Messages in this thread | | | Subject | Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb() | From | Andy Lutomirski <> | Date | Tue, 12 Jan 2016 12:30:46 -0800 |
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On 01/12/2016 09:20 AM, Linus Torvalds wrote: > On Tue, Jan 12, 2016 at 5:57 AM, Michael S. Tsirkin <mst@redhat.com> wrote: >> #ifdef xchgrz >> /* same as xchg but poking at gcc red zone */ >> #define barrier() do { int ret; asm volatile ("xchgl %0, -4(%%" SP ");": "=r"(ret) :: "memory", "cc"); } while (0) >> #endif > > That's not safe in general. gcc might be using its redzone, so doing > xchg into it is unsafe. > > But.. > >> Is this a good way to test it? > > .. it's fine for some basic testing. It doesn't show any subtle > interactions (ie some operations may have different dynamic behavior > when the write buffers are busy etc), but as a baseline for "how fast > can things go" the stupid raw loop is fine. And while the xchg into > the redzoen wouldn't be acceptable as a real implementation, for > timing testing it's likely fine (ie you aren't hitting the problem it > can cause).
I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 64) was better because it avoided stomping on very-likely-to-be-hot write buffers.
--Andy
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