Commit 3d3a2e61 authored Apr 24, 2018 by David Sterba

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 <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>

parent 110a21fe

fs/btrfs/tree-log.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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;

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