Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

From: Paul E. McKenney
Date: Sun Jan 19 2014 - 03:04:47 EST

On Sat, Jan 18, 2014 at 04:57:05PM -0800, Linus Torvalds wrote:
> On Sat, Jan 18, 2014 at 1:22 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > Yes, this requires that -all- updates to the fields in the machine word
> > in question use atomic rmw. Which would not be pretty from a core-code
> > perspective. Hence my suggestion of ceasing Linux-kernel support for
> > DEC Alpha CPUs that don't support byte operations. Also need 16-bit
> > operations as well, of course...
> I'm not seeing this.
> Why the hell would you have byte- or halfword-sized versions of the
> store_release or load_acquire things on alpha anyway?
> What it means is that data structures that do locking or atomics need
> to be "int" or "long" on alpha. That has always been true. What do
> you claim has changed?

OK, another approach would be to never add "select ARCH_USE_QUEUE_RWLOCK"
on Alpha, at least if the queued rwlocks really do want to atomically
manipulate bytes. After all, the Alpha systems that I know about don't
have enough CPUs to make queued rwlocks necessary anyway.

Much simpler solution!

Is this what you were getting at, or am I missing your point?

Thanx, Paul

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