linux-kernel - Re: 3.6rc6 slab corruption.

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Message-ID: <20120920211443.GA27312@konrad-lan.dumpdata.com>
Date:	Thu, 20 Sep 2012 17:14:44 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Suzuki Poulose <suzuki@...ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: 3.6rc6 slab corruption.

On Wed, Sep 19, 2012 at 04:20:25PM -0700, David Rientjes wrote:
> On Wed, 19 Sep 2012, Linus Torvalds wrote:
> 
> > That does look simpler, and avoiding the lock is a good idea. Since we
> > don't support lseek() (or pread/pwrite) on that thing anyway, there's
> > no way to keep the fd open and just re-use it to read the data over
> > and over, so populating it at open time sounds like a good solution
> > with no real downsides.
> > 
> 
> Yeah, my patch is functionally the same as what we currently have with the 
> only exception being that it isn't racy.  I'm wondering if that's what we 
> really want, though, since the data read from the file will remain 
> persistent as long as it is opened.  That obviously happens in my patch 
> because we allocate and copy the buffer at open(), but also happens 
> implicitly with the old code precisely because it's a non-seekable file 
> and *ppos == 0 only once (when not racy).
> 
> So if the API for these xen files is to remain persistent after open() as 
> it currently does, then my patch solves the issue.  However, if the API 

Nah. It was initially a debug option to see how contended the spinlocks
are. Nobody but developers should look at it - and they can deal with
open/close cycle. Thought we should probably provide a nice little
comment in the file mentioning the reason for stale data.

> wants to allow to only open() once and then read the spinlock_stats data 
> continuously, then we'll need the mutex: allocate the file->private_data 
> buffer once at open() for the maximum allowable size and then copy to the 
> buffer from xen's spinlock_stats under the protection of the mutex to 
> read().
> 
> Konrad?

Your patch is way simpler and it does the job better than mine.
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