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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

arch/x86/include/asm/system.h

+3 −3
Original line number Diff line number Diff line
@@ -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)

arch/x86/kernel/entry_64.S

+2 −0
Original line number Diff line number Diff line
@@ -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