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Date: Mon, 22 Oct 2012 19:36:45 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <peterz@infradead.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH 0/2] fix and improvements for percpu-rw-semaphores (was:
 brw_mutex: big read-write mutex)
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> > Ooooh. And I just noticed include/linux/percpu-rwsem.h which does
> > something similar. Certainly it was not in my tree when I started
> > this patch... percpu_down_write() doesn't allow multiple writers,
> > but the main problem it uses msleep(1). It should not, I think.
> > 
> > But. It seems that percpu_up_write() is equally wrong? Doesn't
> > it need synchronize_rcu() before "p->locked = false" ?
> > 
> > (add Mikulas)
> 
> Mikulas said something about doing an updated patch, so I figured I
> would look at his next version.
> 
> 							Thanx, Paul

The best ideas proposed in this thread are:

Using heavy/light barries by Lai Jiangshan. This fixes the missing barrier 
bug, removes the ugly test "#if defined(X86) ..." and makes the read path 
use no barrier instruction on all architectures.

Instead of rcu_read_lock, we can use rcu_read_lock_sched (or 
preempt_disable) - the resulting code is smaller. The critical section is 
so small that there is no problem disabling preemption.

I am sending these two patches. Linus, please apply them if there are no 
objections.

Mikulas