Re: [Qemu-devel] [PATCH 15/16] tcg: Make tb_flush() thread safe

[Richard Henderson]

On 09/24/2016 04:51 AM, Paolo Bonzini wrote:
> ----- Original Message -----
> > From: "Richard Henderson" <address@hidden>
> > To: "Paolo Bonzini" <address@hidden>, address@hidden
> > Cc: "serge fdrv" <address@hidden>, address@hidden, "alex bennee" <address@hidden>, 
> > "sergey fedorov"
> > <address@hidden>
> > Sent: Friday, September 23, 2016 8:06:09 PM
> > Subject: Re: [Qemu-devel] [PATCH 15/16] tcg: Make tb_flush() thread safe
> >
> > On 09/23/2016 12:31 AM, Paolo Bonzini wrote:
> > > +    unsigned tb_flush_req = (unsigned) (uintptr_t) data;
> >
> > Extra cast?
> >
> > > -    tcg_ctx.tb_ctx.tb_flush_count++;
> > > +    atomic_inc(&tcg_ctx.tb_ctx.tb_flush_count);
> >
> > Since this is the only place this value is incremented, and we're under a
> > lock,
> > it should be cheaper to use
> >
> >    atomic_mb_set(&tcg_ctx.tb_ctx.tb_flush_count, tb_flush_req + 1);
>
> atomic_set will do even.  Though it's not really a fast path, which is
> why I went for atomic_inc.

Don't we need the flush to be complete before the new count is seen?  That's 
why I was suggesting the mb_set.


r~