x86-64: remove locked instruction from switch_to()

Impact: micro-optimization

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>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 8e626ea33a1a..fa47b1e6a866 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)