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

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Message-ID: <20140618164359.GA18598@redhat.com>
Date:	Wed, 18 Jun 2014 18:43:59 +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 06/17, Paul E. McKenney wrote:
>
> +		if (drop_boost_mutex) {
> +			rt_mutex_unlock(&rnp->boost_mtx);
>  			complete(&rnp->boost_completion);

Well, I still do not understand this ->boost_completion...

> -	/* Wait until boostee is done accessing mtx before reinitializing. */
> +	/* Wait for boostee to be done w/boost_mtx before reinitializing. */
>  	wait_for_completion(&rnp->boost_completion);

OK, at least we have a comment.

But let me repeat. Thomas has already fixed rt_mutex, unlock is atomic.
It doesn't touch this memory after it makes another lock() possible.

And (contrary to what I said initially) we can rely on this because -rt
converts spinlock_t into rt_mutex ?

Oleg.

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