Patch "fs/dcache.c: avoid soft-lockup in dput()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

fs/dcache.c: avoid soft-lockup in dput()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fs-dcache.c-avoid-soft-lockup-in-dput.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wei Fang <[email protected]>
Date: Wed, 6 Jul 2016 11:32:20 +0800
Subject: fs/dcache.c: avoid soft-lockup in dput()

From: Wei Fang <[email protected]>

commit 47be61845c775643f1aa4d2a54343549f943c94c upstream.

We triggered soft-lockup under stress test which open/access/write/close one file concurrently on more than five different CPUs:

WARN: soft lockup - CPU#0 stuck for 11s! [who:30631]
...

[<ffffffc0003986f8>] dput+0x100/0x298
[<ffffffc00038c2dc>] terminate_walk+0x4c/0x60
[<ffffffc00038f56c>] path_lookupat+0x5cc/0x7a8
[<ffffffc00038f780>] filename_lookup+0x38/0xf0
[<ffffffc000391180>] user_path_at_empty+0x78/0xd0
[<ffffffc0003911f4>] user_path_at+0x1c/0x28
[<ffffffc00037d4fc>] SyS_faccessat+0xb4/0x230

->d_lock trylock may failed many times because of concurrently operations, and dput() may execute a long time.

Fix this by replacing cpu_relax() with cond_resched(). dput() used to be sleepable, so make it sleepable again should be safe.

---
 fs/dcache.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -578,7 +578,6 @@ static struct dentry *dentry_kill(struct
 
 failed:
 	spin_unlock(&dentry->d_lock);
-	cpu_relax();
 	return dentry; /* try again with same dentry */
 }
 

@@ -752,6 +751,8 @@ void dput(struct dentry *dentry)
 		return;
 
 repeat:
+	might_sleep();
+
 	rcu_read_lock();
 	if (likely(fast_dput(dentry))) {
 		rcu_read_unlock();

@@ -783,8 +784,10 @@ repeat:
 
 kill_it:
 	dentry = dentry_kill(dentry);
-	if (dentry)
+	if (dentry) {
+		cond_resched();
 		goto repeat;
+	}
 }
 EXPORT_SYMBOL(dput);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/fs-dcache.c-avoid-soft-lockup-in-dput.patch queue-4.4/fuse-fix-wrong-assignment-of-flags-in-fuse_send_init.patch