Re: [PATCH -next] ashmem: Fix ashmem_shrink deadlock.

From: Robert Love
Date: Thu May 16 2013 - 13:29:14 EST

On Thu, May 16, 2013 at 1:19 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 16 May 2013 13:08:17 -0400 Robert Love <rlove@xxxxxxxxxx> wrote:
>> This problem seems a rare proper use of mutex_trylock.
> Not really. The need for a trylock is often an indication that a
> subsystem has a locking misdesign. That is indeed the case here.

It is exactly the same as PF_MEMALLOC. We've got an effectively
asynchronous event (shrinking) that can occur while you are holding
locks requisite to that shrinking. Given that the shrinkage is best
effort, a trylock actually communicates the intent pretty well: "If
possible, grab this lock and shrink."

I think the idiomatic fix is to introduce a GFP_SHMEM but that seems
overkill. Lots of the GFP flags are really just preventing recursing
into the shrinkage code and it seems ill-designed that we require
developers to know where they might end up. But we can disagree. :)

> Well, it's not exactly a ton of work, but adding a per-ashmem_area lock
> to protect ->file would rather be putting lipstick on a pig. I suppose
> we can put the trylock in there and run away, but it wouldn't hurt to
> drop in a big fat comment somewhere explaining that the driver should be
> migrated to a per-object locking scheme.

Unfortunately I think ashmem_shrink would need to grab the per-object
lock too; it needs to update the ranges. I'm sure we could re-design
this but I don't think it is as easy as simply pushing the locking
into the objects.

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