Re: [PATCH v3 6/8] arm64/efi: Use a mutex to protect the EFI stack and FP/SIMD state

[Will Deacon <will@kernel.org>]

On Fri, Sep 19, 2025 at 03:42:12PM +0200, Ard Biesheuvel wrote:
> On Fri, 19 Sept 2025 at 13:35, Will Deacon <will@kernel.org> wrote:
> >
> > On Thu, Sep 18, 2025 at 12:30:17PM +0200, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@kernel.org>
> > >
> > > Replace the spinlock in the arm64 glue code with a mutex, so that
> > > the CPU can preempted while running the EFI runtime service.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/arm64/kernel/efi.c | 13 ++++++++++---
> > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> > > index 0d52414415f3..4372fafde8e9 100644
> > > --- a/arch/arm64/kernel/efi.c
> > > +++ b/arch/arm64/kernel/efi.c
> > > @@ -166,15 +166,22 @@ asmlinkage efi_status_t efi_handle_corrupted_x18(efi_status_t s, const char *f)
> > >       return s;
> > >  }
> > >
> > > -static DEFINE_RAW_SPINLOCK(efi_rt_lock);
> > > +static DEFINE_MUTEX(efi_rt_lock);
> > >
> > >  bool arch_efi_call_virt_setup(void)
> > >  {
> > >       if (!may_use_simd())
> > >               return false;
> > >
> > > +     /*
> > > +      * This might be called from a non-sleepable context so try to take the
> > > +      * lock but don't block on it. This should never fail in practice, as
> > > +      * all EFI runtime calls are serialized under the efi_runtime_lock.
> > > +      */
> > > +     if (WARN_ON(!mutex_trylock(&efi_rt_lock)))
> > > +             return false;
> >
> > If it will never fail in practice, why do we need the lock at all? Can we
> > just assert that the efi_runtime_lock is held instead and rely on that?
> >
> 
> Excellent point.
> 
> Do you mean a lockdep assert? efi_runtime_lock is a semaphore, so
> there is no is_locked() API that we can BUG() on here.

Yes, I was thinking of lockdep. Even though lockdep doesn't tend to be
enabled in production, just having it in the code is useful documentation
imo.

Will