linux-kernel - Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

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Message-ID: <20151015144052.GD29432@fixme-laptop.cn.ibm.com>
Date:	Thu, 15 Oct 2015 22:40:52 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <dave@...olabs.net>, stable@...r.kernel.org
Subject: Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and
 *cmpxchg a full barrier

On Thu, Oct 15, 2015 at 11:35:44AM +0100, Will Deacon wrote:
> 
> So arm64 is ok. Doesn't lwsync order store->store observability for PPC?
> 

I did some litmus and put the result here. My understanding might be
wrong, and I think Paul can explain the lwsync and store->store order
better ;-)


When a store->lwsync->store pairs with load->lwsync->load, according to
herd, YES.

    PPC W+lwsync+W-R+lwsync+R
    "
      2015-10-15
      herds said (1:r1=0 /\ 1:r2=2) doesn't exist,
      so if P1 observe the write to 'b', it must also observe P0's write
      to 'a'
    "
    {
    0:r1=1; 0:r2=2; 0:r12=a; 0:r13=b;
    1:r1=0; 1:r2=0; 1:r12=a; 1:r13=b;
    }
    
     P0              | P1             ;
     stw r1, 0(r12)  | lwz r2, 0(r13) ;
     lwsync          | lwsync         ;
     stw r2, 0(r13)  | lwz r1, 0(r12) ;
    
    exists
    (1:r1=0 /\ 1:r2=2)


If observation also includes "a write on one CPU -override- another
write on another CPU", then

when a store->lwsync->store pairs(?) with store->sync->load, according
to herd, NO(?).

    PPC W+lwsync+W-W+sync+R
    "
      2015-10-15
      herds said (1:r1=0 /\ b=3) exists sometimes,
      so if P1 observe P0's write to 'b'(by 'overriding' this write to
      'b'), it may not observe P0's write to 'a'.
    "
    {
    0:r1=1; 0:r2=2; 0:r12=a; 0:r13=b;
    1:r1=0; 1:r2=3; 1:r12=a; 1:r13=b;
    }
    
     P0              | P1             ;
     stw r1, 0(r12)  | stw r2, 0(r13) ;
     lwsync          | sync         ;
     stw r2, 0(r13)  | lwz r1, 0(r12) ;
    
    exists
    (1:r1=0 /\ b=3)


Regards,
Boqun

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