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From: Thomas Gleixner <tglx@linutronix.de>
To: Chris Friesen <chris.friesen@windriver.com>
Cc: rt-users <linux-rt-users@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: semantics of reader/writer semaphores in rt patch
Date: Sun, 26 Oct 2014 00:21:31 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.11.1410260019280.5308@nanos> (raw)
In-Reply-To: <alpine.DEB.2.11.1410252354030.5308@nanos>

On Sun, 26 Oct 2014, Thomas Gleixner wrote:
> On Thu, 23 Oct 2014, Chris Friesen wrote:
> > Does the RT kernel just disallow this sort of algorithm?
> 
> Yes. For a good reason. Let's add thread C
> 
> A    	   	B		C
> down_read(X)
> 				down_write(X)
> lock(Y)
> modify data
> unlock(Y)
> wake(B)
> 		down_read(X)
> 
> Due to the mainline rwsem fairness semantics:
> 
> A holds X, C is blocked on A and B is blocked on A.
> 
> Deadlock, without RT and the single reader restriction being involved.
> 
> So RT does not violate ANY of the existing mainline semantics, it just
> imposes a performance impact of not allowing multiple readers.

@peterz: It might be worthwhile to have a CONFIG_LOCKDEP=y dependent
mode which restricts concurrent readers to 1 in mainline to catch this
kind of stuff. Hmm?

Thanks,

	tglx

  reply	other threads:[~2014-10-25 22:21 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <544956B8.2000406@windriver.com>
2014-10-25 22:19 ` Thomas Gleixner
2014-10-25 22:21   ` Thomas Gleixner [this message]
2014-10-26  7:42     ` Peter Zijlstra
2014-10-26  8:47       ` Peter Zijlstra
2014-10-27 15:02   ` Chris Friesen

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