Re: [PATCH] arm64/cpufeature: Convert hook_lock to raw_spin_lock_t in cpu_enable_ssbs()

[Julien Grall]

Hi Will,

On 5/30/19 1:01 PM, Will Deacon wrote:
> On Thu, May 30, 2019 at 12:30:58PM +0100, Julien Grall wrote:
> > cpu_enable_ssbs() is called via stop_machine() as part of the cpu_enable
> > callback. A spin lock is used to ensure the hook is registered before
> > the rest of the callback is executed.
> >
> > On -RT spin_lock() may sleep. However, all the callees in stop_machine()
> > are expected to not sleep. Therefore a raw_spin_lock() is required here.
> >
> > Given this is already done under stop_machine() and the work done under
> > the lock is quite small, the latency should not increase too much.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > ---
> >
> > It was noticed when looking at the current use of spin_lock in
> > arch/arm64. I don't have a platform calling that callback, so I have
> > hacked the code to reproduce the error and check it is now fixed.
> > ---
> >   arch/arm64/kernel/cpufeature.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index ca27e08e3d8a..2a7159fda3ce 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1194,14 +1194,14 @@ static struct undef_hook ssbs_emulation_hook = {
> >   static void cpu_enable_ssbs(const struct arm64_cpu_capabilities *__unused)
> >   {
> >   	static bool undef_hook_registered = false;
> > -	static DEFINE_SPINLOCK(hook_lock);
> > +	static DEFINE_RAW_SPINLOCK(hook_lock);
> >   
> > -	spin_lock(&hook_lock);
> > +	raw_spin_lock(&hook_lock);
> >   	if (!undef_hook_registered) {
> >   		register_undef_hook(&ssbs_emulation_hook);
> >   		undef_hook_registered = true;
> >   	}
> > -	spin_unlock(&hook_lock);
> > +	raw_spin_unlock(&hook_lock);
>
> Makes sense to me. We could probably avoid the lock entirely if we wanted
> to (via atomic_dec_if_positive), but I'm not sure it's really worth it.

I would prefer to remove the lock if it is possible. However, I was 
under the impression the lock is necessary so the hook is registered 
before any CPU attempt to configure the PSTATE.

Cheers,

--
Julien Grall