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Date: Tue, 13 Feb 2018 12:58:51 +0000 From: Will Deacon <will.deacon@....com> To: linux-fsdevel@...r.kernel.org Cc: linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>, Peter Zijlstra <peterz@...radead.org>, Al Viro <viro@...iv.linux.org.uk> Subject: [PATCH] fs: dcache: Avoid livelock between d_alloc_parallel and __d_add 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> Signed-off-by: Will Deacon <will.deacon@....com> --- fs/dcache.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/fs/dcache.c b/fs/dcache.c index 7c38f39958bc..b243deec298c 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2474,7 +2474,7 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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 @@ struct dentry *d_alloc_parallel(struct dentry *parent, 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); -- 2.1.4
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