Hi Jens

On 11/13/18 12:26 AM, Jens Axboe wrote:

On 11/12/18 2:35 AM, jianchao.wang wrote:

quoted

Hi Jens

On 11/10/18 11:13 PM, Jens Axboe wrote:

quoted

We only really need the barrier if we're going to be sleeping,
if we're just polling we're fine with the __set_current_state()
variant.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/block_dev.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index c039abfb2052..e7550b1f9670 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c

@@ -237,12 +237,23 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 
 	qc = submit_bio(&bio);
 	for (;;) {
-		set_current_state(TASK_UNINTERRUPTIBLE);
+		/*
+		 * Using non-atomic task state for polling
+		 */
+		__set_current_state(TASK_UNINTERRUPTIBLE);
+
 		if (!READ_ONCE(bio.bi_private))
 			break;

When we don't use polling, the blkdev_bio_end_io_simple may come on a different cpu before
submit_bio returns 
For example,
__blkdev_direct_IO_simple
  qc = submit_bio(&bio) 
                                          blkdev_bio_end_io_simple
                                            WRITE_ONCE(bio.bi_private, NULL)
                                            wake_up_process(waiter)
  __set_current_state(TASK_UNINTERRUPTIBLE)                              
  READ_ONCE(bio.bi_private)

While I agree that you are right, that's an existing issue. The store
barrier implied by set_current_state() doesn't fix that.

How about this variant:

https://urldefense.proofpoint.com/v2/url?u=http-3A__git.kernel.dk_cgit_linux-2Dblock_commit_-3Fh-3Dmq-2Dperf-26id-3D036ba7a8d1334889c0fe55d4858d78f4e8022f12&d=DwICaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=pjbk_9rxuy7OS1pCkQ9MAHzFf4eT27Yt4RL8jASDJCA&s=AnmYMIcGpIILUrjDFsabj61pcTyMCRHB1Pu8XXi47t0&e=

which then changes the later wake up helper patch to be:
 
https://urldefense.proofpoint.com/v2/url?u=http-3A__git.kernel.dk_cgit_linux-2Dblock_commit_-3Fh-3Dmq-2Dperf-26id-3Df8c3f188425967adb040cfefb799b0a5a1df769d&d=DwICaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=pjbk_9rxuy7OS1pCkQ9MAHzFf4eT27Yt4RL8jASDJCA&s=PdajuUul55e6p0FED_AzzWt5Gip0ekLi1eGYBMLiZPo&e=

The wake up path has contained a full memory barrier with the raw_spin_lock and following smp_mb__after_spinlock

wake_up_process
  -> try_to_wake_up

	raw_spin_lock_irqsave(&p->pi_lock, flags);
	smp_mb__after_spinlock();

So a smp_rmb() could be enough. :)

Thanks
Jianchao