linux-kernel - Re: [PATCH 1/2] break out page allocation warning code

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Date:	Thu, 28 Apr 2011 16:48:20 -0700
From:	john stultz <johnstul@...ibm.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] break out page allocation warning code

On Thu, 2011-04-28 at 15:48 -0700, David Rientjes wrote:
> On Wed, 27 Apr 2011, john stultz wrote:
> 
> > So thinking further, this can be simplified by adding the seqlock first,
> > and then retaining the task_locking only in the set_task_comm path until
> > all comm accessors are converted to using get_task_comm.
> > 
> 
> On second thought, I think it would be better to just retain using a 
> spinlock but instead of using alloc_lock, introduce a new spinlock to 
> task_struct for the sole purpose of protecting comm.
> 
> And, instead, of using get_task_comm() to write into a preallocated 
> buffer, I think it would be easier in the vast majority of cases that 
> you'll need to convert to just provide task_comm_lock(p) and 
> task_comm_unlock(p) so that p->comm can be dereferenced safely.  

So my concern with this is that it means one more lock that could be
mis-nested. By keeping the locking isolated to the get/set_task_comm, we
can be sure that won't happen. 

Also tracking new current->comm references will be easier if we just
don't allow new ones. Validating that all the comm references are
correctly locked becomes more difficult if we need locking at each use
site.

Further, since I'm not convinced that we never reference current->comm
from irq context, if we go with spinlocks, we're going to have to
disable irqs in the read path as well. seqlocks were nice for that
aspect.

> get_task_comm() could use that interface itself and then write into a 
> preallocated buffer.
> 
> The problem with using get_task_comm() everywhere is it requires 16 
> additional bytes to be allocated on the stack in hundreds of locations 
> around the kernel which may or may not be safe.

True. Although is this maybe a bit overzealous?

Maybe I can make sure not to add any mid-layer stack nesting by limiting
the scope of the 16bytes to just around where it is used.  This would
ensure we're only adding 16bytes to any current usage.

Other ideas?

thanks
-john

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