linux-kernel - Re: [PATCH v2 5/7] arm64/efi: Use a semaphore to protect the EFI stack and FP/SIMD state

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Message-ID: <20250908153720.GF4067720@noisy.programming.kicks-ass.net>
Date: Mon, 8 Sep 2025 17:37:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ard Biesheuvel <ardb@...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,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v2 5/7] arm64/efi: Use a semaphore to protect the EFI
 stack and FP/SIMD state

On Fri, Sep 05, 2025 at 03:54:55PM +0200, Ard Biesheuvel wrote:
> On Fri, 5 Sept 2025 at 15:44, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Fri, Sep 05, 2025 at 03:30:41PM +0200, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@...nel.org>
> > >
> > > Replace the spinlock in the arm64 glue code with a semaphore, so that
> > > the CPU can preempted while running the EFI runtime service.
> >
> > Gotta ask, why a semaphore and not a mutex?
> 
> Because mutex_trylock() is not permitted in interrupt context.

Ah, true. Might make for a good comment near there.