linux-kernel - Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock

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Date:	Thu, 2 Feb 2012 12:42:06 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for
 simple self-deadlock

On Thu, Feb 02, 2012 at 11:56:38AM -0800, Josh Triplett wrote:
> On Thu, Feb 02, 2012 at 08:20:17AM -0800, Paul E. McKenney wrote:
> > On Wed, Feb 01, 2012 at 04:55:54PM -0800, Josh Triplett wrote:
> > > On Wed, Feb 01, 2012 at 11:41:21AM -0800, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > 
> > > > It is illegal to have a grace period within a same-flavor RCU read-side
> > > > critical section, so this commit adds lockdep-RCU checks to splat when
> > > > such abuse is encountered.  This commit does not detect more elaborate
> > > > RCU deadlock situations.  These situations might be a job for lockdep
> > > > enhancements.
> > > 
> > > Since doing so also violates the prohibition on blocking within an RCU
> > > read-side critical section, wouldn't it suffice to call might_sleep() or
> > > equivalent, which also detects other problems?  (Obviously this doesn't
> > > apply to SRCU, but it applies to the other variants of RCU.)
> > 
> > Yes, but...
> > 
> > The advantage of the lockdep-RCU splat is that it gives you a better
> > hint as to where the RCU read-side critical section was entered, which
> > is very helpful when tracking these down, especially when they are
> > intermittent.
> 
> Ah, fair enough.
> 
> > And yes, I should also well check for the other variants of RCU read-side
> > critical section (other than RCU).  Done.
> 
> Oh?  What hadn't you checked for?

Things like synchronize_sched() in rcu_read_lock() critical section
and vice versa.

> > I also glued the strings together to promote grepability as you suggest
> > later.  (But I leave it to you to get checkpatch.pl upgraded -- it currently
> > warns about long lines, but not about strings split across lines.)
> 
> It theoretically shouldn't warn about long lines that consist only of a
> quoted string possibly followed by ',' or ');'; it has a check to ignore
> those.  After you glued the strings together, what did you end up with?

You are quite right -- it ignores the lines with long strings.

> As for adding a warning about strings broken across lines, that seems
> sensible.  Some quick grepping suggests that doing so would catch a pile
> of existing code, too.  Patch to follow momentarily.

Indeed, I might not be the only one to overgeneralize from the 80-character
warning.  ;-)

							Thanx, Paul

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