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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

-- 
Pavel Begunkov