linux-kernel - [patch 0/2] x86/fpu: Make fpu

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Date:   Tue, 27 Oct 2020 11:09:49 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [patch 0/2] x86/fpu: Make fpu_lock() RT compliant

FPU serialization uses

    preempt_disable(); local_bh_disable();

which is silly because local_bh_disable() implicitly disables
preemption. So this can be reduced to local_bh_disable().

But this does not work on RT kernels because local_bh_disable() only
serializes bottom half related processing via a local lock, but does not
disable preemption.

On RT kernels preempt_disable() is the right choice because on RT bottom
half processing is always in thread context, so preempt_disable()
implicitly protects against bottom half processing there.

Thanks,

	tglx