linux-kernel - [PATCH v6 3/3] dax: Wake up all waiters after invalidating dax entry

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Date:   Wed, 28 Apr 2021 15:03:14 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, virtio-fs@...hat.com,
        dan.j.williams@...el.com
Cc:     vgoyal@...hat.com, willy@...radead.org, jack@...e.cz,
        groug@...d.org, Sergio Lopez <slp@...hat.com>
Subject: [PATCH v6 3/3] dax: Wake up all waiters after invalidating dax entry

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 wake these waiters.

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.

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@...hat.com>
Fixes: ac401cc78242 ("dax: New fault locking")
Reviewed-by: Jan Kara <jack@...e.cz>
Suggested-by: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
---
 fs/dax.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 56eb1c759ca5..df5485b4bddf 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -675,7 +675,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
 	mapping->nrexceptional--;
 	ret = 1;
 out:
-	put_unlocked_entry(&xas, entry, WAKE_NEXT);
+	put_unlocked_entry(&xas, entry, WAKE_ALL);
 	xas_unlock_irq(&xas);
 	return ret;
 }
-- 
2.25.4