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Subject[PATCH v4] io_uring: don't wait when under-submitting
Date
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.

In such cases adjust min_complete, so it won't wait for more than
what have been submitted in the current io_uring_enter() call. It
may be less than total in-flight, but that up to a user to handle.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---

v2: cap min_complete if submitted partially (Jens Axboe)
v3: update commit message (Jens Axboe)
v4: fix behavioural change when submitting more than have (Jens Axboe)

fs/io_uring.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5ad652fa24b8..167fbcc8be0b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4463,11 +4463,15 @@ 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 (flags & IORING_ENTER_GETEVENTS) {
unsigned nr_events = 0;

min_complete = min(min_complete, ctx->cq_entries);
+ if (submitted != to_submit)
+ min_complete = min(min_complete, (u32)submitted);

if (ctx->flags & IORING_SETUP_IOPOLL) {
ret = io_iopoll_check(ctx, &nr_events, min_complete);
@@ -4475,7 +4479,7 @@ 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);
--
2.24.0
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