/0x1b0
[   80.281190]  __lock_acquire+0xe3/0x58b0
[   80.285725]  ? __lock_acquire+0xbb0/0x58b0
[   80.290514]  ? lockdep_hardirqs_on_prepare+0x3e0/0x3e0
[   80.296348]  lock_acquire+0x181/0x6a0
[   80.300707]  ? blk_mq_wait_for_tag_readers+0xdd/0x150
[   80.306453]  ? lock_release+0x680/0x680
[   80.310983]  ? find_held_lock+0x2c/0x110
[   80.315604]  ? lock_is_held_type+0x98/0x110
[   80.320484]  down_write+0x84/0x130
[   80.324586]  ? blk_mq_wait_for_tag_readers+0xdd/0x150
[   80.330330]  ? down_write_killable_nested+0x150/0x150
[   80.336076]  blk_mq_wait_for_tag_readers+0xdd/0x150
[   80.341650]  elevator_exit+0x75/0xe0
[   80.345924]  elevator_switch_mq+0xda/0x4d0
[   80.350717]  elevator_switch+0x5d/0xa0
[   80.355161]  elv_iosched_store+0x31b/0x3c0
[   80.359956]  ? elevator_init_mq+0x350/0x350
[   80.364834]  ? lock_is_held_type+0x98/0x110
[   80.369714]  queue_attr_store+0x8b/0xd0
[   80.374248]  ? sysfs_file_ops+0x170/0x170
[   80.378951]  kernfs_fop_write_iter+0x2cb/0x460
[   80.384094]  new_sync_write+0x355/0x5e0
[   80.388627]  ? new_sync_read+0x5d0/0x5d0
[   80.393243]  ? ksys_write+0xe9/0x1b0
[   80.397517]  ? lock_release+0x680/0x680
[   80.402047]  ? __cond_resched+0x15/0x30
[   80.406580]  ? inode_security+0x56/0xf0
[   80.411115]  ? lock_is_held_type+0x98/0x110
[   80.415999]  vfs_write+0x5b2/0x860
[   80.420096]  ksys_write+0xe9/0x1b0
[   80.424193]  ? __ia32_sys_read+0xb0/0xb0
[   80.428811]  ? syscall_enter_from_user_mode+0x27/0x70
[   80.434556]  ? trace_hardirqs_on+0x1c/0x110
[   80.439438]  do_syscall_64+0x33/0x40
[   80.443709]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   80.449458] RIP: 0033:0x7fad1b2794e7
[   80.453733] Code: 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
[   80.473169] RSP: 002b:00007ffd987bdc38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   80.481429] RAX: ffffffffffffffda RBX: 000000000000000c RCX: 00007fad1b2794e7
[   80.489253] RDX: 000000000000000c RSI: 000055a049b83d30 RDI: 0000000000000001
[   80.497071] RBP: 000055a049b83d30 R08: 000000000000000a R09: 00007fad1b3100c0
[   80.504890] R10: 00007fad1b30ffc0 R11: 0000000000000246 R12: 000000000000000c
[   80.512715] R13: 00007fad1b34c520 R14: 000000000000000c R15: 00007fad1b34c720
[   80.520576] ------------[ cut here ]------------
[   80.525896] DEBUG_RWSEMS_WARN_ON(sem->magic != sem): count = 0x1, magic = 0x0, owner = 0xffff8881251cb240, curr 0xffff8881251cb240, list not empty
[   80.539706] WARNING: CPU: 3 PID: 1229 at kernel/locking/rwsem.c:1311 up_write+0x365/0x530
[   80.548579] Modules linked in: xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT nf_nat_tftp nf_conntrack_tftp tun bridge stp llc nft_objref nf_conntrack_netbios_ns nf_conntrack_broadcast nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_tables ebtable_nat ebtable_broute ip6table_nat ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_mangle iptable_raw iptable_security ip_set nfnetlink rfkill target_core_user ebtable_filter ebtables target_core_mod ip6table_filter ip6_tables iptable_filter sunrpc intel_rapl_msr intel_rapl_common x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass rapl intel_cstate iTCO_wdt intel_uncore intel_pmc_bxt pcspkr iTCO_vendor_support joydev i2c_i801 i2c_smbus lpc_ich ses enclosure mei_me ipmi_ssif mei ioatdma wmi acpi_ipmi ipmi_si ipmi_devintf ipmi_msghandler acpi_power_meter acpi_pad
[   80.548690]  zram ip_tables ast drm_vram_helper drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops cec drm_ttm_helper ttm crct10dif_pclmul crc32_pclmul crc32c_intel drm ghash_clmulni_intel igb mpt3sas dca i2c_algo_bit xhci_pci raid_class xhci_pci_renesas scsi_transport_sas fuse
[   80.661295] CPU: 3 PID: 1229 Comm: check Not tainted 5.12.0-rc4+ #8
[   80.668258] Hardware name: Supermicro Super Server/X10SRL-F, BIOS 2.0 12/17/2015
[   80.676345] RIP: 0010:up_write+0x365/0x530
