From 3d3a2e610ea5e7c6d4f9481ecce5d8e2d8317843 Mon Sep 17 00:00:00 2001 From: David Sterba Date: Tue, 24 Apr 2018 14:53:56 +0200 Subject: [PATCH] btrfs: add barriers to btrfs_sync_log before log_commit_wait wakeups 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... --- 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 Link: https://lkml.kernel.org/r/6ee068d8-1a69-3728-00d1-d86293d43c9f@suse.com Reviewed-by: Nikolay Borisov Signed-off-by: David Sterba --- fs/btrfs/tree-log.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index 8f23a94..2009cea 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -3116,8 +3116,11 @@ 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: @@ -3128,8 +3131,11 @@ 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; -- 2.7.4