[PATCH 3.2 144/152] dcache: Fix locking bugs in backported "deal with deadlock in d_walk()"

Ben Hutchings
2015-02-17T01:46:53Z

From: Ben Hutchings <ben@decadent.org.uk>

Steven Rostedt reported:
> Porting -rt to the latest 3.2 stable tree I triggered this bug:
>
> =====================================
> [ BUG: bad unlock balance detected! ]
> -------------------------------------
> rm/1638 is trying to release lock (rcu_read_lock) at:
> [<c04fde6c>] rcu_read_unlock+0x0/0x23
> but there are no more locks to release!
>
> other info that might help us debug this:
> 2 locks held by rm/1638:
> #0: (&sb->s_type->i_mutex_key#9/1){+.+.+.}, at: [<c04f93eb>] do_rmdir+0x5f/0xd2
> #1: (&sb->s_type->i_mutex_key#9){+.+.+.}, at: [<c04f9329>] vfs_rmdir+0x49/0xac
>
> stack backtrace:
> Pid: 1638, comm: rm Not tainted 3.2.66-test-rt96+ #2
> Call Trace:
> [<c083f390>] ? printk+0x1d/0x1f
> [<c0463cdf>] print_unlock_inbalance_bug+0xc3/0xcd
> [<c04653a8>] lock_release_non_nested+0x98/0x1ec
> [<c046228d>] ? trace_hardirqs_off_caller+0x18/0x90
> [<c0456f1c>] ? local_clock+0x2d/0x50
> [<c04fde6c>] ? d_hash+0x2f/0x2f
> [<c04fde6c>] ? d_hash+0x2f/0x2f
> [<c046568e>] lock_release+0x192/0x1ad
> [<c04fde83>] rcu_read_unlock+0x17/0x23
> [<c04ff344>] shrink_dcache_parent+0x227/0x270
> [<c04f9348>] vfs_rmdir+0x68/0xac
> [<c04f9424>] do_rmdir+0x98/0xd2
> [<c04f03ad>] ? fput+0x1a3/0x1ab
> [<c084dd42>] ? sysenter_exit+0xf/0x1a
> [<c0465b58>] ? trace_hardirqs_on_caller+0x118/0x149
> [<c04fa3e0>] sys_unlinkat+0x2b/0x35
> [<c084dd13>] sysenter_do_call+0x12/0x12
>
>
>
>
> There's a path to calling rcu_read_unlock() without calling
> rcu_read_lock() in have_submounts().
>
> goto positive;
>
> positive:
> if (!locked && read_seqretry(&rename_lock, seq))
> goto rename_retry;
>
> rename_retry:
> rcu_read_unlock();
>
> in the above path, rcu_read_lock() is never done before calling
> rcu_read_unlock();

I reviewed locking contexts in all three functions that I changed when
backporting "deal with deadlock in d_walk()". It's actually worse
than this:

- We don't hold this_parent->d_lock at the 'positive' label in
have_submounts(), but it is unlocked after 'rename_retry'.
- There is an rcu_read_unlock() after the 'out' label in
select_parent(), but it's not held at the 'goto out'.

Fix all three lock imbalances.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
---
fs/dcache.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1035,7 +1035,7 @@ ascend:
return 0; /* No mount points found in tree */
positive:
if (!locked && read_seqretry(&rename_lock, seq))
- goto rename_retry;
+ goto rename_retry_unlocked;
if (locked)
write_sequnlock(&rename_lock);
return 1;
@@ -1045,6 +1045,7 @@ rename_retry:
rcu_read_unlock();
if (locked)
goto again;
+rename_retry_unlocked:
locked = 1;
write_seqlock(&rename_lock);
goto again;
@@ -1109,6 +1110,7 @@ resume:
*/
if (found && need_resched()) {
spin_unlock(&dentry->d_lock);
+ rcu_read_lock();
goto out;
}