Re: [PATCH stable-6.1 1/1] io_uring: fix waiters missing wake ups

[Jens Axboe <axboe@kernel.dk>]

On 1/24/25 2:23 PM, Pavel Begunkov wrote:
> On 1/24/25 20:47, Jens Axboe wrote:
>> On 1/24/25 11:53 AM, Pavel Begunkov wrote:
>>> [ upstream commit 3181e22fb79910c7071e84a43af93ac89e8a7106 ]
>>>
>>> There are reports of mariadb hangs, which is caused by a missing
>>> barrier in the waking code resulting in waiters losing events.
>>>
>>> The problem was introduced in a backport
>>> 3ab9326f93ec4 ("io_uring: wake up optimisations"),
>>> and the change restores the barrier present in the original commit
>>> 3ab9326f93ec4 ("io_uring: wake up optimisations")
>>>
>>> Reported by: Xan Charbonnet <xan@charbonnet.com>
>>> Fixes: 3ab9326f93ec4 ("io_uring: wake up optimisations")
>>> Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1093243#99
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>>   io_uring/io_uring.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>> index 9b58ba4616d40..e5a8ee944ef59 100644
>>> --- a/io_uring/io_uring.c
>>> +++ b/io_uring/io_uring.c
>>> @@ -592,8 +592,10 @@ static inline void __io_cq_unlock_post_flush(struct io_ring_ctx *ctx)
>>>       io_commit_cqring(ctx);
>>>       spin_unlock(&ctx->completion_lock);
>>>       io_commit_cqring_flush(ctx);
>>> -    if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN))
>>> +    if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN)) {
>>> +        smp_mb();
>>>           __io_cqring_wake(ctx);
>>> +    }
>>>   }
>>
>> We could probably just s/__io_cqring_wake/io_cqring_wake here to get
>> the same effect. Not that it really matters, it's just simpler.
> 
> Right, I noticed but am keeping it closer to the original
> in case we'd need to port more in the future.

Yep that's fine, let's just go with this one as-is.

-- 
Jens Axboe