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From: Ingo Molnar <mingo@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	linux-crypto@vger.kernel.org, linux-pm@vger.kernel.org,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ayush Jain <Ayush.Jain3@amd.com>,
	Herbert Xu <herbert@gondor.apana.org.au>
Subject: Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs_disabled()
Date: Mon, 19 May 2025 10:05:58 +0200	[thread overview]
Message-ID: <aCrmZnSokvmqfel3@gmail.com> (raw)
In-Reply-To: <20250518200114.GA1764@sol>


* Eric Biggers <ebiggers@kernel.org> wrote:

> > # echo PANIC > /sys/kernel/debug/provoke-crash/DIRECT
> > 
> > Another case that likely executes with IRQs disabled (but I haven't
> > double checked) is reset_system(), which may return with an error, or
> > reboot/poweroff the machine and never return.
> 
> That makes sense to me.  preempt_disable() and preempt_enable() are already
> allowed when IRQs are disabled, and I'm not sure why local_bh_disable() and
> local_bh_enable() are different.

Because local_bh_enable() may run softirq handlers immediately if 
there's pending softirqs, which shouldn't be done in hardirq context.

This is a key optimization of the Linux networking code, which uses 
BH-off/BH-on sections instead of IRQS-off/IRQS-on critical sections, 
for performance reasons.

Thanks,

	Ingo

  reply	other threads:[~2025-05-19  8:06 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-05-16 23:18 [PATCH 0/3] x86: Don't support kernel-mode FPU with hardirqs disabled Eric Biggers
2025-05-16 23:18 ` [PATCH 1/3] x86/fpu: Add fpu_save_state() for __save_processor_state() Eric Biggers
2025-05-16 23:18 ` [PATCH 2/3] x86/pm: Use fpu_save_state() in __save_processor_state() Eric Biggers
2025-05-16 23:18 ` [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs_disabled() Eric Biggers
2025-05-17  7:09   ` Ingo Molnar
2025-05-17 18:39     ` Eric Biggers
2025-05-18  6:34       ` Ingo Molnar
2025-05-18 13:18         ` Ard Biesheuvel
2025-05-18 20:01           ` Eric Biggers
2025-05-19  8:05             ` Ingo Molnar [this message]
2025-05-19  9:49               ` Ard Biesheuvel
2025-05-19 12:57                 ` Ingo Molnar
2025-05-19 13:50                   ` Ard Biesheuvel
2025-05-20  7:42                     ` Ingo Molnar
2025-05-17  1:30 ` [PATCH 0/3] x86: Don't support kernel-mode FPU with hardirqs disabled Eric Biggers

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