linux-kernel - Re: [PATCH] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU

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Date:   Sun, 10 Oct 2021 22:33:05 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     mpe@...erman.id.au, linux-kernel@...r.kernel.org,
        paulmck@...nel.org, Daniel Lustig <dlustig@...dia.com>,
        will@...nel.org, peterz@...radead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        alexander.shishkin@...ux.intel.com, hpa@...or.com,
        parri.andrea@...il.com, mingo@...nel.org, vincent.weaver@...ne.edu,
        tglx@...utronix.de, jolsa@...hat.com, acme@...hat.com,
        eranian@...gle.com, Paul Walmsley <paul.walmsley@...ive.com>,
        stern@...land.harvard.edu, linux-arch@...r.kernel.org
Subject: Re: [PATCH] tools/memory-model: Provide extra ordering for
 unlock+lock pair on the same CPU

On Fri, Oct 08, 2021 at 09:32:58AM -0700, Palmer Dabbelt wrote:
> On Thu, 07 Oct 2021 23:54:23 PDT (-0700), boqun.feng@...il.com wrote:
> > On Fri, Oct 08, 2021 at 04:30:37PM +1100, Michael Ellerman wrote:
> > > Boqun Feng <boqun.feng@...il.com> writes:
> > > > (Add linux-arch in Cc list)
> > > >
> > > > Architecture maintainers, this patch is about strengthening our memory
> > > > model a little bit, your inputs (confirmation, ack/nack, etc.) are
> > > > appreciated.
> > > 
> > > Hi Boqun,
> > > 
> > > I don't feel like I'm really qualified to give an ack here, you and the
> > > other memory model folk know this stuff much better than me.
> > > 
> > > But I have reviewed it and it matches my understanding of how our
> > > barriers work, so it looks OK to me.
> > > 
> > > Reviewed-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)
> 
> I'm basically in the same spot.  I think I said something to that effect
> somewhere in the thread, but I'm not sure if it got picked up so
> 
> Acked-by: Palmer Dabbelt <palmerdabbelt@...gle.com> (RISC-V)
> 

Thanks!

> (I don't feel comfortable reviewing it so I'm acking it, not sure if I'm
> just backwards about what all this means though ;)).
> 
> IIUC this change will mean the RISC-V port is broken, but I'm happy to fix

No, the RISC-V port is not broken, this patch only strengthen the
unlock(A)+lock(B) to TSO ordering, as per the previous discussion:

	https://lore.kernel.org/lkml/5412ab37-2979-5717-4951-6a61366df0f2@nvidia.com/

RISC-V's current lock implementation is fine, and it's still OK if
RISC-V still to queued spinlock, since as Dan said in that email thread,
the following code still provides TSO ordering:

	FENCE RW, W
	store A
	ll/sc B
	FENCE R, RW

Regards,
Boqun

> it.  Were you guys trying to target this for 5.16?