lkml.org 
[lkml]   [2018]   [Feb]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] fs: dcache: Avoid livelock between d_alloc_parallel and __d_add
Date
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>
Signed-off-by: Will Deacon <will.deacon@arm.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
\
 
 \ /
  Last update: 2018-02-13 13:59    [W:0.036 / U:0.276 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site