linux-kernel - Re: semantics of reader/writer semaphores in rt patch

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Message-ID: <alpine.DEB.2.11.1410260019280.5308@nanos>
Date:	Sun, 26 Oct 2014 00:21:31 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chris Friesen <chris.friesen@...driver.com>
cc:	rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: semantics of reader/writer semaphores in rt patch

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
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