From: Benjamin LaHaise <bcrl@kvack.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: linux-kernel@vger.kernel.org
Subject: [x86-64] remove locked instruction from switch_to()
Date: Sat, 10 Jan 2009 16:26:25 -0500 [thread overview]
Message-ID: <20090110212625.GE13467@kvack.org> (raw)
The patch below removes an unnecessary locked instruction from switch_to().
TIF_FORK is only ever set in copy_thread() on initial process creation, and
gets cleared during the first scheduling of the process. As such, it is
safe to use an unlocked test for the flag within switch_to()./
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org
diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 2ed3f0f..ee2e589 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -96,15 +96,15 @@ do { \
"thread_return:\n\t" \
"movq %%gs:%P[pda_pcurrent],%%rsi\n\t" \
"movq %P[thread_info](%%rsi),%%r8\n\t" \
- LOCK_PREFIX "btr %[tif_fork],%P[ti_flags](%%r8)\n\t" \
"movq %%rax,%%rdi\n\t" \
- "jc ret_from_fork\n\t" \
+ "testl %[_tif_fork],%P[ti_flags](%%r8)\n\t" \
+ "jnz ret_from_fork\n\t" \
RESTORE_CONTEXT \
: "=a" (last) \
: [next] "S" (next), [prev] "D" (prev), \
[threadrsp] "i" (offsetof(struct task_struct, thread.sp)), \
[ti_flags] "i" (offsetof(struct thread_info, flags)), \
- [tif_fork] "i" (TIF_FORK), \
+ [_tif_fork] "i" (_TIF_FORK), \
[thread_info] "i" (offsetof(struct task_struct, stack)), \
[pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent)) \
: "memory", "cc" __EXTRA_CLOBBER)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index b86f332..16b4587 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -247,6 +247,7 @@ ENTRY(native_usergs_sysret64)
*/
/* rdi: prev */
ENTRY(ret_from_fork)
+ LOCK ; btr $TIF_FORK,TI_flags(%r8)
CFI_DEFAULT_STACK
push kernel_eflags(%rip)
CFI_ADJUST_CFA_OFFSET 8
next reply other threads:[~2009-01-10 21:44 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-01-10 21:26 Benjamin LaHaise [this message]
2009-01-10 22:36 ` [x86-64] remove locked instruction from switch_to() Ingo Molnar
2009-01-11 4:00 ` [PATCH] x86-64: " Benjamin LaHaise
2009-01-11 4:05 ` Ingo Molnar
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