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FAILED: patch "[PATCH] fuse: fix blocked_waitq wakeup" failed to apply to 3.18-stable tree

PATCH [email protected] 1 participant 1 message Nov 15, 2018

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The patch below does not apply to the 3.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

Quoted 1 line

From 908a572b80f6e9577b45e81b3dfe2e22111286b8 Mon Sep 17 00:00:00 2001

From: Miklos Szeredi <[email protected]>
Date: Fri, 28 Sep 2018 16:43:22 +0200
Subject: [PATCH] fuse: fix blocked_waitq wakeup

Using waitqueue_active() is racy. Make sure we issue a wake_up() unconditionally after storing into fc->blocked. After that it's okay to optimize with waitqueue_active() since the first wake up provides the necessary barrier for all waiters, not the just the woken one.

Signed-off-by Miklos Szeredi <[email protected]>
Fixes 3c18ef8117f0 ("fuse: optimize wake_up")
Cc <[email protected]> # v3.10
fs/fuse/dev.c +11 -4
index 34976b42f3e1..51eb602a435b 100644
@@ -391,12 +391,19 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 	if (test_bit(FR_BACKGROUND, &req->flags)) {
 		spin_lock(&fc->lock);
 		clear_bit(FR_BACKGROUND, &req->flags);
-		if (fc->num_background == fc->max_background)
+		if (fc->num_background == fc->max_background) {
 			fc->blocked = 0;
-
-		/* Wake up next waiter, if any */
-		if (!fc->blocked && waitqueue_active(&fc->blocked_waitq))
 			wake_up(&fc->blocked_waitq);
+		} else if (!fc->blocked) {
+			/*
+			 * Wake up next waiter, if any.  It's okay to use
+			 * waitqueue_active(), as we've already synced up
+			 * fc->blocked with waiters with the wake_up() call
+			 * above.
+			 */
+			if (waitqueue_active(&fc->blocked_waitq))
+				wake_up(&fc->blocked_waitq);
+		}
 
 		if (fc->num_background == fc->congestion_threshold && fc->sb) {
 			clear_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);