Re: signals not reliably interrupting io_uring_enter anymore

[Jens Axboe <axboe@kernel.dk>]

On 7/3/20 7:52 PM, Jens Axboe wrote:
> On 7/3/20 7:13 PM, Andres Freund wrote:
>> Hi, 
>>
>> On July 3, 2020 5:48:21 PM PDT, Jens Axboe <axboe@kernel.dk> wrote:
>>> On 7/3/20 6:15 PM, Andres Freund wrote:
>>>> Hi,
>>>>
>>>> On 2020-07-03 17:00:49 -0700, Andres Freund wrote:
>>>>> I haven't yet fully analyzed the problem, but after updating to
>>>>> cdd3bb54332f82295ed90cd0c09c78cd0c0ee822 io_uring using postgres
>>> does
>>>>> not work reliably anymore.
>>>>>
>>>>> The symptom is that io_uring_enter(IORING_ENTER_GETEVENTS) isn't
>>>>> interrupted by signals anymore. The signal handler executes, but
>>>>> afterwards the syscall is restarted. Previously io_uring_enter
>>> reliably
>>>>> returned EINTR in that case.
>>>>>
>>>>> Currently postgres relies on signals interrupting io_uring_enter().
>>> We
>>>>> probably can find a way to not do so, but it'd not be entirely
>>> trivial.
>>>>>
>>>>> I suspect the issue is
>>>>>
>>>>> commit ce593a6c480a22acba08795be313c0c6d49dd35d (tag:
>>> io_uring-5.8-2020-07-01, linux-block/io_uring-5.8)
>>>>> Author: Jens Axboe <axboe@kernel.dk>
>>>>> Date:   2020-06-30 12:39:05 -0600
>>>>>
>>>>>     io_uring: use signal based task_work running
>>>>>
>>>>> as that appears to have changed the error returned by
>>>>> io_uring_enter(GETEVENTS) after having been interrupted by a signal
>>> from
>>>>> EINTR to ERESTARTSYS.
>>>>>
>>>>>
>>>>> I'll check to make sure that the issue doesn't exist before the
>>> above
>>>>> commit.
>>>>
>>>> Indeed, on cd77006e01b3198c75fb7819b3d0ff89709539bb the PG issue
>>> doesn't
>>>> exist, which pretty much confirms that the above commit is the issue.
>>>>
>>>> What was the reason for changing EINTR to ERESTARTSYS in the above
>>>> commit? I assume trying to avoid returning spurious EINTRs to
>>> userland?
>>>
>>> Yeah, for when it's running task_work. I wonder if something like the
>>> below will do the trick?
>>>
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 700644a016a7..0efa73d78451 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -6197,11 +6197,11 @@ static int io_cqring_wait(struct io_ring_ctx
>>> *ctx, int min_events,
>>> 	do {
>>> 		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
>>> 						TASK_INTERRUPTIBLE);
>>> -		/* make sure we run task_work before checking for signals */
>>> -		if (current->task_works)
>>> -			task_work_run();
>>> 		if (signal_pending(current)) {
>>> -			ret = -ERESTARTSYS;
>>> +			if (current->task_works)
>>> +				ret = -ERESTARTSYS;
>>> +			else
>>> +				ret = -EINTR;
>>> 			break;
>>> 		}
>>> 		if (io_should_wake(&iowq, false))
>>> @@ -6210,7 +6210,7 @@ static int io_cqring_wait(struct io_ring_ctx
>>> *ctx, int min_events,
>>> 	} while (1);
>>> 	finish_wait(&ctx->wait, &iowq.wq);
>>>
>>> -	restore_saved_sigmask_unless(ret == -ERESTARTSYS);
>>> +	restore_saved_sigmask_unless(ret == -EINTR);
>>>
>>> 	return READ_ONCE(rings->cq.head) == READ_ONCE(rings->cq.tail) ? ret :
>>> 0;
>>> }
>>
>> I'll try in a bit. Suspect however that there'd be trouble if there
>> were both an actual signal and task work pending?
> 
> Yes, I have that worry too. We'd really need to check if we have an
> actual signal pending - if we do, we still do -EINTR. If not, then we
> just do -ERESTARTSYS and restart the system call after task_work has
> been completed. Half-assed approach below, I suspect this won't _really_
> work without appropriate locking. Which would be unfortunate.
> 
> Either that, or we'd need to know if an actual signal was delivered when
> we get re-entered due to returning -ERESTARTSYS. If it was just
> task_work being run, then we're fine. But if an actual signal was
> pending, then we'd need to return -EINTR.
> 
> CC'ing Oleg to see if he has any good ideas here.

This might be simpler:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 700644a016a7..8f54fd5085bf 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6197,11 +6197,11 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	do {
 		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
 						TASK_INTERRUPTIBLE);
-		/* make sure we run task_work before checking for signals */
-		if (current->task_works)
-			task_work_run();
 		if (signal_pending(current)) {
-			ret = -ERESTARTSYS;
+			if (!sigisemptyset(&current->pending.signal))
+				ret = -EINTR;
+			else
+				ret = -ERESTARTSYS;
 			break;
 		}
 		if (io_should_wake(&iowq, false))
@@ -6210,7 +6210,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	} while (1);
 	finish_wait(&ctx->wait, &iowq.wq);
 
-	restore_saved_sigmask_unless(ret == -ERESTARTSYS);
+	restore_saved_sigmask_unless(ret == -EINTR);
 
 	return READ_ONCE(rings->cq.head) == READ_ONCE(rings->cq.tail) ? ret : 0;
 }

-- 
Jens Axboe