linux-kernel - Re: [PATCH] vfs: Fix RCU path walk failiures due to uninitialized nameidata seq number for root directory

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Date:	Sat, 16 Apr 2011 07:43:52 +0200
From:	Sedat Dilek <sedat.dilek@...glemail.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, shaohua.li@...el.com,
	alex.shi@...el.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] vfs: Fix RCU path walk failiures due to uninitialized
 nameidata seq number for root directory

On Fri, Apr 15, 2011 at 11:54 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Apr 15, 2011 at 2:09 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> On 4/15/2011 11:39 AM, Tim Chen wrote:
>>>
>>> During RCU walk in path_lookupat and path_openat, the rcu lookup
>>> frequently failed because when root directory was looked up, seq number
>>> was not properly set in nameidata.  We dropped out of RCU walk in
>>> nameidata_drop_rcu due to mismatch in directory entry's seq number.  We
>>> reverted to slow path walk that need to take references.
>>
>> Thanks Tim. Adding Andrew, Linus too. IMHO this fix is quite important to
>> actually make the fabled RCU dcache work -- without it it's just slower
>> because
>> it will fallback nearly allways.
>
> Well, only for absolute paths, but yes.
>
> I think all my benchmarking was for thing like "git diff", which are
> all about the relative paths and wouldn't have triggered this case.
>
> So this patch does look correct, and yes, should also be stable material for 38.
>
> And it's pretty fundamental. But I'd like to get a few more acks
> exactly because it's so fundamental. Al and Nick sadly are both gone.
> Anybody else feel like they know something about the path lookup code?
>
>                             Linus
>

I see this patch was pushed.

What's with fs-synchronize_rcu-when-unregister_filesystem-success-not-failure.patch
from [1]?

- Sedat -

[1] https://patchwork.kernel.org/patch/707322/
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