In __afs_break_callback() we might check ->cb_nr_mmap and if it's non-zero
do queue_work(&vnode->cb_work). In afs_drop_open_mmap() we decrement
->cb_nr_mmap and do flush_work(&vnode->cb_work) if it reaches zero.
The trouble is, there's nothing to prevent __afs_break_callback() from
seeing ->cb_nr_mmap before the decrement and do queue_work() after both
the decrement and flush_work(). If that happens, we might be in trouble -
vnode might get freed before the queued work runs.
__afs_break_callback() is always done under ->cb_lock, so let's make
sure that ->cb_nr_mmap can change from non-zero to zero while holding
->cb_lock (the spinlock component of it - it's a seqlock and we don't
need to mess with the counter).
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/afs/file.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/fs/afs/file.c b/fs/afs/file.c
index 3d33b221d9ca..ef2cc8f565d2 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -417,13 +417,17 @@ static void afs_add_open_mmap(struct afs_vnode *vnode)
static void afs_drop_open_mmap(struct afs_vnode *vnode)
{
- if (!atomic_dec_and_test(&vnode->cb_nr_mmap))
+ if (atomic_add_unless(&vnode->cb_nr_mmap, -1, 1))
return;
down_write(&vnode->volume->open_mmaps_lock);
- if (atomic_read(&vnode->cb_nr_mmap) == 0)
+ read_seqlock_excl(&vnode->cb_lock);
+ // the only place where ->cb_nr_mmap may hit 0
+ // see __afs_break_callback() for the other side...
+ if (atomic_dec_and_test(&vnode->cb_nr_mmap))
list_del_init(&vnode->cb_mmap_link);
+ read_sequnlock_excl(&vnode->cb_lock);
up_write(&vnode->volume->open_mmaps_lock);
flush_work(&vnode->cb_work);
--
2.39.2
