linux-kernel - [tip:x86/fpu] x86/fpu: Remove preempt_disable() in fpu_

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Message-ID: <tip-60e528d6ce3f60a058bbb64f8acb2a07f84b172a@git.kernel.org>
Date:   Sat, 13 Apr 2019 13:48:01 -0700
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, mingo@...hat.com, luto@...nel.org,
        x86@...nel.org, tglx@...utronix.de, dave.hansen@...el.com,
        nstange@...e.de, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, hpa@...or.com, bp@...e.de,
        rkrcmar@...hat.com, Jason@...c4.com, pbonzini@...hat.com,
        riel@...riel.com, kvm@...r.kernel.org
Subject: [tip:x86/fpu] x86/fpu: Remove preempt_disable() in fpu__clear()

Commit-ID:  60e528d6ce3f60a058bbb64f8acb2a07f84b172a
Gitweb:     https://git.kernel.org/tip/60e528d6ce3f60a058bbb64f8acb2a07f84b172a
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Wed, 3 Apr 2019 18:41:32 +0200
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Tue, 9 Apr 2019 19:27:46 +0200

x86/fpu: Remove preempt_disable() in fpu__clear()

The preempt_disable() section was introduced in commit

  a10b6a16cdad8 ("x86/fpu: Make the fpu state change in fpu__clear() scheduler-atomic")

and it was said to be temporary.

fpu__initialize() initializes the FPU struct to its initial value and
then sets ->initialized to 1. The last part is the important one.
The content of the state does not matter because it gets set via
copy_init_fpstate_to_fpregs().

A preemption here has little meaning because the registers will always be
set to the same content after copy_init_fpstate_to_fpregs(). A softirq
with a kernel_fpu_begin() could also force to save FPU's registers after
fpu__initialize() without changing the outcome here.

Remove the preempt_disable() section in fpu__clear(), preemption here
does not hurt.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Dave Hansen <dave.hansen@...el.com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: kvm ML <kvm@...r.kernel.org>
Cc: Nicolai Stange <nstange@...e.de>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Rik van Riel <riel@...riel.com>
Cc: x86-ml <x86@...nel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-4-bigeasy@linutronix.de
---
 arch/x86/kernel/fpu/core.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1d3ae7988f7f..1940319268ae 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -366,11 +366,9 @@ void fpu__clear(struct fpu *fpu)
 	 * Make sure fpstate is cleared and initialized.
 	 */
 	if (static_cpu_has(X86_FEATURE_FPU)) {
-		preempt_disable();
 		fpu__initialize(fpu);
 		user_fpu_begin();
 		copy_init_fpstate_to_fpregs();
-		preempt_enable();
 	}
 }