linux-kernel - Re: rt_mutex_timed_lock() vs hrtimer

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Message-ID: <20060801001258.GA130@oleg>
Date:	Tue, 1 Aug 2006 04:12:58 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Esben Nielsen <nielsen.esben@...glemail.com>,
	linux-kernel@...r.kernel.org
Subject: Re: rt_mutex_timed_lock() vs hrtimer_wakeup() race ?

On 07/30, Steven Rostedt wrote:
>
> On Sun, 2006-07-30 at 08:36 +0400, Oleg Nesterov wrote:
> > 
> > Another question, task_blocks_on_rt_mutex() does get_task_struct() and checks
> > owner->pi_blocked_on != NULL under owner->pi_lock. Why ? RT_MUTEX_HAS_WAITERS
> > bit is set, we are holding ->wait_lock, so the 'owner' can't go away until
> > we drop ->wait_lock.
> 
> That's probably true that the owner can't disappear before we let go of
> the wait_lock, since the owner should not be disappearing while holding
> locks.  But you are missing the point to why we are grabbing the
> pi_lock.  We are preventing a race that can have us do unneeded work
> (see below).

Yes, I see. But ...

> ===================================================================
> --- linux-2.6.18-rc2.orig/kernel/rtmutex.c	2006-07-30 18:04:12.000000000 -0400
> +++ linux-2.6.18-rc2/kernel/rtmutex.c	2006-07-30 18:07:08.000000000 -0400
> @@ -433,25 +433,26 @@ static int task_blocks_on_rt_mutex(struc
>	...
>  	else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
>  		spin_lock_irqsave(&owner->pi_lock, flags);
> -		if (owner->pi_blocked_on) {
> +		if (owner->pi_blocked_on)
>  			boost = 1;
> -			/* gets dropped in rt_mutex_adjust_prio_chain()! */
> -			get_task_struct(owner);
> -		}
>  		spin_unlock_irqrestore(&owner->pi_lock, flags);

In that case ->pi_lock can't buy anything. With or without ->pi_lock this
check is equally racy, so spin_lock() only adds unneeded work?

Thanks!

Oleg.

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