From: Vivek Goyal <vgoyal@redhat.com>
To: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-nvdimm@lists.01.org, virtio-fs@redhat.com,
dan.j.williams@intel.com
Cc: vgoyal@redhat.com, miklos@szeredi.hu, jack@suse.cz,
willy@infradead.org, slp@redhat.com
Subject: [PATCH v5 3/3] dax: Wake up all waiters after invalidating dax entry
Date: Wed, 28 Apr 2021 12:50:40 -0400 [thread overview]
Message-ID: <20210428165040.1856202-4-vgoyal@redhat.com> (raw)
In-Reply-To: <20210428165040.1856202-1-vgoyal@redhat.com>
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@redhat.com>
Fixes: ac401cc78242 ("dax: New fault locking")
Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
fs/dax.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/dax.c b/fs/dax.c
index e84dd240c35c..42d7406b2cea 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
prev parent reply other threads:[~2021-04-28 16:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-28 16:50 [PATCH v5 0/3] dax: Fix missed wakeup in put_unlocked_entry() Vivek Goyal
2021-04-28 16:50 ` [PATCH v5 1/3] dax: Add an enum for specifying dax wakup mode Vivek Goyal
2021-04-28 17:03 ` Vivek Goyal
2021-04-28 16:50 ` [PATCH v5 2/3] dax: Add a wakeup mode parameter to put_unlocked_entry() Vivek Goyal
2021-04-28 16:50 ` Vivek Goyal [this message]
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