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SubjectRe: [PATCH 1/2] break out page allocation warning code
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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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