4.9-stable review patch. If anyone has any objections, please let me know.
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From: Will Deacon will.deacon@arm.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@infradead.org Cc: Al Viro viro@zeniv.linux.org.uk Reported-by: Naresh Madhusudana naresh.madhusudana@arm.com Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Matthew Wilcox mawilcox@microsoft.com Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.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);