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

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Message-ID: <CAMj1kXHDtTNMzih7OoTYU0vN4M3mOmFL3YOfaPUKReyJQA6uAQ@mail.gmail.com>
Date: Fri, 19 Sep 2025 15:42:12 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Mark Rutland <mark.rutland@....com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Catalin Marinas <catalin.marinas@....com>, 
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v3 6/8] arm64/efi: Use a mutex to protect the EFI stack
 and FP/SIMD state

On Fri, 19 Sept 2025 at 13:35, Will Deacon <will@...nel.org> wrote:
>
> On Thu, Sep 18, 2025 at 12:30:17PM +0200, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@...nel.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@...nel.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.