Re: [tip:x86/idle] x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers

[Ingo Molnar]

* tip-bot for H. Peter Anvin <tip...@zytor.com> wrote:

> Commit-ID:  7e98b71920464b8d15fa95c74366416cd3c88861
> Gitweb:     http://git.kernel.org/tip/7e98b71920464b8d15fa95c74366416cd3c88861
> Author:     H. Peter Anvin <h...@linux.intel.com>
> AuthorDate: Thu, 19 Dec 2013 11:58:16 -0800
> Committer:  H. Peter Anvin <h...@linux.intel.com>
> CommitDate: Thu, 19 Dec 2013 11:58:16 -0800
> 
> x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers
> 
> Use static_cpu_has() to conditionalize the CLFLUSH workaround, and add
> memory barriers around it since the documentation is explicit that
> CLFLUSH is only ordered with respect to MFENCE.
> 
> Signed-off-by: H. Peter Anvin <h...@linux.intel.com>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Len Brown <len.br...@intel.com>
> Link: 
> http://lkml.kernel.org/r/ca%2b55afzgxcml7j8cevqpyzh0w81uvoaavmgctmouz7cz1yy...@mail.gmail.com
> ---
>  arch/x86/include/asm/mwait.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 361b02e..19b71c4 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -43,8 +43,11 @@ static inline void __mwait(unsigned long eax, unsigned 
> long ecx)
>  static inline void mwait_idle_with_hints(unsigned long eax, unsigned long 
> ecx)
>  {
>       if (!current_set_polling_and_test()) {
> -             if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
> +             if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
> +                     mb();
>                       clflush((void *)&current_thread_info()->flags);
> +                     mb();
> +             }

So since this is a v3.14 commit, lets get it right, ok?

The first mb() looks superfluous, because see 
current_set_polling_and_test():

static inline bool __must_check current_set_polling_and_test(void)
{
        __current_set_polling();

        /*
         * Polling state must be visible before we test NEED_RESCHED,
         * paired by resched_task()
         */
        smp_mb();

        return unlikely(tif_need_resched());
}

So it already has a (MFENCE) barrier after ->flags is modified.

So using a comment - something like the patch below, should be enough 
as well, agreed?

Thanks,

        Ingo

Signed-off-by: Ingo Molnar <mi...@kernel.org>

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 19b71c4..d4f5c9a 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -44,7 +44,10 @@ static inline void mwait_idle_with_hints(unsigned long eax, 
unsigned long ecx)
 {
        if (!current_set_polling_and_test()) {
                if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
-                       mb();
+                       /*
+                        * There's no need for an extra barrier here: 
current_set_polling_and_test()
+                        * already has an smp_mb() after ->flags gets modified, 
see sched.h.
+                        */
                        clflush((void *)&current_thread_info()->flags);
                        mb();
                }
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/