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Subject: Re: [PATCH 1/1] io_uring: don't wait when under-submitting
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <5caa38be87f069eb4cc921d58ee1a98ff5d53978.1576223348.git.asml.silence@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <21ca72b0-c35d-96b7-399f-d4034d976c27@kernel.dk>
Date:   Fri, 13 Dec 2019 11:22:46 -0700
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On 12/13/19 12:51 AM, Pavel Begunkov wrote:
> There is no reliable way to submit and wait in a single syscall, as
> io_submit_sqes() may under-consume sqes (in case of an early error).
> Then it will wait for not-yet-submitted requests, deadlocking the user
> in most cases.

Why not just cap the wait_nr? If someone does to_submit = 8, wait_nr = 8,
and we only submit 4, just wait for 4? Ala:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 81219a631a6d..4a76ccbb7856 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5272,6 +5272,10 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 		submitted = io_submit_sqes(ctx, to_submit, f.file, fd,
 					   &cur_mm, false);
 		mutex_unlock(&ctx->uring_lock);
+		if (submitted <= 0)
+			goto done;
+		if (submitted != to_submit && min_complete > submitted)
+			min_complete = submitted;
 	}
 	if (flags & IORING_ENTER_GETEVENTS) {
 		unsigned nr_events = 0;
@@ -5284,7 +5288,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			ret = io_cqring_wait(ctx, min_complete, sig, sigsz);
 		}
 	}
-
+done:
 	percpu_ref_put(&ctx->refs);
 out_fput:
 	fdput(f);

-- 
Jens Axboe