[Virtio-fs] [PATCH] dax: Fix missed wakeup in put_unlocked_entry()

[Vivek Goyal]

On Fri, Apr 16, 2021 at 12:56:05PM -0700, Dan Williams wrote:
> On Fri, Apr 16, 2021 at 10:35 AM Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > 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 32M to reproduce
> > the problem consistently.
> >
> > This is not a complete patch. I am just proposing this partial fix to
> > highlight the issue and trying to figure out how it should be fixed.
> > Should it be fixed in generic dax code or should filesystem (fuse/virtiofs)
> > take care of this.
> >
> > 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.
> >
> > invalidate_inode_pages2()
> >   invalidate_inode_pages2_range()
> >     invalidate_exceptional_entry2()
> >       dax_invalidate_mapping_entry_sync()
> >         __dax_invalidate_entry() {
> >                 xas_lock_irq(&xas);
> >                 entry = get_unlocked_entry(&xas, 0);
> >                 ...
> >                 ...
> >                 dax_disassociate_entry(entry, mapping, trunc);
> >                 xas_store(&xas, NULL);
> >                 ...
> >                 ...
> >                 put_unlocked_entry(&xas, entry);
> >                 xas_unlock_irq(&xas);
> >         }
> >
> > 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 "entry=0"
> > 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
> > up.
> >
> > In my case I am seeing that dax page fault path itself is waiting
> > on grab_mapping_entry() and also invalidate_inode_page2() is
> > waiting in get_unlocked_entry() but entry has already been cleaned
> > up and nobody woke up these processes. Atleast I think that's what
> > is happening.
> >
> > This patch wakes up a process even if entry=0. And deadlock does not
> > happen. I am running into some OOM issues, that will debug.
> >
> > So my question is that is it a dax issue and should it be fixed in
> > dax layer. Or should it be handled in fuse to make sure that
> > multiple instances of invalidate_inode_pages2() on same inode
> > don't make progress in parallel and introduce enough locking
> > around it.
> >
> > Right now fuse_finish_open() calls invalidate_inode_pages2() without
> > any locking. That allows it to make progress in parallel to dax
> > fault path as well as allows multiple instances of invalidate_inode_pages2()
> > to run in parallel.
> >
> > Not-yet-signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> > ---
> >  fs/dax.c |    7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > Index: redhat-linux/fs/dax.c
> > ===================================================================
> > --- redhat-linux.orig/fs/dax.c  2021-04-16 12:50:40.141363317 -0400
> > +++ redhat-linux/fs/dax.c       2021-04-16 12:51:42.385926390 -0400
> > @@ -266,9 +266,10 @@ static void wait_entry_unlocked(struct x
> >
> >  static void put_unlocked_entry(struct xa_state *xas, void *entry)
> >  {
> > -       /* If we were the only waiter woken, wake the next one */
> > -       if (entry && !dax_is_conflict(entry))
> > -               dax_wake_entry(xas, entry, false);
> > +       if (dax_is_conflict(entry))
> > +               return;
> > +
> > +       dax_wake_entry(xas, entry, false);
> 

Hi Dan,

> How does this work if entry is NULL? dax_entry_waitqueue() will not
> know if it needs to adjust the index.

Wake waiters both at current index as well PMD adjusted index. It feels
little ugly though.

> I think the fix might be to
> specify that put_unlocked_entry() in the invalidate path needs to do a
> wake_up_all().

Doing a wake_up_all() when we invalidate an entry, sounds good. I will give
it a try.

Thanks
Vivek