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

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Date:	Sun, 26 Oct 2014 08:42:57 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Chris Friesen <chris.friesen@...driver.com>,
	rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: semantics of reader/writer semaphores in rt patch

On Sun, Oct 26, 2014 at 12:21:31AM +0200, Thomas Gleixner wrote:
> 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?

There were patches by ego that fix lockdep's read side tracking. I need
to find a few spare days to look at those :/
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