Re: [PATCH 0/2] revert changes to zcache_do_preload()

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 08/24/2012 07:28 AM, Minchan Kim wrote:
> On Thu, Aug 23, 2012 at 05:10:00PM -0500, Seth Jennings wrote:
>> On 08/23/2012 03:56 PM, Minchan Kim wrote:
>>> Hi Seth,
>>>
>>> On Thu, Aug 23, 2012 at 10:33:09AM -0500, Seth Jennings wrote:
>>>> This patchset fixes a regression in 3.6 by reverting two dependent
>>>> commits that made changes to zcache_do_preload().
>>>>
>>>> The commits undermine an assumption made by tmem_put() in
>>>> the cleancache path that preemption is disabled.  This change
>>>> introduces a race condition that can result in the wrong page
>>>> being returned by tmem_get(), causing assorted errors (segfaults,
>>>> apparent file corruption, etc) in userspace.
>>>>
>>>> The corruption was discussed in this thread:
>>>> https://lkml.org/lkml/2012/8/17/494
>>>
>>> I think changelog isn't enough to explain what's the race.
>>> Could you write it down in detail?
>>
>> I didn't come upon this solution via code inspection, but
>> rather through discovering that the issue didn't exist in
>> v3.5 and just looking at the changes since then.
> 
> Okay, then, why do you think the patchsets are culprit?
> I didn't look the cleanup patch series of Xiao at that time
> so I can be wrong but as I just look through patch of
> "zcache: optimize zcache_do_preload", I can't find any fault
> because zcache_put_page checks irq_disable so we don't need
> to disable preemption so it seems that patch is correct to me.
> If the race happens by preemption, BUG_ON in zcache_put_page
> should catch it.

Confused me too!

And the first patch just do the cleanup, it is not different
before the patch and after the patch, what i missed?

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