I am seeing missed wakeups which ultimately lead to a deadlock when I am
using virtiofs with DAX enabled and running "make -j". I had to mount
virtiofs as rootfs and also reduce to dax window size to 256M to reproduce
the problem consistently.
So here is the problem. put_unlocked_entry() wakes up waiters only
if entry is not null as well as !dax_is_conflict(entry). But if I
call multiple instances of invalidate_inode_pages2() in parallel,
then I can run into a situation where there are waiters on
this index but nobody will wait these.
entry = get_unlocked_entry(&xas, 0);
dax_disassociate_entry(entry, mapping, trunc);
Say a fault in in progress and it has locked entry at offset say "0x1c".
Now say three instances of invalidate_inode_pages2() are in progress
(A, B, C) and they all try to invalidate entry at offset "0x1c". Given
dax entry is locked, all tree instances A, B, C will wait in wait queue.
When dax fault finishes, say A is woken up. It will store NULL entry
at index "0x1c" and wake up B. When B comes along it will find
at page offset 0x1c and it will call put_unlocked_entry(&xas, 0). And
this means put_unlocked_entry() will not wake up next waiter, given
the current code. And that means C continues to wait and is not woken
This patch fixes the issue by waking up all waiters when a dax entry
has been invalidated. This seems to fix the deadlock I am facing
and I can make forward progress.
Reported-by: Sergio Lopez <slp(a)redhat.com>
Fixes: ac401cc78242 ("dax: New fault locking")
Suggested-by: Dan Williams <dan.j.williams(a)intel.com>
Signed-off-by: Vivek Goyal <vgoyal(a)redhat.com>
fs/dax.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/dax.c b/fs/dax.c
index f19d76a6a493..cc497519be83 100644
@@ -676,7 +676,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
ret = 1;
- put_unlocked_entry(&xas, entry, WAKE_NEXT);
+ put_unlocked_entry(&xas, entry, WAKE_ALL);