Re: signals not reliably interrupting io_uring_enter anymore

[Jens Axboe <axboe@kernel.dk>]

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;
 }

-- 
Jens Axboe