linux-kernel - Re: 3.6rc6 slab corruption.

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Message-ID: <20120920211814.GB27312@konrad-lan.dumpdata.com>
Date:	Thu, 20 Sep 2012 17:18:14 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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>
Subject: Re: 3.6rc6 slab corruption.

On Wed, Sep 19, 2012 at 07:46:57PM -0700, David Rientjes wrote:
> On Thu, 20 Sep 2012, Raghavendra K T wrote:
> 
> > Only problem, I find is histogram data expands dynamically (because it
> > changes). I think having static allocation of 352 bytes as suggested
> > Linus is a good idea.
> > 
> 
> Certainly, but it's a different topic and would be a subsequent patch to 
> either my patch or Konrad's patch.  Before that's done, I think we should 
> fix the race condition that currently exists either by:
> 
>  - merging my patch (which I can sign-off and write a changelog for if
>    Konrad agrees), or

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 
>  - merging Konrad's patch and introducing a mutex so that it's possible to 
>    do many reads to collect statistics after opening the file a single 
>    time with a single fd.
> 
> Since these files are incapable of doing lseek, it would seem that my 
> patch would be best and you'd simply want to close() + open() to read new 
> data, which also guarantees that all readers get the same information.  

Yup.
> The only reason I hesitate on that and will defer to Konrad's opinion is 
> because the way the code is currently written looks like it was intended 
> to copy the data are read() rather than open(); in other words, it almost 
> seems as if they were made to be non-seekable after the u32_array_read() 
> implementation was complete and it was at one time possible to do an 
> lseek(SEEK_SET).

The "users" (looks at himself and Raghavendra) can deal with the
open/close, open/close cycle. The only thing that I would add extra is
to add the explanation you provided in the comment of the file in case
somebody expects something else.
> 
> After that's fixed, and to address your concern, we can simply do the 
> allocation of file->private_data for the maximum size possible when the 
> file is created as a follow-up patch.
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