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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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.

>> 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.

Sebastian

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