linux-kernel - Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin

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Date:   Tue, 27 Feb 2018 13:06:35 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Daniel Lustig <dlustig@...dia.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <albert@...ive.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Akira Yokosawa <akiyks@...il.com>,
        Ingo Molnar <mingo@...nel.org>, linux-riscv@...ts.infradead.org
Subject: Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and
 spin_unlock()

On Mon, Feb 26, 2018 at 04:24:27PM +0000, Will Deacon wrote:
> On Mon, Feb 26, 2018 at 08:06:59AM -0800, Linus Torvalds wrote:
> > On Mon, Feb 26, 2018 at 6:21 AM, Luc Maranget <luc.maranget@...ia.fr> wrote:
> > >
> > > That is, locks are not implemented from more basic primitive but are specified.
> > > The specification can be described as behaving that way:
> > >   - A lock behaves as a read-modify-write. the read behaving as a read-acquire
> > 
> > This is wrong, or perhaps just misleading.
> > 
> > The *whole* r-m-w acts as an acquire. Not just the read part. The
> > write is very much part of it.
> > 
> > Maybe that's what you meant, but it read to me as "just the read part
> > of the rmw behaves as a read-acquire".
> > 
> > Because it is very important that the _write_ part of the rmw is also
> > ordered wrt everything that is inside the spinlock.
> > 
> > So doing a spinlock as
> > 
> >  (a) read-locked-acquire
> >    modify
> >  (c) write-conditional
> > 
> > would be wrong, because the accesses inside the spinlock are ordered
> > not just wrt the read-acquire, they have to be ordered wrt the write
> > too.
> > 
> > So it is closer to say that it's the _write_ of the r-m-w sequence
> > that has the acquire semantics, not the read.
> 
> Strictly speaking, that's not what we've got implemented on arm64: only
> the read part of the RmW has Acquire semantics, but there is a total
> order on the lock/unlock operations for the lock. For example, if one
> CPU does:
> 
> spin_lock(&lock);
> WRITE_ONCE(foo, 42);
> 
> then another CPU could do:
> 
> if (smp_load_acquire(&foo) == 42)
> 	BUG_ON(!spin_is_locked(&lock));
> 

Hmm.. this is new to me. So the write part of spin_lock() and the
WRITE_ONCE() will not get reordered? Could you explain more about this
or point where I should look in the document? I understand the write
part of spin_lock() must be committed earlier than the WRITE_ONCE()
committed due to the ll/sc, but I thought the ordering of their arrivals
in memory system is undefined/arbitary.

Regards,
Boqun

> and that could fire. Is that relied on somewhere?
> 
> Will

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