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Message-ID: <20160714183743.GK30154@twins.programming.kicks-ass.net>
Date: Thu, 14 Jul 2016 20:37:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...nel.org, linux-kernel@...r.kernel.org
Cc: oleg@...hat.com, tj@...nel.org, paulmck@...ux.vnet.ibm.com,
john.stultz@...aro.org, dimitrysh@...gle.com, romlem@...gle.com,
ccross@...gle.com, tkjos@...gle.com
Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob
On Thu, Jul 14, 2016 at 08:25:47PM +0200, Peter Zijlstra wrote:
> @@ -19,6 +20,13 @@ int __percpu_init_rwsem(struct percpu_rw
>
> /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
> rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
> + if (bias == PERCPU_RWSEM_WRITER) {
> + /*
> + * Disable rcu_sync() and force slow path.
> + */
> + sem->rss.gp_count++;
> + sem->rss.gp_state = !0;
> + }
> __init_rwsem(&sem->rw_sem, name, rwsem_key);
> init_waitqueue_head(&sem->writer);
> sem->state = readers_slow;
So this seemed like a better deal than calling rcu_sync_enter(), because
that would still incur a (pointless) synchronize_sched() at init time
and people do tend to complain about things like that.
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