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* [PATCH] io_uring: fix race with shadow drain deferrals
@ 2019-11-20 23:07 Jens Axboe
  2019-11-20 23:58 ` Jens Axboe
  0 siblings, 1 reply; 19+ messages in thread
From: Jens Axboe @ 2019-11-20 23:07 UTC (permalink / raw)
  To: io-uring; +Cc: Jackie Liu

When we go and queue requests with drain, we check if we need to defer
based on sequence. This is done safely under the lock, but then we drop
the lock before actually inserting the shadow. If the original request
is found on the deferred list by another completion in the mean time,
it could have been started AND completed by the time we insert the
shadow, which will stall the queue.

After re-grabbing the completion lock, check if the original request is
still in the deferred list. If it isn't, then we know that someone else
already found and issued it. If that happened, then our job is done, we
can simply free the shadow.

Cc: Jackie Liu <[email protected]>
Fixes: 4fe2c963154c ("io_uring: add support for link with drain")
Signed-off-by: Jens Axboe <[email protected]>

---

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6175e2e195c0..6fb25ae53817 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2973,6 +2973,7 @@ static void io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
 	int ret;
 	int need_submit = false;
 	struct io_ring_ctx *ctx = req->ctx;
+	struct io_kiocb *tmp;
 
 	if (unlikely(req->flags & REQ_F_FAIL_LINK)) {
 		ret = -ECANCELED;
@@ -3011,8 +3012,30 @@ static void io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
 
 	/* Insert shadow req to defer_list, blocking next IOs */
 	spin_lock_irq(&ctx->completion_lock);
-	trace_io_uring_defer(ctx, shadow, true);
-	list_add_tail(&shadow->list, &ctx->defer_list);
+	if (ret) {
+		/*
+		 * We dropped the lock since deciding we needed to defer this
+		 * request. We must re-check under the lock, if it's now gone
+		 * from the list, that means that another completion came in
+		 * and submitted it since we decided we needed to defer. If
+		 * that's the case, simply drop the shadow, there's nothing
+		 * more we need to do here.
+		 */
+		list_for_each_entry(tmp, &ctx->defer_list, list) {
+			if (tmp == req)
+				break;
+		}
+		if (tmp != req) {
+			__io_free_req(shadow);
+			shadow = NULL;
+		}
+	}
+	if (shadow) {
+		trace_io_uring_defer(ctx, shadow, true);
+		list_add_tail(&shadow->list, &ctx->defer_list);
+	} else {
+		need_submit = false;
+	}
 	spin_unlock_irq(&ctx->completion_lock);
 
 	if (need_submit)

-- 
Jens Axboe


^ permalink raw reply related	[flat|nested] 19+ messages in thread

end of thread, other threads:[~2019-11-21 15:26 UTC | newest]

Thread overview: 19+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2019-11-20 23:07 [PATCH] io_uring: fix race with shadow drain deferrals Jens Axboe
2019-11-20 23:58 ` Jens Axboe
2019-11-21  1:32   ` Jackie Liu
2019-11-21  1:35     ` Jackie Liu
2019-11-21  1:40       ` Jens Axboe
2019-11-21  1:49         ` Jens Axboe
2019-11-21  1:57           ` Jackie Liu
2019-11-20 23:14             ` Jens Axboe
     [not found]               ` <[email protected]>
2019-11-20 23:03                 ` Jens Axboe
2019-11-21  8:54           ` [PATCH] io_uring: drain next sqe instead of shadowing Pavel Begunkov
     [not found]             ` <[email protected]>
2019-11-21  9:43               ` Pavel Begunkov
     [not found]                 ` <[email protected]>
2019-11-21 12:40                   ` Pavel Begunkov
     [not found]                     ` <[email protected]>
2019-11-21 13:47                       ` Jens Axboe
     [not found]                         ` <[email protected]>
2019-11-21 13:54                           ` Jens Axboe
     [not found]                         ` <[email protected]>
2019-11-21 14:28                           ` Pavel Begunkov
2019-11-21 13:53                             ` Jens Axboe
2019-11-21 15:23                               ` Pavel Begunkov
2019-11-21 13:50                                 ` Jens Axboe
2019-11-21  1:39     ` [PATCH] io_uring: fix race with shadow drain deferrals Jens Axboe

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