linux-kernel - Re: [PATCH tip/core/rcu 08/17] rcu: Allow post-unlock reference for rt

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Date:	Wed, 9 Jul 2014 09:50:09 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	<linux-kernel@...r.kernel.org>, <mingo@...nel.org>,
	<dipankar@...ibm.com>, <akpm@...ux-foundation.org>,
	<mathieu.desnoyers@...icios.com>, <josh@...htriplett.org>,
	<niv@...ibm.com>, <tglx@...utronix.de>, <peterz@...radead.org>,
	<rostedt@...dmis.org>, <dhowells@...hat.com>,
	<edumazet@...gle.com>, <dvhart@...ux.intel.com>,
	<fweisbec@...il.com>, <oleg@...hat.com>, <sbw@....edu>
Subject: Re: [PATCH tip/core/rcu 08/17] rcu: Allow post-unlock reference for
 rt_mutex

On 07/08/2014 06:38 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> The current approach to RCU priority boosting uses an rt_mutex strictly
> for its priority-boosting side effects.  The rt_mutex_init_proxy_locked()
> function is used by the booster to initialize the lock as held by the
> boostee.  The booster then uses rt_mutex_lock() to acquire this rt_mutex,
> which priority-boosts the boostee.  When the boostee reaches the end
> of its outermost RCU read-side critical section, it checks a field in
> its task structure to see whether it has been boosted, and, if so, uses
> rt_mutex_unlock() to release the rt_mutex.  The booster can then go on
> to boost the next task that is blocking the current RCU grace period.
> 
> But reasonable implementations of rt_mutex_unlock() might result in the
> boostee referencing the rt_mutex's data after releasing it. 

XXXX_unlock(lock_ptr) should not reference to the lock_ptr after it has unlocked the lock. (*)
So I think this patch is unneeded. Although its adding overhead is at slow-patch,
but it adds REVIEW-burden.

And although the original rt_mutex_unlock() violates the rule(*) when the fast-cmpxchg-path,
but it is fixed now.

It is the lock-subsystem's responsible to do this. I prefer to add the wait_for_complete()
stuff until the future when the boostee needs to re-access the booster after rt_mutex_unlock()
instead of now.

Thanks,
Lai
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