Commit 47be6184 authored Jul 06, 2016 by Wei Fang Committed by Al Viro Jul 24, 2016

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



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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Wei Fang <fangwei1@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent 285b102d

fs/dcache.c

+5 −2

Original line number	Diff line number	Diff line
		@@ -596,7 +596,6 @@ static struct dentry dentry_kill(struct dentry dentry)

		failed:
		spin_unlock(&dentry->d_lock);
		cpu_relax();
		return dentry; /* try again with same dentry */
		}

		@@ -770,6 +769,8 @@ void dput(struct dentry *dentry)
		return;

		repeat:
		might_sleep();

		rcu_read_lock();
		if (likely(fast_dput(dentry))) {
		rcu_read_unlock();
		@@ -803,9 +804,11 @@ repeat:

		kill_it:
		dentry = dentry_kill(dentry);
		if (dentry)
		if (dentry) {
		cond_resched();
		goto repeat;
		}
		}
		EXPORT_SYMBOL(dput);

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