linux-kernel - Re: [PATCH] exit: Use read lock for do_notify

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Date:	Fri, 10 Apr 2015 10:22:45 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Michal Hocko <mhocko@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Ionut Alexa <ionut.m.alexa@...il.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Kirill Tkhai <tkhai@...dex.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] exit: Use read lock for do_notify_parent() instead of
 write lock


* Peter Zijlstra <peterz@...radead.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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