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@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 5.5+
Don't see it queued, so up, in case it was forgotten
>
>>
>> Reported-by: Abaci <abaci@xxxxxxxxxxxxxxxxx>
>> Fixes: 6c503150ae33 ("io_uring: patch up IOPOLL overflow_flush sync")
>> Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
>> ---
>> 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
