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

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Date:	Tue, 10 Jun 2014 19:07:16 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 06/10, Thomas Gleixner wrote:
>
> +static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
> +	__releases(lock->wait_lock)
> +{
> +	unsigned long owner, *p = (unsigned long *) &lock->owner;
> +
> +	owner = (unsigned long) rt_mutex_owner(lock);
> +	clear_rt_mutex_waiters(lock);
> +	raw_spin_unlock(&lock->wait_lock);
> +	/*
> +	 * If a new waiter comes in between the unlock and the cmpxchg
> +	 * we have two situations:
> +	 *
> +	 * unlock(wait_lock);
> +	 *					lock(wait_lock);
> +	 * cmpxchg(p, owner, 0) == owner
> +	 *					mark_rt_mutex_waiters(lock);
> +	 *					acquire(lock);
> +	 * or:
> +	 *
> +	 * unlock(wait_lock);
> +	 *					lock(wait_lock);
> +	 *					mark_rt_mutex_waiters(lock);
> +	 *
> +	 * cmpxchg(p, owner, 0) != owner
> +	 *					enqueue_waiter();
> +	 *					unlock(wait_lock);
> +	 * lock(wait_lock);
> +	 * wake waiter();
> +	 * unlock(wait_lock);
> +	 *					lock(wait_lock);
> +	 *					acquire(lock);
> +	 */
> +	return rt_mutex_cmpxchg(p, owner, 0);

Wait, but this looks like a typo. rt_mutex_cmpxchg() needs "struct rt_mutex *",
not "long *". It seems that you should simply kill "*p" above.

Oleg.

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