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Date: Sat, 7 Mar 2015 10:21:26 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michel Lespinasse <walken@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running
Message-ID: <20150307092126.GO23367@worktop.ger.corp.intel.com>
References: <1425714331.2475.388.camel@j-VirtualBox>
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On Fri, Mar 06, 2015 at 11:45:31PM -0800, Jason Low wrote:
>  static noinline
>  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
>  {
>  	long count;
>  
>  	rcu_read_lock();
> +	while (sem->owner == owner) {
> +		/*
> +		 * Ensure we emit the owner->on_cpu, dereference _after_
> +		 * checking sem->owner still matches owner, if that fails,
> +		 * owner might point to free()d memory, if it still matches,
> +		 * the rcu_read_lock() ensures the memory stays valid.
> +		 */
> +		barrier();
> +
> +		/* abort spinning when need_resched or owner is not running */
> +		if (!owner->on_cpu || need_resched()) {
>  			rcu_read_unlock();
>  			return false;
>  		}


Thanks, looks good; do we want to change the mutex code (again) to more
closely resemble this too? It still has the while (true) instead of the
while(lock->owner != owner).