From a3a0e43fd77013819e4b6f55e37e0efe8e35d805 Mon Sep 17 00:00:00 2001 From: Jens Axboe Date: Tue, 20 Aug 2019 11:03:11 -0600 Subject: [PATCH] io_uring: don't enter poll loop if we have CQEs pending 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 --- 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 36f04d0b197b..e7a43a354d91 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -679,6 +679,13 @@ 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 */ @@ -818,6 +825,14 @@ 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 @@ -2449,13 +2464,6 @@ 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.