linux-kernel - Re: Are jump labels broken on 6.11-rc1?

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Message-ID: <20240807150503.GF6051@frogsfrogsfrogs>
Date: Wed, 7 Aug 2024 08:05:03 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Chandan Babu R <chandanbabu@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	xfs <linux-xfs@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: Are jump labels broken on 6.11-rc1?

On Wed, Aug 07, 2024 at 04:55:53PM +0200, Thomas Gleixner wrote:
> On Wed, Aug 07 2024 at 16:34, Peter Zijlstra wrote:
> > On Wed, Aug 07, 2024 at 04:03:12PM +0200, Thomas Gleixner wrote:
> >
> >> > +	if (static_key_dec(key, true)) // dec-not-one
> >> 
> >> Eeew.
> >
> > :-) I knew you'd hate on that
> 
> So you added it just to make me grumpy enough to fix it for you, right?

FWIW with peter's 'ugly' patch applied, fstests didn't cough up any
static key complaints overnight.

> >> +/*
> >> + * Fastpath: Decrement if the reference count is greater than one
> >> + *
> >> + * Returns false, if the reference count is 1 or -1 to force the caller
> >> + * into the slowpath.
> >> + *
> >> + * The -1 case is to handle a decrement during a concurrent first enable,
> >> + * which sets the count to -1 in static_key_slow_inc_cpuslocked(). As the
> >> + * slow path is serialized the caller will observe 1 once it acquired the
> >> + * jump_label_mutex, so the slow path can succeed.
> >> + */
> >> +static bool static_key_dec_not_one(struct static_key *key)
> >> +{
> >> +	int v = static_key_dec(key, true);
> >> +
> >> +	return v != 1 && v != -1;
> >
> > 	if (v < 0)
> > 		return false;
> 
> Hmm. I think we should do:
> 
> #define KEY_ENABLE_IN_PROGRESS		-1
> 
> or even a more distinct value like (INT_MIN / 2)
> 
> and replace all the magic -1 numbers with it. Then the check becomes
> explicit:
> 
>         if (v == KEY_ENABLE_IN_PROGRESS)
>         	return false;
> 
> > 	/*
> > 	 * Notably, 0 (underflow) returns true such that it bails out
> > 	 * without doing anything.
> > 	 */
> > 	return v != 1;
> >
> > Perhaps?
> 
> Sure.
> 
> >> +}
> >> +
> >> +/*
> >> + * Slowpath: Decrement and test whether the refcount hit 0.
> >> + *
> >> + * Returns true if the refcount hit zero, i.e. the previous value was one.
> >> + */
> >> +static bool static_key_dec_and_test(struct static_key *key)
> >> +{
> >> +	int v = static_key_dec(key, false);
> >> +
> >> +	lockdep_assert_held(&jump_label_mutex);
> >> +	return v == 1;
> >>  }
> >
> > But yeah, this is nicer!
> 
> :)

It probably goes without saying that if either of you send a cleaned up
patch with all these changes baked in, I will test it for you all. :)

--D

> 
> Thanks,
> 
>         tglx
>