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authorWill Deacon <will.deacon@arm.com>2018-02-19 14:55:54 +0000
committerAl Viro <viro@zeniv.linux.org.uk>2018-02-25 12:51:09 -0500
commit015555fd4d2930bc0c86952c46ad88b3392f66e4 (patch)
tree9b88022b9acc5dbb20c3bb767cfda68355abb5ae /fs/dcache.c
parent3b821409632ab778d46e807516b457dfa72736ed (diff)
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@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>
Diffstat (limited to 'fs/dcache.c')
-rw-r--r--fs/dcache.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/fs/dcache.c b/fs/dcache.c
index 32aaab21e648..bde3b6662601 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2479,7 +2479,7 @@ 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)) {
@@ -2500,6 +2500,12 @@ 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);