linux-kernel - Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock

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Message-ID: <20121024171855.GA22371@redhat.com>
Date:	Wed, 24 Oct 2012 19:18:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock_sched

On 10/24, Paul E. McKenney wrote:
>
> static inline void percpu_up_read(struct percpu_rw_semaphore *p)
> {
> 	/*
> 	 * Decrement our count, but protected by RCU-sched so that
> 	 * the writer can force proper serialization.
> 	 */
> 	rcu_read_lock_sched();
> 	this_cpu_dec(*p->counters);
> 	rcu_read_unlock_sched();
> }

Yes, the explicit lock/unlock makes the new assumptions about
synchronize_sched && barriers unnecessary. And iiuc this could
even written as

	rcu_read_lock_sched();
	rcu_read_unlock_sched();

	this_cpu_dec(*p->counters);


> Of course, it would be nice to get rid of the extra synchronize_sched().
> One way to do this is to use SRCU, which allows blocking operations in
> its read-side critical sections (though also increasing read-side overhead
> a bit, and also untested):
>
> ------------------------------------------------------------------------
>
> struct percpu_rw_semaphore {
> 	bool locked;
> 	struct mutex mtx; /* Could also be rw_semaphore. */
> 	struct srcu_struct s;
> 	wait_queue_head_t wq;
> };

but in this case I don't understand

> static inline void percpu_up_write(struct percpu_rw_semaphore *p)
> {
> 	/* Allow others to proceed, but not yet locklessly. */
> 	mutex_unlock(&p->mtx);
>
> 	/*
> 	 * Ensure that all calls to percpu_down_read() that did not
> 	 * start unambiguously after the above mutex_unlock() still
> 	 * acquire the lock, forcing their critical sections to be
> 	 * serialized with the one terminated by this call to
> 	 * percpu_up_write().
> 	 */
> 	synchronize_sched();

how this synchronize_sched() can help...

Oleg.

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