Re: [Qemu-devel] [PATCH 01/15] atomic: introduce atomic operations

[Paolo Bonzini <pbonzini@redhat.com>]

Il 08/08/2012 15:09, Stefan Hajnoczi ha scritto:
>> > NAK. We don't want random inline assembly implementations of locking
>> > primitives in QEMU, they are way too hard to keep working with all the
>> > possible host architectures we support. I spent some time a while back
>> > getting rid of the (variously busted) versions we had previously.
>> >
>> > If you absolutely must use atomic ops, use the gcc builtins. For
>> > preference, stick to higher level and less error-prone abstractions.
> We're spoilt for choice here:
> 
> 1. Atomic built-ins from gcc
> 2. glib atomics
> 
> No need to roll our own or copy the implementation from the kernel.

To some extent we need to because:

1. GCC atomics look ugly, :) do not provide rmb/wmb, and in some
versions of GCC mb is known to be (wrongly) a no-op.

2. glib atomics do not provide mb/rmb/wmb either, and
g_atomic_int_get/g_atomic_int_set are inefficient: they add barriers
everywhere, while it is clearer if you put barriers manually, and you
often do not need barriers in the get side.  glib atomics also do not
provide xchg.

I agree however that a small wrapper around GCC atomics is much better
than assembly.  Assembly can be limited to the memory barriers (where we
already have it anyway).

Paolo