linux-kernel - Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

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Message-ID: <20140612172844.GA15795@redhat.com>
Date:	Thu, 12 Jun 2014 19:28:44 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
	unprotected when accessed by /proc)

On 06/11, Paul E. McKenney wrote:
>
> On Wed, Jun 11, 2014 at 07:59:34PM +0200, Oleg Nesterov wrote:
> > On 06/11, Paul E. McKenney wrote:
> > >
> > > I was thinking of ->boost_completion as the way to solve it easily, but
> > > what did you have in mind?
> >
> > I meant, rcu_boost() could probably just do "mtx->owner = t", we know that
> > it was unlocked by us and nobody else can use it until we set
> > t->rcu_boost_mutex.
>
> My concern with this is that rcu_read_unlock_special() could hypothetically
> get preempted (either by kernel or hypervisor), so that it might be a long
> time until it makes its reference.  But maybe that reference would be
> harmless in this case.

Confused... Not sure I understand what did you mean, and certainly I do not
understand how this connects to the proxy-locking method.

Could you explain?

> > And if we move it into rcu_node, then we can probably kill ->rcu_boost_mutex,
> > rcu_read_unlock_special() could check rnp->boost_mutex->owner == current.
>
> If this was anywhere near a hot code path, I would be sorely tempted.

Ah, but I didn't mean perfomance. I think it is always good to try to remove
something from task_struct, it is huge. I do not mean sizeof() in the first
place, the very fact that I can hardly understand the purpose of a half of its
members makes me sad ;)

Oleg.

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