linux-kernel - Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from

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Message-ID: <20170608103244.1b4b24c9@roar.ozlabs.ibm.com>
Date:   Thu, 8 Jun 2017 10:32:44 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, will.deacon@....com,
        oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu
Subject: Re: [RFC][PATCH 5/5] powerpc: Remove SYNC from _switch

On Wed, 07 Jun 2017 18:15:06 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> Now that the scheduler's rq->lock is RCsc and thus provides full
> transitivity between scheduling actions. And since we cannot migrate
> current, a task needs a switch-out and a switch-in in order to
> migrate, in which case the RCsc provides all the ordering we need.

Hi Peter,

I'm actually just working on removing this right now too, so
good timing.

I think we can't "just" remove it, because it is required to order
MMIO on powerpc as well.

But what I have done is to comment that some other primitives are
already providing the hwsync for other, so we don't have to add
another one in _switch.

Thanks,
Nick

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/powerpc/kernel/entry_64.S |    8 --------
>  1 file changed, 8 deletions(-)
> 
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -488,14 +488,6 @@ _GLOBAL(_switch)
>  	std	r23,_CCR(r1)
>  	std	r1,KSP(r3)	/* Set old stack pointer */
>  
> -#ifdef CONFIG_SMP
> -	/* We need a sync somewhere here to make sure that if the
> -	 * previous task gets rescheduled on another CPU, it sees all
> -	 * stores it has performed on this one.
> -	 */
> -	sync
> -#endif /* CONFIG_SMP */
> -
>  	/*
>  	 * If we optimise away the clear of the reservation in system
>  	 * calls because we know the CPU tracks the address of the
> 
>