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: <CAMj1kXE49HFQHYJ=bgz2hPoa7ZocqbeL+pWGtpxxBU2g6_7+SA@mail.gmail.com>
Date: Fri, 5 Sep 2025 15:54:55 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Peter Zijlstra <peterz@...radead.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, 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.