Re: [PATCH v3] io_uring: fix possible deadlock in io_uring_poll

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

On 05/02/2021 12:58, Pavel Begunkov wrote:
> On 05/02/2021 08:34, Hao Xu wrote:
>> This might happen if we do epoll_wait on a uring fd while reading/writing
>> the former epoll fd in a sqe in the former uring instance.
>> So let's don't flush cqring overflow list, just do a simple check.
> 
> I haven't tested it but tried out identical before.
> 
> Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
> Cc: stable@vger.kernel.org # 5.5+

Don't see it queued, so up, in case it was forgotten

> 
>>
>> Reported-by: Abaci <abaci@linux.alibaba.com>
>> Fixes: 6c503150ae33 ("io_uring: patch up IOPOLL overflow_flush sync")
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>  fs/io_uring.c | 17 +++++++++++++++--
>>  1 file changed, 15 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 38c6cbe1ab38..5f42ad6f2155 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -8718,8 +8718,21 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
>>  	smp_rmb();
>>  	if (!io_sqring_full(ctx))
>>  		mask |= EPOLLOUT | EPOLLWRNORM;
>> -	io_cqring_overflow_flush(ctx, false, NULL, NULL);
>> -	if (io_cqring_events(ctx))
>> +
>> +	/*
>> +	 * Don't flush cqring overflow list here, just do a simple check.
>> +	 * Otherwise there could possible be ABBA deadlock:
>> +	 *      CPU0                    CPU1
>> +	 *      ----                    ----
>> +	 * lock(&ctx->uring_lock);
>> +	 *                              lock(&ep->mtx);
>> +	 *                              lock(&ctx->uring_lock);
>> +	 * lock(&ep->mtx);
>> +	 *
>> +	 * Users may get EPOLLIN meanwhile seeing nothing in cqring, this
>> +	 * pushs them to do the flush.
>> +	 */
>> +	if (io_cqring_events(ctx) || test_bit(0, &ctx->cq_check_overflow))
>>  		mask |= EPOLLIN | EPOLLRDNORM;
>>  
>>  	return mask;
>>
> 

-- 
Pavel Begunkov