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Subject[PATCH AUTOSEL 5.2 58/76] io_uring: don't enter poll loop if we have CQEs pending
Date
From: Jens Axboe <axboe@kernel.dk>

[ Upstream commit a3a0e43fd77013819e4b6f55e37e0efe8e35d805 ]

We need to check if we have CQEs pending before starting a poll loop,
as those could be the events we will be spinning for (and hence we'll
find none). This can happen if a CQE triggers an error, or if it is
found by eg an IRQ before we get a chance to find it through polling.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/io_uring.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5bb01d84f38d3..83e3cede11220 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -618,6 +618,13 @@ static void io_put_req(struct io_kiocb *req)
io_free_req(req);
}

+static unsigned io_cqring_events(struct io_cq_ring *ring)
+{
+ /* See comment at the top of this file */
+ smp_rmb();
+ return READ_ONCE(ring->r.tail) - READ_ONCE(ring->r.head);
+}
+
/*
* Find and free completed poll iocbs
*/
@@ -756,6 +763,14 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, unsigned *nr_events,
do {
int tmin = 0;

+ /*
+ * Don't enter poll loop if we already have events pending.
+ * If we do, we can potentially be spinning for commands that
+ * already triggered a CQE (eg in error).
+ */
+ if (io_cqring_events(ctx->cq_ring))
+ break;
+
/*
* If a submit got punted to a workqueue, we can have the
* application entering polling for a command before it gets
@@ -2232,13 +2247,6 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit)
return submit;
}

-static unsigned io_cqring_events(struct io_cq_ring *ring)
-{
- /* See comment at the top of this file */
- smp_rmb();
- return READ_ONCE(ring->r.tail) - READ_ONCE(ring->r.head);
-}
-
/*
* Wait until events become available, if we don't already have some. The
* application must reap them itself, as they reside on the shared cq ring.
--
2.20.1
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