linux-kernel - Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit reader rt locks

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Message-ID: <1337103096.27694.94.camel@twins>
Date:	Tue, 15 May 2012 19:31:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit
 reader rt locks

On Tue, 2012-05-15 at 13:25 -0400, Steven Rostedt wrote:
> On Tue, 2012-05-15 at 11:42 -0400, Steven Rostedt wrote:
> > On Tue, 2012-05-15 at 17:06 +0200, Peter Zijlstra wrote:
> > > On Tue, 2012-05-15 at 10:03 -0400, Steven Rostedt wrote:
> > > > 
> > > > where readers may nest (the same task may grab the same rwsem for
> > > > read multiple times), but only one task may hold the rwsem at any
> > > > given
> > > > time (for read or write).
> > > 
> > > Humm, that sounds iffy, rwsem isn't a recursive read lock only rwlock_t
> > > is.
> > 
> > In that case, current -rt is broken. As it has it being a recursive lock
> > (without my patch).

Nah not broken, just pointless. A recursive lock that's not used
recursively is fine.

> 
> Why wouldn't it be recursive. If two different tasks are allowed to grab
> a read lock at the same time, why can't the same task grab a read lock
> twice? As long as it releases it the same amount of times.
> 
> Now you can't grab a read lock if you have the write lock.

rwsem is fifo-fair, if a writer comes in between the second read
acquisition (even by the same task) would block and you'd be a deadlock
since the write won't succeed since you're still holding a reader.
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