Re: [Bug] WARNING in static_key_disable_cpuslocked

[Jason Baron]

On 3/6/24 5:16 PM, Josh Poimboeuf wrote:
> On Wed, Mar 06, 2024 at 03:12:07PM -0500, Jason Baron wrote:
> > On 3/6/24 2:31 PM, Josh Poimboeuf wrote:
> > > On Wed, Mar 06, 2024 at 10:54:20AM -0500, Steven Rostedt wrote:
> > > > Now I guess the question is, why is something trying to disable something
> > > > that is not enabled? Is the above scenario OK? Or should the users of
> > > > static_key also prevent this?
> > >
> > > Apparently that's an allowed scenario, as the jump label code seems to
> > > be actively trying to support it.  Basically the last one "wins".
> > >
> > > See for example:
> > >
> > >     1dbb6704de91 ("jump_label: Fix concurrent static_key_enable/disable()")
> > >
> > > Also the purpose of the first atomic_read() is to do a quick test before
> > > grabbing the jump lock.  So instead of grabbing the jump lock earlier,
> > > it should actually do the first test atomically:
> >
> > Makes sense but the enable path can also set key->enabled to -1.
>
> Ah, this code is really subtle :-/
>
> > So I think a concurrent disable could then see the -1 in tmp and still
> > trigger the WARN.
>
> I think this shouldn't be possible, for the same reason that
> static_key_slow_try_dec() warns on -1:  key->enabled can only be -1
> during the first enable.  And disable should never be called before
> then.

hmm, right but I think in this case the reproducer is writing to a sysfs 
file to enable/disable randomly so i'm not sure if there is anything 
that would enforce that ordering. I guess you could try the reproducer, 
I haven't really looked at it in any detail.

The code in question here is in mm/vmscan.c which actually already takes 
the local 'state_mutex' for some cases. So that could be extended I 
think easily to avoid this warning.

> > So I think we could change the WARN to be:
> > WARN_ON_ONCE(tmp != 0 && tmp != -1). And also add a similar check
> > for enable if we have enable vs enable racing?
>
> My patch subtly changed the "key->enabled > 0" to "key->enabled != 0".
> If I change that back then it should be fine.
>
> > Although it seems like the set key->enabled to -1 while used in the inc/dec
> > API isn't really doing anything in the enable/disable part here?
> > But then the key->enabled I think has to move in front of the
> > jump_label_update() to make that part work right...
>
> Yeah, this code needs better comments.  Let me turn it into a proper
> patch.