linux-kernel - [PATCH 5/5] x86, fpu: sanitize the usage of use_eager_fpu() in switch_fpu

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Message-ID: <20140824194747.GA27458@redhat.com>
Date:	Sun, 24 Aug 2014 21:47:47 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Bean Anderson <bean@...lsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 5/5] x86, fpu: sanitize the usage of use_eager_fpu() in
	switch_fpu_prepare()

switch_fpu_prepare() calls use_eager_fpu() 3 times, and every call
is confusing.

1st: "tsk_used_math(new) && use_eager_fpu()" is pointless, eagerfpu
     means that used_math() is also true.

2nd: !fpu.preload && use_eager_fpu() is not possible, we do not need
     to check use_eager_fpu() before use_eager_fpu(). IOW, !preload
     alway means !use_eager_fpu(), so this check only adds more
     confusion.

3rd: If __thread_has_fpu(old) == F then use_eager_fpu() is false too,
     no need to check it before fpu_lazy_restore(). IOW, the "else"
     branch is simply impossible if use_eager_fpu().

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/include/asm/fpu-internal.h |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index ae6bd35..7b95a87 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -424,8 +424,8 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 	 * If the task has used the math, pre-load the FPU on xsave processors
 	 * or if the past 5 consecutive context-switches used math.
 	 */
-	fpu.preload = tsk_used_math(new) && (use_eager_fpu() ||
-					     new->thread.fpu_counter > 5);
+	fpu.preload = use_eager_fpu() ||
+			(tsk_used_math(new) && new->thread.fpu_counter > 5);
 	if (__thread_has_fpu(old)) {
 		if (!__save_init_fpu(old))
 			cpu = ~0;
@@ -437,14 +437,14 @@ static inline fpu_switch_t switch_fpu_prepare(struct task_struct *old, struct ta
 			new->thread.fpu_counter++;
 			__thread_set_has_fpu(new);
 			prefetch(new->thread.fpu.state);
-		} else if (!use_eager_fpu())
+		} else
 			stts();
 	} else {
 		old->thread.fpu_counter = 0;
 		old->thread.fpu.last_cpu = ~0;
 		if (fpu.preload) {
 			new->thread.fpu_counter++;
-			if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
+			if (fpu_lazy_restore(new, cpu))
 				fpu.preload = 0;
 			else
 				prefetch(new->thread.fpu.state);
-- 
1.5.5.1

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