linux-kernel - Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110607184857.GC23214@somewhere>
Date:	Tue, 7 Jun 2011 20:49:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Milton Miller <miltonm@....com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent
 states

On Tue, Jun 07, 2011 at 11:34:14AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 07, 2011 at 02:58:13PM +0200, Frederic Weisbecker wrote:
> > On Mon, Jun 06, 2011 at 09:40:05PM -0700, Paul E. McKenney wrote:> 
> > > The bit I am missing is how to distinguish between spinlocks (where
> > > sleeping is illegal) and mutexes (where sleeping is perfectly fine).
> > > We could teach lockdep the difference, I suppose, but it is not clear
> > > to me that it is worth it.
> > 
> > Ah, in fact it doesn't pass through any lockdep check.
> > 
> > It's only a function called might_sleep() that is placed in functions
> > that can sleep. And inside might_sleep() it checks whether it is in a preemptible
> > area. So it's actually locking-agnostic, it only relies on the preempt_count
> > and some more for the preempt rcu cases.
> > 
> > I think it is called CONFIG_DEBUG_SPINLOCK_SLEEP because it was first used
> > for spinlock debugging purposes. But then it has a broader use now: sleep
> > inside preemptible section, sleep inside interrupts, sleep inside rcu.
> 
> But the __might_sleep() function can only differentiate between
> spinlocks and sleeplocks if CONFIG_PREEMPT=y.

It doesn't differentiate between locks but checks on the lowest level
by looking at the preempt count. But yeah it only works if CONFIG_PREEMPT,
which is why I proposed to inc/dec the preempt count also when we have
that DEBUG_SPINLOCK_SLEEP.

> 
> > It certainly deserves a rename, like CONFIG_DEBUG_ILLEGAL_SLEEP.
> 
> Hmmm...  It already checks for sleeping in the middle of a
> preempt_disable() as well as in a spinlock critical section.
> So the need for a rename is independent of any RCU checking.

Sure, rcu just adds itself to the pile of users of might_sleep(), thus
it would be a nice cleanup to rename the option to something more
generic. But that rename is not necessary to improve RCU checking.

> 
> > > In contrast, with RCU, this is straightforward -- check for rcu_sched
> > > and rcu_bh, but not SRCU.
> 
> Actually it makes sense to keep the checks in rcu_note_context_switch(),
> as there are places that call schedule() directly without a might_sleep().
> Perhaps having checks in both places is the correct approach?

In this case it makes more sense to add your checks in schedule_debug(),
so that we don't wait for a context switch to detect the bug.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/