4.4-stable review patch. If anyone has any objections, please let me know.
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From: David Sterba dsterba@suse.com
[ 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@linuxfoundation.org ---
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@suse.com Link: https://lkml.kernel.org/r/6ee068d8-1a69-3728-00d1-d86293d43c9f@suse.com Reviewed-by: Nikolay Borisov nborisov@suse.com Signed-off-by: David Sterba dsterba@suse.com
Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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;