linux-kernel - Re: v4.2-rc dcache regression, probably 75a6f82a0d10

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Message-ID: <20150731205036.GA3752@nautica>
Date:	Fri, 31 Jul 2015 22:50:36 +0200
From:	Dominique Martinet <asmadeus@...ewreck.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Dominique Martinet <dominique.martinet@....fr>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: v4.2-rc dcache regression, probably 75a6f82a0d10

Hugh Dickins wrote on Fri, Jul 31, 2015:
> It will indeed be weird and odd if it confirms that DCACHE_DISCONNECTED
> revert is good.  I agree that Dominique's 4bf46a272647 seems now more
> likely, if still unlikely; but that was included in v4.1, and I saw
> no problem with v4.1 once the rmap_walk() skip was fixed.

I think it could, actually, and that neither commits are actually bad --
just that they affect timing enough to raise an issue between d_delete
(I guess?) and link_path_walk (see last mail in other thread[1])

It's probably an old race that was very hard to hit because of cache
coherency.
Basically, before the wmb/rmb, the dentry was always updated closely to
its flags, so the other CPU would "usually" get both updates at the same
time; the barriers make it so the updates are split and it's possible to
get it, and would explain why I could pick 4bf46a2726 as "the one"

I'm not sure why the problem wouldn't arise on tmpfs though.

Hugh, could you try the reproducer I gave in the other thread[2] on both
filesystems maybe?
I need to let the thing run for a while, might need to tune params as
well. I was trying to fine tune cpu affinity with less threads but it's
not getting anywhere.

I'll also check if it's getting even easier to reproduce with
75a6f82a0d10 (or a recent kernel), who knows... How fast do you hit the
bug with the commit?

Thanks,
-- 
Dominique

[1] https://marc.info/?l=linux-fsdevel&m=143835651005259&w=2
[2] https://marc.info/?l=linux-fsdevel&m=143825706609188&w=2 
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