Re: [x86-64] remove locked instruction from switch_to()

Ingo Molnar
2009-01-10T22:36:44Z

* Benjamin LaHaise <bcrl@kvack.org> wrote:

> 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

nice!

Small detail, it does not apply to latest -git cleanly:

hunk #1 FAILED at 247.
1 out of 1 hunk FAILED -- rejects in file arch/x86/kernel/entry_64.S

Probably trivial - would you mind to repost?

Ingo