Subject: Re: [PATCH 1/2] break out page allocation warning code
From: john stultz <johnstul@us.ibm.com>
To: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Dave Hansen <dave@linux.vnet.ibm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Nazarewicz <mina86@mina86.com>,
        Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <1303950728.2971.35.camel@work-vm>
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Date: Wed, 27 Apr 2011 18:29:53 -0700
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On Wed, 2011-04-27 at 17:32 -0700, john stultz wrote:
> On Wed, 2011-04-27 at 16:51 -0700, David Rientjes wrote:
> > On Tue, 26 Apr 2011, john stultz wrote:
> > > In the meantime, I'll put some effort into trying to protect unlocked
> > > current->comm acccess using get_task_comm() where possible. Won't happen
> > > in a day, and help would be appreciated. 
> > > 
> > 
> > We need to stop protecting ->comm with ->alloc_lock since it is used for 
> > other members of task_struct that may or may not be held in a function 
> > that wants to read ->comm.  We should probably introduce a seqlock.
> 
> Agreed. My initial approach is to consolidate accesses to use
> get_task_comm(), with special case to skip the locking if tsk==current,
> as well as a lock free __get_task_comm() for cases where its not current
> being accessed and the task locking is already done.
> 
> Once that's all done, the next step is to switch to a seqlock (or
> possibly RCU if Dave is still playing with that idea), internally in the
> get_task_comm implementation and then yank the special __get_task_comm. 

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.

I'll be sending out some initial patches for review shortly.

thanks
-john


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