linux-kernel - Re: [Bug] WARNING in static_key_disable

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Message-ID: <20240306221650.sw3lha7kca2quv63@treble>
Date: Wed, 6 Mar 2024 14:16:50 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jason Baron <jbaron@...mai.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, Sam Sun <samsun1006219@...il.com>,
	linux-kernel@...r.kernel.org, syzkaller@...glegroups.com,
	xrivendell7@...il.com, ardb@...nel.org, peterz@...radead.org,
	linux-mm@...ck.org, akpm@...ux-foundation.org,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [Bug] WARNING in static_key_disable_cpuslocked

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.

> 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.

-- 
Josh