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

From: Andrew Morton
Date: Thu May 16 2013 - 12:46:13 EST

On Thu, 16 May 2013 09:44:49 -0400 Robert Love <rlove@xxxxxxxxxx> wrote:

> On Thu, May 16, 2013 at 4:15 AM, Raul Xiong <raulxiong@xxxxxxxxx> wrote:
> > The issue happens in such sequence:
> > ashmem_mmap acquired ashmem_mutex --> ashmem_mutex:shmem_file_setup
> > called kmem_cache_alloc --> shrink due to low memory --> ashmem_shrink
> > tries to acquire the same ashmem_mutex -- it blocks here.
> >
> > I think this reports the bug clearly. Please have a look.
> There is no debate about the nature of the bug. Only the fix.
> My mutex_trylock patch fixes the problem. I prefer that solution.
> Andrew's suggestion of GFP_ATOMIC won't work as we'd have to propagate
> that down into shmem and elsewhere.

s/won't work/impractical/

A better approach would be to add a new __GFP_NOSHRINKERS, but it's all
variations on a theme.

> Using PF_MEMALLOC will work. You'd want to define something like:
> static int set_memalloc(void)
> {
> if (current->flags & PF_MEMALLOC)
> return 0;
> current->flags |= PF_MEMALLOC;
> return 1;
> }
> static void clear_memalloc(int memalloc)
> {
> if (memalloc)
> current->flags &= ~PF_MEMALLOC;
> }
> and then set/clear PF_MEMALLOC around every memory allocation and
> function that descends into a memory allocation. As said I prefer my
> solution but if someone wants to put together a patch with this
> approach, fine by me.

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

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...

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?

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