linux-kernel - Re: Mutex vs semaphores scheduler bug

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Message-Id: <1255793527.8152.4.camel@laptop>
Date:	Sat, 17 Oct 2009 17:32:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	Török Edwin <edwin@...mav.net>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	aCaB <acab@...mav.net>, Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Mutex vs semaphores scheduler bug

On Fri, 2009-10-16 at 00:44 +0100, David Howells wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > The problem appears to be that rwsem doesn't allow lock-stealing
> 
> With good reason.  rwsems can be read or write locked for a long time - so if
> readers can jump the queue on read-locked rwsems, then writer starvation is a
> real possibility.  I carefully implemented it so that it is a strict FIFO to
> avoid certain problems I was having.

Well, it kinda sucks that rwsem is slower than a mutex.

What about allowing writer stealing when the next contending task is a
writer?

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