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

From: Robert Love
Date: Thu May 16 2013 - 13:08:28 EST

On Thu, May 16, 2013 at 12:45 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> A better approach would be to add a new __GFP_NOSHRINKERS, but it's all
> variations on a theme.

I don't like this proposal, either. Many of the existing GFP flags
already exist to prevent recurse into that flag's respective shrinker.

This problem seems a rare proper use of mutex_trylock.

> The mutex_trylock(ashmem_mutex) will actually have the best
> performance, because it skips the least amount of memory reclaim
> opportunities.


> But it still sucks! The real problem is that there exists a lock
> called "ashmem_mutex", taken by both the high-level mmap() and by the
> low-level shrinker. And taken by everything else too! The ashmem
> locking is pretty crude...

The locking is "crude" because I optimized for space, not time, and
there was (and is) no indication we were suffering lock contention due
to the global lock. I haven't thought through the implications of
pushing locking into the ashmem_area and ashmem_range objects, but it
does look like we'd end up often grabbing all of the locks ...

> What is the mutex_lock() in ashmem_mmap() actually protecting? I don't
> see much, apart from perhaps some incidental races around the contents
> of the file's ashmem_area, and those could/should be protected by a
> per-object lock, not a global one?

... but not, as you note, in ashmem_mmap. The main race there is
around the allocation of asma->file. That could definitely be a lock
local to ashmem_area. I'm OK if anyone wants to take that on but it
seems a lot of work for a driver with an unclear future.

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