linux-kernel - [PATCH 4.9 142/329] fs: dcache: Avoid livelock between d_alloc_parallel and __d

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Message-Id: <20180528100254.388311994@linuxfoundation.org>
Date:   Mon, 28 May 2018 12:01:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Naresh Madhusudana <naresh.madhusudana@....com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Will Deacon <will.deacon@....com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.9 142/329] fs: dcache: Avoid livelock between d_alloc_parallel and __d_add

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@....com>

[ Upstream commit 015555fd4d2930bc0c86952c46ad88b3392f66e4 ]

If d_alloc_parallel runs concurrently with __d_add, it is possible for
d_alloc_parallel to continuously retry whilst i_dir_seq has been
incremented to an odd value by __d_add:

CPU0:
__d_add
	n = start_dir_add(dir);
		cmpxchg(&dir->i_dir_seq, n, n + 1) == n

CPU1:
d_alloc_parallel
retry:
	seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
	hlist_bl_lock(b);
		bit_spin_lock(0, (unsigned long *)b); // Always succeeds

CPU0:
	__d_lookup_done(dentry)
		hlist_bl_lock
			bit_spin_lock(0, (unsigned long *)b); // Never succeeds

CPU1:
	if (unlikely(parent->d_inode->i_dir_seq != seq)) {
		hlist_bl_unlock(b);
		goto retry;
	}

Since the simple bit_spin_lock used to implement hlist_bl_lock does not
provide any fairness guarantees, then CPU1 can starve CPU0 of the lock
and prevent it from reaching end_dir_add(dir), therefore CPU1 cannot
exit its retry loop because the sequence number always has the bottom
bit set.

This patch resolves the livelock by not taking hlist_bl_lock in
d_alloc_parallel if the sequence counter is odd, since any subsequent
masked comparison with i_dir_seq will fail anyway.

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Reported-by: Naresh Madhusudana <naresh.madhusudana@....com>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Matthew Wilcox <mawilcox@...rosoft.com>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/dcache.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2474,7 +2474,7 @@ struct dentry *d_alloc_parallel(struct d
 
 retry:
 	rcu_read_lock();
-	seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
+	seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
 	r_seq = read_seqbegin(&rename_lock);
 	dentry = __d_lookup_rcu(parent, name, &d_seq);
 	if (unlikely(dentry)) {
@@ -2495,6 +2495,12 @@ retry:
 		rcu_read_unlock();
 		goto retry;
 	}
+
+	if (unlikely(seq & 1)) {
+		rcu_read_unlock();
+		goto retry;
+	}
+
 	hlist_bl_lock(b);
 	if (unlikely(parent->d_inode->i_dir_seq != seq)) {
 		hlist_bl_unlock(b);