linux-kernel - Re: [PATCH 5/7] module: take rcu_read_lock

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Date:	Thu, 15 Mar 2012 09:18:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dennis1.Chen@....com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	NeilBrown <neilb@...e.de>, Kay Sievers <kay.sievers@...y.org>,
	Joe Perches <joe@...ches.com>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/7] module: take rcu_read_lock_sched() for /proc read

On Thu, 2012-03-15 at 22:48 +0800, Cong Wang wrote:
> In the seq file operations, it still hold module_mutex for just reading
> the module list, switch to rcu_read_lock_sched() too.
> 

Why is it needed ?

1) insmod/rmmod/lsmod is hardly in fast path.

2) after a mutex_lock(), a task is still preemptible
   after rcu_read_lock_sched(), preemption is disabled

   with NR_CPUS=4096, getting module refcounts is _very_ slow

RCU doesnt fit every workload.



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