Re: [PATCH] dax: Fix missed wakeup in put_unlocked_entry()

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Apr 16, 2021 at 10:35 AM Vivek Goyal <vgoyal@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@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);

How does this work if entry is NULL? dax_entry_waitqueue() will not
know if it needs to adjust the index. I think the fix might be to
specify that put_unlocked_entry() in the invalidate path needs to do a
wake_up_all().