Re: [PATCH] mm: memcontrol: correct the comment in mem_cgroup_swapout()

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Jun 19, 2015 at 07:18:31PM +0200, Sebastian Andrzej Siewior wrote:
> On 06/19/2015 07:11 PM, Johannes Weiner wrote:
> > On Fri, Jun 19, 2015 at 06:34:18PM +0200, Sebastian Andrzej Siewior wrote:
> >> Clark stumbled over a VM_BUG_ON() in -RT which was then was removed by
> >> Johannes in commit f371763a79d ("mm: memcontrol: fix false-positive
> >> VM_BUG_ON() on -rt"). The comment before that patch was a tiny bit
> >> better than it is now. While the patch claimed to fix a false-postive on
> >> -RT this was not the case. None of the -RT folks ACKed it and it was not a
> >> false positive report. That was a *real* problem.
> > 
> > The real problem is that irqs_disabled() on -rt is returning false
> > negatives.  Having it return false within a spin_lock_irq() section is
> > broken.
> 
> As I explained it in
> 	http://www.spinics.net/lists/linux-rt-users/msg13499.html
> it is not.

I missed this email, sorry about that.

> >> This patch updates the comment that is improper because it refers to
> >> "disabled preemption" as a consequence of that lock being taken. A
> >> spin_lock() disables preemption, true, but in this case the code relies on
> >> the fact that the lock _also_ disables interrupts once it is acquired. And
> >> this is the important detail (which was checked the VM_BUG_ON()) which needs
> >> to be pointed out. This is the hint one needs while looking at the code. It
> >> was explained by Johannes on the list that the per-CPU variables are protected
> >> by local_irq_save(). The BUG_ON() was helpful. This code has been workarounded
> >> in -RT in the meantime. I wouldn't mind running into more of those if the code
> >> in question uses *special* kind of locking since now there is no no
> >> verification (in terms of lockdep or BUG_ON()).
> > 
> > I'd be happy to re-instate the VM_BUG_ON that checks for disabled
> > interrupts as before, that was the most obvious documentation.
> 
> sure thing, patch follows in a jiffy or two.

Thanks.

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