linux-kernel - Re: [PATCH -rt 1/9] preempt rcu: check for underflow

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Message-Id: <1185810528.2636.17.camel@imap.mvista.com>
Date:	Mon, 30 Jul 2007 08:48:48 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH -rt 1/9] preempt rcu: check for underflow

On Mon, 2007-07-30 at 11:22 +0200, Ingo Molnar wrote:
> * Daniel Walker <dwalker@...sta.com> wrote:
> 
> > +	/*
> > +	 * If our rcu_read_lock_nesting went negative, likely
> > +	 * something is wrong..
> > +	 */
> > +	WARN_ON(current->rcu_read_lock_nesting < 0);
> 
> have you actually caught any rcu locking problem this way? Double 
> unlocks should be caught by lockdep already, at a higher level.
> 
> in any case i've added a slightly different form of this change to the 
> -rt queue that will also check for counter overflows. But i'm not sure 
> we want to litter the code with trivial checks like this, so i'm keeping 
> it separate and if it does not trigger anything real i'll remove it.

I haven't caught anything with it, but this code would have made it much
easier to catch the single rcu unlock in sys_sched_yield() which was
silent in PREEMPT_RT, and hung !PREEMPT_RT ..

It's fine with me, if you have another method.

Daniel

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