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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Dave Hansen <dave@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] break out page allocation warning code
Cc: kosaki.motohiro@jp.fujitsu.com, David Rientjes <rientjes@google.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>,
        John Stultz <johnstul@us.ibm.com>
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Hi

(Cc to  John Stultz who/proc/<pid>/comm author. I think we need to hear his opinion)

> On Tue, 2011-04-19 at 14:21 -0700, David Rientjes wrote:
> > On Tue, 19 Apr 2011, KOSAKI Motohiro wrote:
> > > The rule is,
> > > 
> > > 1) writing comm
> > > 	need task_lock
> > > 2) read _another_ thread's comm
> > > 	need task_lock
> > > 3) read own comm
> > > 	no need task_lock
> > 
> > That was true a while ago, but you now need to protect every thread's 
> > ->comm with get_task_comm() or ensuring task_lock() is held to protect 
> > against /proc/pid/comm which can change other thread's ->comm.  That was 
> > different before when prctl(PR_SET_NAME) would only operate on current, so 
> > no lock was needed when reading current->comm.
> 
> Everybody still goes through set_task_comm() to _set_ it, though.  That
> means that the worst case scenario that we get is output truncated
> (possibly to nothing).  We already have at least one existing user in
> mm/ (kmemleak) that thinks this is OK.  I'd tend to err in the direction
> of taking a truncated or empty task name to possibly locking up the
> system.
> 
> There are also plenty of instances of current->comm going in to the
> kernel these days.  I count 18 added since 2.6.37.
> 
> As for a long-term fix, locks probably aren't the answer.  Would
> something like this completely untested patch work?  It would have the
> added bonus that it keeps tsk->comm users working for the moment.  We
> could eventually add an rcu_read_lock()-annotated access function.

The concept is ok to me. but AFAIK some caller are now using ARRAY_SIZE(tsk->comm).
or sizeof(tsk->comm). Probably callers need to be changed too.

Thanks.


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