4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: David Sterba <dsterba@xxxxxxxx>
[ Upstream commit 3d3a2e610ea5e7c6d4f9481ecce5d8e2d8317843 ]
Currently the code assumes that there's an implied barrier by the
sequence of code preceding the wakeup, namely the mutex unlock.
As Nikolay pointed out:
I think this is wrong (not your code) but the original assumption that
the RELEASE semantics provided by mutex_unlock is sufficient.
According to memory-barriers.txt:
Section 'LOCK ACQUISITION FUNCTIONS' states:
(2) RELEASE operation implication:
Memory operations issued before the RELEASE will be completed before the
RELEASE operation has completed.
Memory operations issued after the RELEASE *may* be completed before the
RELEASE operation has completed.
(I've bolded the may portion)
The example given there:
As an example, consider the following:
*A = a;
*B = b;
ACQUIRE
*C = c;
*D = d;
RELEASE
*E = e;
*F = f;
The following sequence of events is acceptable:
ACQUIRE, {*F,*A}, *E, {*C,*D}, *B, RELEASE
So if we assume that *C is modifying the flag which the waitqueue is checking,
and *E is the actual wakeup, then those accesses can be re-ordered...
IMHO this code should be considered broken...
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
To be on the safe side, add the barriers. The synchronization logic
around log using the mutexes and several other threads does not make it
easy to reason for/against the barrier.
CC: Nikolay Borisov <nborisov@xxxxxxxx>
Link: https://lkml.kernel.org/r/6ee068d8-1a69-3728-00d1-d86293d43c9f@xxxxxxxx
Reviewed-by: Nikolay Borisov <nborisov@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/btrfs/tree-log.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2961,8 +2961,11 @@ out_wake_log_root:
mutex_unlock(&log_root_tree->log_mutex);
/*
- * The barrier before waitqueue_active is implied by mutex_unlock
+ * The barrier before waitqueue_active is needed so all the updates
+ * above are seen by the woken threads. It might not be necessary, but
+ * proving that seems to be hard.
*/
+ smp_mb();
if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
wake_up(&log_root_tree->log_commit_wait[index2]);
out:
@@ -2973,8 +2976,11 @@ out:
mutex_unlock(&root->log_mutex);
/*
- * The barrier before waitqueue_active is implied by mutex_unlock
+ * The barrier before waitqueue_active is needed so all the updates
+ * above are seen by the woken threads. It might not be necessary, but
+ * proving that seems to be hard.
*/
+ smp_mb();
if (waitqueue_active(&root->log_commit_wait[index1]))
wake_up(&root->log_commit_wait[index1]);
return ret;
