linux-kernel - Re: [PATCH RESEND v3 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

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Date:	Wed, 14 Oct 2015 11:10:00 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <dave@...olabs.net>, stable@...r.kernel.org
Subject: Re: [PATCH RESEND v3 1/6] powerpc: atomic: Make *xchg and *cmpxchg
 a full barrier

On Mon, 2015-10-12 at 22:30 +0800, Boqun Feng wrote:
> According to memory-barriers.txt, xchg, cmpxchg and their atomic{,64}_
> versions all need to imply a full barrier, however they are now just
> RELEASE+ACQUIRE, which is not a full barrier.
> 
> So replace PPC_RELEASE_BARRIER and PPC_ACQUIRE_BARRIER with
> PPC_ATOMIC_ENTRY_BARRIER and PPC_ATOMIC_EXIT_BARRIER in
> __{cmp,}xchg_{u32,u64} respectively to guarantee a full barrier
> semantics of atomic{,64}_{cmp,}xchg() and {cmp,}xchg().
> 
> This patch is a complement of commit b97021f85517 ("powerpc: Fix
> atomic_xxx_return barrier semantics").
> 
> Cc: <stable@...r.kernel.org> # 3.4.y-
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  arch/powerpc/include/asm/cmpxchg.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)

Hi Boqun,

Thanks for fixing this. In future you should send a patch like this as a
separate patch. I've not been paying attention to it because I assumed it was
part of your full series and was still under discussion like the other patches.

I don't think we've seen any crashes caused by this have we? So I guess I'll
put it in next to let it get some wider testing rather than sending it straight
to Linus.

To be clear you're doing:

> -	PPC_RELEASE_BARRIER
> +	PPC_ATOMIC_ENTRY_BARRIER

Which is correct but doesn't actually change anything at the moment, because
both macros turn into LWSYNC.

On the other hand:

> -	PPC_ACQUIRE_BARRIER
> +	PPC_ATOMIC_EXIT_BARRIER

Is changing an isync (which is then patched to lwsync on some cpus), with a sync.


Also I'm not clear what your stable line means:

> Cc: <stable@...r.kernel.org> # 3.4.y-

Do you mean 3.4 and anything after? I usually write that as 3.4+, but I'm not
sure if that's the correct syntax either.


cheers


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