linux-kernel - [tip:x86/asm] x86/asm/entry/64: Simplify looping around preempt_schedule

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Message-ID: <tip-36acef2510853e2831047ca9e22d333ba7a1047b@git.kernel.org>
Date:	Thu, 2 Apr 2015 05:26:12 -0700
From:	tip-bot for Denys Vlasenko <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	hpa@...or.com, luto@...capital.net, bp@...en8.de, mingo@...nel.org,
	linux-kernel@...r.kernel.org, oleg@...hat.com,
	torvalds@...ux-foundation.org, ast@...mgrid.com, wad@...omium.org,
	fweisbec@...il.com, keescook@...omium.org, tglx@...utronix.de,
	rostedt@...dmis.org, dvlasenk@...hat.com
Subject: [tip:x86/asm] x86/asm/entry/64:
  Simplify looping around preempt_schedule_irq()

Commit-ID:  36acef2510853e2831047ca9e22d333ba7a1047b
Gitweb:     http://git.kernel.org/tip/36acef2510853e2831047ca9e22d333ba7a1047b
Author:     Denys Vlasenko <dvlasenk@...hat.com>
AuthorDate: Tue, 31 Mar 2015 19:00:07 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 1 Apr 2015 13:17:39 +0200

x86/asm/entry/64: Simplify looping around preempt_schedule_irq()

At the 'exit_intr' label we test whether interrupt/exception was in
kernel. If it did, we jump to the preemption check. If preemption
does happen (IOW if we call preempt_schedule_irq()), we go back to
'exit_intr'.

But it's pointless, we already know that the test succeeded last
time, preemption doesn't change the fact that interrupt/exception
was in the kernel.

We can go back directly to checking PER_CPU_VAR(__preempt_count) instead.

This makes the 'exit_intr' label unused, drop it.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Alexei Starovoitov <ast@...mgrid.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Will Drewry <wad@...omium.org>
Link: http://lkml.kernel.org/r/1427821211-25099-5-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/entry_64.S | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 9f8d01f..bad285d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -654,7 +654,6 @@ ret_from_intr:
 	CFI_DEF_CFA_REGISTER	rsp
 	CFI_ADJUST_CFA_OFFSET	RBP
 
-exit_intr:
 	testl $3,CS(%rsp)
 	je retint_kernel
 	/* Interrupt came from user space */
@@ -741,12 +740,12 @@ retint_kernel:
 #ifdef CONFIG_PREEMPT
 	/* Interrupts are off */
 	/* Check if we need preemption */
-	cmpl	$0,PER_CPU_VAR(__preempt_count)
-	jnz	1f
 	bt	$9,EFLAGS(%rsp)	/* interrupts were off? */
 	jnc	1f
+0:	cmpl	$0,PER_CPU_VAR(__preempt_count)
+	jnz	1f
 	call	preempt_schedule_irq
-	jmp	exit_intr
+	jmp	0b
 1:
 #endif
 	/*
--
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