Re: [PATCH -next v3] block: remove init_mutex and open-code blk_iolatency_try_init

Hi,

在 2023/08/10 21:43, Yu Kuai 写道:

在 2023/08/10 11:51, Li Lingfeng 写道:

quoted

From: Li Lingfeng <redacted>

Commit a13696b83da4 ("blk-iolatency: Make initialization lazy") adds
a mutex named "init_mutex" in blk_iolatency_try_init for the race
condition of initializing RQ_QOS_LATENCY.
Now a new lock has been add to struct request_queue by commit 
a13bd91be223
("block/rq_qos: protect rq_qos apis with a new lock"). And it has been
held in blkg_conf_open_bdev before calling blk_iolatency_init.
So it's not necessary to keep init_mutex in blk_iolatency_try_init, just
remove it.

Since init_mutex has been removed, blk_iolatency_try_init can be
open-coded back to iolatency_set_limit() like ioc_qos_write().

Reviewed-by: Yu Kuai <redacted>

Thanks,
Kuai

quoted

Signed-off-by: Li Lingfeng <redacted>
---
   v1->v2: open-code blk_iolatency_try_init()
   v2->v3: add lockdep check
  block/blk-iolatency.c | 35 +++++++++++------------------------
  1 file changed, 11 insertions(+), 24 deletions(-)

diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index fd5fec989e39..c16aef4be036 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c

@@ -824,29 +824,6 @@ static void iolatency_clear_scaling(struct 

blkcg_gq *blkg)
      }
  }
-static int blk_iolatency_try_init(struct blkg_conf_ctx *ctx)
-{
-    static DEFINE_MUTEX(init_mutex);
-    int ret;
-
-    ret = blkg_conf_open_bdev(ctx);
-    if (ret)
-        return ret;
-
-    /*
-     * blk_iolatency_init() may fail after rq_qos_add() succeeds 
which can
-     * confuse iolat_rq_qos() test. Make the test and init atomic.
-     */
-    mutex_lock(&init_mutex);
-
-    if (!iolat_rq_qos(ctx->bdev->bd_queue))
-        ret = blk_iolatency_init(ctx->bdev->bd_disk);
-
-    mutex_unlock(&init_mutex);
-
-    return ret;
-}
-
  static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char 
*buf,
                   size_t nbytes, loff_t off)
  {

@@ -861,7 +838,17 @@ static ssize_t iolatency_set_limit(struct 

kernfs_open_file *of, char *buf,
      blkg_conf_init(&ctx, buf);
-    ret = blk_iolatency_try_init(&ctx);
+    ret = blkg_conf_open_bdev(&ctx);
+    if (ret)
+        goto out;
+
+    /*
+     * blk_iolatency_init() may fail after rq_qos_add() succeeds 
which can
+     * confuse iolat_rq_qos() test. Make the test and init atomic.
+     */

The original mutex and above comments is used to avoid the problem that
blk_iolatency_init() is not atomic:

t1:			t2:
if (!iolat_rq_qos)
// not exist
  blk_iolatency_init
   rq_qos_add
   blkcg_activate_policy
   // failed
			if (!iolat_rq_qos)
			// now exist
   rq_qos_del
			...
			// continue while rq_qos is deleted

Now that this problem doesn't exist, I think it's ok just to remove this
comment.

Thanks,
Kuai

quoted

+    lockdep_assert_held(ctx.bdev->bd_queue->rq_qos_mutex);
+    if (!iolat_rq_qos(ctx.bdev->bd_queue))
+        ret = blk_iolatency_init(ctx.bdev->bd_disk);
      if (ret)
          goto out;

.