linux-kernel - Re: [PATCH] Percpu tag allocator

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Date:	Thu, 13 Jun 2013 12:13:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Kent Overstreet <koverstreet@...gle.com>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Jens Axboe <axboe@...nel.dk>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Subject: Re: [PATCH] Percpu tag allocator

On Thu, 13 Jun 2013 12:06:10 -0700 Tejun Heo <tj@...nel.org> wrote:

> Hello, Andrew, Kent.
> 
> On Wed, Jun 12, 2013 at 04:38:54PM -0700, Andrew Morton wrote:
> ...
> > > +unsigned percpu_tag_alloc(struct percpu_tag_pool *pool, gfp_t gfp)
> > > +{
> > > +	DEFINE_WAIT(wait);
> > > +	struct percpu_tag_cpu_freelist *tags;
> > > +	unsigned long flags;
> > > +	unsigned tag, this_cpu;
> > > +
> > > +	while (1) {
> > > +		local_irq_save(flags);
> ...
> > > +		schedule();
> > > +	}
> > 
> > Does this loop need a try_to_freeze()?
> 
> I don't think so.  Kernel tasks should never enter freezer without it
> explicitly knowing it.  It should be something evident in the
> top-level control flow.  Freezer acts as a giant lock and entering
> freezer deep underneath where the task could be holding random number
> of resources and locks can easily develop into a deadlock.

As I understand it, if a task is stuck in this loop at freeze time, the
whole freeze attempt will fail.  But it's been a long time since I
thought about or worked on this stuff.

Another issue is device takedown ordering - this thread is blocked
waiting for tags to be returned by IO completion, so there may be
issues where the hardware has been shut down.

I really don't know - I'm flagging it as something which should be
thought about, tested, etc.

> If this allocation wait is gonna be visible to userland, what's
> necessary probably would be making the sleeping interruptible.  The
> freezer will then make the alloc fail and control should return to the
> signal delivery path where it'll be frozen without holding any
> resources.

Maybe.  Interruptible sleeps here will be a bit of a nuisance with
signals.  Poke Rafael ;)

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