Date: Fri, 10 Apr 2015 10:22:45 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Kirill Tkhai <ktkhai@parallels.com>, linux-kernel@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Michal Hocko <mhocko@suse.cz>,
        Rik van Riel <riel@redhat.com>, Ionut Alexa <ionut.m.alexa@gmail.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Kirill Tkhai <tkhai@yandex.ru>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] exit: Use read lock for do_notify_parent() instead of
 write lock
Message-ID: <20150410082245.GA25842@gmail.com>
References: <1428602348.12166.29.camel@parallels.com>
 <20150409182838.GZ5029@twins.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Apr 09, 2015 at 08:59:08PM +0300, Kirill Tkhai wrote:
> 
> I've not really read your email yet, however:
> 
> > Also, in the future we may think about new rwlock primitive, which atomically
> > drops write lock and acquires read lock. Something like this for example:
> > 
> > include/asm-generic/qrwlock.h:
> > static inline void queue_reduce_locked_write_to_read(struct qrwlock *lock)
> > {
> > 	smp_mb__before_atomic();
> > 	atomic_add(_QR_BIAS - _QW_LOCKED, &lock->cnts);
> > }
> 
> we actually have that for the rwsems: downgrade_write(). So the
> consistent naming would be: queue_downgrade_write().

and 'downgrade_write_lock()' on the rwlock side.

Thanks,

	Ingo
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