[   80.681155] Code: 02 00 0f 85 73 01 00 00 ff 34 24 48 8b 55 00 4d 89 f0 48 c7 c6 c0 45 69 a0 4c 8b 4c 24 10 48 c7 c7 00 46 69 a0 e8 65 e2 e2 01 <0f> 0b 59 e9 c3 fe ff ff 4c 8d 75 58 c6 05 cd ef 67 03 01 48 b8 00
[   80.700595] RSP: 0018:ffff888125affb08 EFLAGS: 00010286
[   80.706513] RAX: 0000000000000000 RBX: 1ffff11024b5ff65 RCX: 0000000000000000
[   80.714339] RDX: 0000000000000004 RSI: 0000000000000008 RDI: ffffed1024b5ff57
[   80.722169] RBP: ffff888144da20c0 R08: 0000000000000001 R09: ffff8888114ee4a7
[   80.730000] R10: ffffed110229dc94 R11: 0000000000000001 R12: ffff888144da20c8
[   80.737826] R13: ffff888144da2128 R14: ffff8881251cb240 R15: ffffffffa19a5668
[   80.745653] FS:  00007fad1b184740(0000) GS:ffff8888114c0000(0000) knlGS:0000000000000000
[   80.754432] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   80.760872] CR2: 00007fad1b34c5b0 CR3: 000000015e552003 CR4: 00000000001706e0
[   80.768700] Call Trace:
[   80.771849]  ? downgrade_write+0x370/0x370
[   80.776645]  ? down_write_killable_nested+0x150/0x150
[   80.782393]  blk_mq_wait_for_tag_readers+0xe5/0x150
[   80.787977]  elevator_exit+0x75/0xe0
[   80.792255]  elevator_switch_mq+0xda/0x4d0
[   80.797051]  elevator_switch+0x5d/0xa0
[   80.801503]  elv_iosched_store+0x31b/0x3c0
[   80.806295]  ? elevator_init_mq+0x350/0x350
[   80.811177]  ? lock_is_held_type+0x98/0x110
[   80.816064]  queue_attr_store+0x8b/0xd0
[   80.820605]  ? sysfs_file_ops+0x170/0x170
[   80.825310]  kernfs_fop_write_iter+0x2cb/0x460
[   80.830452]  new_sync_write+0x355/0x5e0
[   80.834994]  ? new_sync_read+0x5d0/0x5d0
[   80.839621]  ? ksys_write+0xe9/0x1b0
[   80.843893]  ? lock_release+0x680/0x680
[   80.848427]  ? __cond_resched+0x15/0x30
[   80.852967]  ? inode_security+0x56/0xf0
[   80.857510]  ? lock_is_held_type+0x98/0x110
[   80.862398]  vfs_write+0x5b2/0x860
[   80.866497]  ksys_write+0xe9/0x1b0
[   80.870596]  ? __ia32_sys_read+0xb0/0xb0
[   80.875214]  ? syscall_enter_from_user_mode+0x27/0x70
[   80.880962]  ? trace_hardirqs_on+0x1c/0x110
[   80.885850]  do_syscall_64+0x33/0x40
[   80.890132]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   80.895886] RIP: 0033:0x7fad1b2794e7
[   80.900160] Code: 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
[   80.919598] RSP: 002b:00007ffd987bdc38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   80.927856] RAX: ffffffffffffffda RBX: 000000000000000c RCX: 00007fad1b2794e7
[   80.935684] RDX: 000000000000000c RSI: 000055a049b83d30 RDI: 0000000000000001
[   80.943512] RBP: 000055a049b83d30 R08: 000000000000000a R09: 00007fad1b3100c0
[   80.951335] R10: 00007fad1b30ffc0 R11: 0000000000000246 R12: 000000000000000c
[   80.959163] R13: 00007fad1b34c520 R14: 000000000000000c R15: 00007fad1b34c720
[   80.966993] irq event stamp: 34619
[   80.971096] hardirqs last  enabled at (34619): [<ffffffffa01e3044>] _raw_spin_unlock_irq+0x24/0x30
[   80.980753] hardirqs last disabled at (34618): [<ffffffffa01e2e33>] _raw_spin_lock_irq+0x43/0x50
[   80.990233] softirqs last  enabled at (34494): [<ffffffff9e1af289>] __irq_exit_rcu+0x1b9/0x270
[   80.999542] softirqs last disabled at (34487): [<ffffffff9e1af289>] __irq_exit_rcu+0x1b9/0x270
[   81.008841] ---[ end trace c154275a45048f81 ]---

-- 
Best Regards,
Shin'ichiro Kawasaki

On 3/26/21 1:44 AM, Shinichiro Kawasaki wrote:

I applied this series on v5.12-rc4 and ran blktests block/005 with a HDD behind
SAS-HBA, and I observed kernel INFO and WARNING [1]. It looks like that
tags->iter_rwsem is not initialized. I found blk_mq_init_tags() has two paths
to "return tags". I think when blk_mq_init_tags() returns at the first path to
"return tags", tags->iter_rwsem misses the initialization. To confirm it, I
moved the init_rwsem() before the first "return tags", then saw the kernel
messages disappeared (use-after-free disappeared also).

Hi Shinichiro,

Thanks for the quick feedback. I agree with your analysis and will fix
this in v4 of this patch series.

Bart.