On Fri, Jun 19, 2015 at 07:36:12PM +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.
>
> 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
> verification (in terms of lockdep or BUG_ON()) and therefore I bring the
> VM_BUG_ON() check back in.
>
> The two functions after the comment could also have a "local_irq_save()"
> dance around them in order to serialize access to the per-CPU variables.
> This has been avoided because the interrupts should be off.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Much better, thanks.
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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