linux-kernel - Re: [BUG] staging: android: ashmem: Deadlock during ashmem

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Date:	Tue, 30 Apr 2013 18:59:44 +0530
From:	Shankar Brahadeeswaran <shankoo77@...il.com>
To:	Robert Love <rlove@...gle.com>
Cc:	Dan Carpenter <dan.carpenter@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bjorn Bringert <bringert@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>, devel@...verdev.osuosl.org,
	Hugh Dickins <hughd@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Anjana V Kumar <anjanavk12@...il.com>
Subject: Re: [BUG] staging: android: ashmem: Deadlock during ashmem_shrink

Hi Robert,
Thanks for the patch. In the first email in this thread I was
proposing the same solution and had asked whether doing this has any
side effects.
That is how this discussion started. I did some experiments and have
got the answers for that. Just for every ones benefit I've re-worded
the question again
and put the details of the experiment below.

Question:
On occasions when we return because of the lock unavailability, what
could be the worst case number of ashmem pages that are left
unfreed (lru_count). Will it be very huge and have side effects?

To get the answer for this question, I added some instrumentation code
to ashmem_shrink function on top of the patch. I ran Android monkey
tests with lot of memory hungry applications so as to hit the Low
Memory situation more frequently. After running this for almost a day
I did not see a situation where the shrinker did not have the mutex.
In fact what I found is that (in this use case at-least) most of the
time the "lru_count" is zero, which means the application has not
unpinned the pages. So the shrinker has no job to do (basically
shrink_slab does not call ashmem_shrinker second time). So worst case
if we hit a scenario where the shrinker is called I'm sure the
lru_count would be very low. So even if the shrinker returns without
freeing them (because of unavailability of the lock) its not going to
be costly.

After this experiment, I too think that this patch (returning from
ashmem_shrink if the lock is not available) is good enough and does
not seem to have any major side effects.

PS: Any plans of submitting this patch formally?

Warm Regards,
Shankar
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