linux-kernel - Re: [PATCH RT 1/2] rwsem-rt: Do not allow readers to nest

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Date:	Wed, 25 Feb 2015 13:15:57 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Carsten Emde <C.Emde@...dl.org>,
	John Kacur <jkacur@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RT 1/2] rwsem-rt: Do not allow readers to nest

On 02/18/2015 09:13 PM, Steven Rostedt wrote:

>> Here the same thing but without cmpxchg(). _If_ after an increment the
>> value is negative then we take slowpath. Otherwise we have the lock.
> 
> OK, so I need to make it so it can nest with trylock. I have to look at
> the patch again because it has been a while.

I have reverted the patch and can confirm that cpufreq works again.

I did some testing on vanilla and -RT:
- down_read(l) + down_read(l)
  this triggers a lockdep warning about a possible deadlock the lock is
  obtained.

- down_read(l) + down_read_trylock()
  this passes without a warning.

So I think we good now.

> An RW sem must not do two down_read()s on the same lock (it's fine for
> a trylock if it has a fail safe for it). The reason is, the second
> down_read() will block if there's a writer waiting. Thus you are
> guaranteed a deadlock if you have the lock for read, a write comes in
> and waits, and you grab the RW sem again, because it will block, and
> the writer is waiting for the reader to release. Thus you have a
> deadlock.

I fully understand. However nesting is allowed according to the code in
vanilla and now again in -RT. Lockdep complains properly so we should
catch people doing it wrong in both trees.

> I'll have to revisit this. I also need to revisit the multi readers
> (although Thomas hates it, but he even admitted there's a better way to
> do it. Now only if I could remember what that was ;-)

Okay. For now I keep the revert since it looks sane and simple.

> 
> Thanks,
> 
> -- Steve

Sebastian
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