Commit 7106a5ab authored by Benjamin LaHaise's avatar Benjamin LaHaise Committed by Ingo Molnar
Browse files

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: default avatarBenjamin LaHaise <bcrl@kvack.org>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent c5976504
......@@ -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)
......
......@@ -408,6 +408,8 @@ END(save_paranoid)
ENTRY(ret_from_fork)
DEFAULT_FRAME
LOCK ; btr $TIF_FORK,TI_flags(%r8)
push kernel_eflags(%rip)
CFI_ADJUST_CFA_OFFSET 8
popf # reset kernel eflags
......
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