Messages in this thread | | | Date | Thu, 5 Nov 2015 16:48:20 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v11 1/5] xen: move xen_setup_runstate_info and get_runstate_snapshot to drivers/xen/time.c |
| |
Hi,
> +static u64 get64(const u64 *p) > +{ > + u64 ret; > + > + if (BITS_PER_LONG < 64) { > + u32 *p32 = (u32 *)p; > + u32 h, l; > + > + /* > + * Read high then low, and then make sure high is > + * still the same; this will only loop if low wraps > + * and carries into high. > + * XXX some clean way to make this endian-proof? > + */ > + do { > + h = p32[1]; > + barrier(); > + l = p32[0]; > + barrier(); > + } while (p32[1] != h);
I realise this is simply a move of existing code, but it may be better to instead have:
do { h = READ_ONCE(p32[1]); l = READ_ONCE(p32[0]); } while (READ_ONCE(p32[1] != h);
Which ensures that each load is a single access (though it almost certainly would be anyway), and prevents the compiler from having to reload any other memory locations (which the current barrier() usage forces).
> + > + ret = (((u64)h) << 32) | l; > + } else > + ret = *p;
Likewise, this would be better as READ_ONCE(*p), to force a single access.
> + > + return ret; > +}
> + do { > + state_time = get64(&state->state_entry_time); > + barrier(); > + *res = *state; > + barrier();
You can also have:
*res = READ_ONCE(*state);
That will which will handle the barriers implicitly.
Thanks, Mark.
> + } while (get64(&state->state_entry_time) != state_time); > +}
| |