Re: [PATCH] dax: Fix missed PMD wakeups

From: Jan Kara <jack@suse.cz>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Boaz Harrosh <openosd@gmail.com>, stable <stable@vger.kernel.org>,
	Robert Barror <robert.barror@intel.com>,
	Seema Pandit <seema.pandit@intel.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] dax: Fix missed PMD wakeups
Date: Mon, 29 Jul 2019 14:02:28 +0200	[thread overview]
Message-ID: <20190729120228.GC17833@quack2.suse.cz> (raw)
In-Reply-To: <CAPcyv4hA+44EHpGN9F5eQD5Y_AuyPTKmovNWvccAFGhF_O2JMg@mail.gmail.com>

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On Tue 16-07-19 20:39:46, Dan Williams wrote:
> On Fri, Jul 12, 2019 at 2:14 AM Jan Kara <jack@suse.cz> wrote:
> >
> > On Thu 11-07-19 08:25:50, Matthew Wilcox wrote:
> > > On Thu, Jul 11, 2019 at 07:13:50AM -0700, Matthew Wilcox wrote:
> > > > However, the XA_RETRY_ENTRY might be a good choice.  It doesn't normally
> > > > appear in an XArray (it may appear if you're looking at a deleted node,
> > > > but since we're holding the lock, we can't see deleted nodes).
> > >
> > ...
> >
> > > @@ -254,7 +267,7 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
> > >  static void put_unlocked_entry(struct xa_state *xas, void *entry)
> > >  {
> > >       /* If we were the only waiter woken, wake the next one */
> > > -     if (entry)
> > > +     if (entry && dax_is_conflict(entry))
> >
> > This should be !dax_is_conflict(entry)...
> >
> > >               dax_wake_entry(xas, entry, false);
> > >  }
> >
> > Otherwise the patch looks good to me so feel free to add:
> >
> > Reviewed-by: Jan Kara <jack@suse.cz>
> 
> Looks good, and passes the test case. Now pushed out to
> libnvdimm-for-next for v5.3 inclusion:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm.git/commit/?h=libnvdimm-for-next&id=23c84eb7837514e16d79ed6d849b13745e0ce688

Thanks for picking up the patch but you didn't apply the fix I've mentioned
above. So put_unlocked_entry() is not waking up anybody anymore... Since
this got already to Linus' tree, I guess a separate fixup patch is needed
(attached).

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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From 950204f7dfdb06198f40820be4d33ce824508f11 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Mon, 29 Jul 2019 13:57:49 +0200
Subject: [PATCH] dax: Fix missed wakup in put_unlocked_entry()

The condition checking whether put_unlocked_entry() needs to wake up
following waiter got broken by commit 23c84eb78375 ("dax: Fix missed
wakeup with PMD faults"). We need to wake the waiter whenever the passed
entry is valid (i.e., non-NULL and not special conflict entry). This
could lead to processes never being woken up when waiting for entry
lock. Fix the condition.

CC: stable@vger.kernel.org
Fixes: 23c84eb78375 ("dax: Fix missed wakeup with PMD faults")
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/dax.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index a237141d8787..b64964ef44f6 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -266,7 +266,7 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
 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))
+	if (entry && !dax_is_conflict(entry))
 		dax_wake_entry(xas, entry, false);
 }
 
-- 
2.16.4

