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From:   "Pavel Begunkov (Silence)" <asml.silence@gmail.com>
To:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH] io_uring: remove wait loop spurious wakeups
Date:   Tue,  8 Oct 2019 02:18:42 +0300
Message-Id: <936cd758d6c694fe1b8b9de050e24cfecdc2e60d.1570489620.git.asml.silence@gmail.com>
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From: Pavel Begunkov <asml.silence@gmail.com>

Any changes interesting to tasks waiting in io_cqring_wait() are
commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_refs()
also tries to do that but with no reason, that means spurious wakeups
every io_free_req() and io_uring_enter().

Just use percpu_ref_put() instead.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c934f91c51e9..89d77a626063 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -591,14 +591,6 @@ static void io_cqring_add_event(struct io_ring_ctx *ctx, u64 user_data,
 	io_cqring_ev_posted(ctx);
 }
 
-static void io_ring_drop_ctx_refs(struct io_ring_ctx *ctx, unsigned refs)
-{
-	percpu_ref_put_many(&ctx->refs, refs);
-
-	if (waitqueue_active(&ctx->wait))
-		wake_up(&ctx->wait);
-}
-
 static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
 				   struct io_submit_state *state)
 {
@@ -646,7 +638,7 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
 	req->result = 0;
 	return req;
 out:
-	io_ring_drop_ctx_refs(ctx, 1);
+	percpu_ref_put(&ctx->refs);
 	return NULL;
 }
 
@@ -654,7 +646,7 @@ static void io_free_req_many(struct io_ring_ctx *ctx, void **reqs, int *nr)
 {
 	if (*nr) {
 		kmem_cache_free_bulk(req_cachep, *nr, reqs);
-		io_ring_drop_ctx_refs(ctx, *nr);
+		percpu_ref_put_many(&ctx->refs, *nr);
 		*nr = 0;
 	}
 }
@@ -663,7 +655,7 @@ static void __io_free_req(struct io_kiocb *req)
 {
 	if (req->file && !(req->flags & REQ_F_FIXED_FILE))
 		fput(req->file);
-	io_ring_drop_ctx_refs(req->ctx, 1);
+	percpu_ref_put(&req->ctx->refs);
 	kmem_cache_free(req_cachep, req);
 }
 
@@ -3630,7 +3622,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 		}
 	}
 
-	io_ring_drop_ctx_refs(ctx, 1);
+	percpu_ref_put(&ctx->refs);
 out_fput:
 	fdput(f);
 	return submitted ? submitted : ret;
-- 
2.23.0