linux-kernel - Re: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache

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Message-ID: <20180414080206.GV30522@ZenIV.linux.org.uk>
Date:   Sat, 14 Apr 2018 09:02:06 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Jeff Mahoney <jeffm@...e.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] fs/dcache.c: re-add cond_resched() in
 shrink_dcache_parent()

On Sat, Apr 14, 2018 at 10:00:29AM +0300, Nikolay Borisov wrote:
> 
> 
> On 14.04.2018 00:14, Andrew Morton wrote:
> > On Fri, 13 Apr 2018 13:28:23 -0700 Khazhismel Kumykov <khazhy@...gle.com> wrote:
> > 
> >> shrink_dcache_parent may spin waiting for a parallel shrink_dentry_list.
> >> In this case we may have 0 dentries to dispose, so we will never
> >> schedule out while waiting for the parallel shrink_dentry_list to
> >> complete.
> >>
> >> Tested that this fixes syzbot reports of stalls in shrink_dcache_parent()
> > 
> > Well I guess the patch is OK as a stopgap, but things seem fairly
> > messed up in there.  shrink_dcache_parent() shouldn't be doing a
> > busywait, waiting for the concurrent shrink_dentry_list().
> > 
> > Either we should be waiting (sleeping) for the concurrent operation to
> > complete or we should just bail out of shrink_dcache_parent(), perhaps
> > with 
> > 
> > 	if (list_empty(&data.dispose))
> > 		break;
> > 
> > or similar.  Dunno.
> 
> I agree, however, not being a dcache expert I'd refrain from touching
> it, since it seems to be rather fragile. Perhaps Al could take a look in
> there?

"Bail out" is definitely a bad idea, "sleep"... what on?  Especially
since there might be several evictions we are overlapping with...