linux-kernel - Re: [PATCH 2/2] s390: convert to GENERIC

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Date:   Mon, 03 Aug 2020 21:27:36 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Sven Schnelle <svens@...ux.ibm.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH 2/2] s390: convert to GENERIC_VDSO

Heiko Carstens <hca@...ux.ibm.com> writes:

> On Mon, Aug 03, 2020 at 06:05:24PM +0200, Thomas Gleixner wrote:
>> +/**
>> + * vdso_update_begin - Start of a VDSO update section
>> + *
>> + * Allows architecture code to safely update the architecture specific VDSO
>> + * data.
>> + */
>> +void vdso_update_begin(void)
>> +{
>> +	struct vdso_data *vdata = __arch_get_k_vdso_data();
>> +
>> +	raw_spin_lock(&timekeeper_lock);
>> +	vdso_write_begin(vdata);
>> +}
>
> I would assume that this only works if vdso_update_begin() is called
> with irqs disabled, otherwise it could deadlock, no?

Yes.

> Maybe something like:
>
> void vdso_update_begin(unsigned long *flags)
> {
> 	struct vdso_data *vdata = __arch_get_k_vdso_data();
>
> 	raw_spin_lock_irqsave(&timekeeper_lock, *flags);
> 	vdso_write_begin(vdata);

Shudder. Why not returning flags?

> }
>
> void vdso_update_end(unsigned long *flags)

Ditto, why pointer and not value?

> {
> 	struct vdso_data *vdata = __arch_get_k_vdso_data();
>
> 	vdso_write_end(vdata);
> 	__arch_sync_vdso_data(vdata);
> 	raw_spin_unlock_irqrestore(&timekeeper_lock, *flags);
> }
>
> ? Just wondering.

Thought about that briefly, but then hated the flags thing and delegated
it to the caller. Lockdep will yell if that lock is taken with
interrupts enabled :)

But aside of the pointer vs. value thing, I'm fine with doing it in the
functions.

Thanks,

        tglx