Date: Tue, 17 May 2011 07:56:06 -0400 From: John Baldwin <jhb@FreeBSD.org> To: Andriy Gapon <avg@FreeBSD.org> Cc: Max Laier <max@love2party.net>, FreeBSD current <freebsd-current@FreeBSD.org>, neel@FreeBSD.org, Peter Grehan <grehan@FreeBSD.org> Subject: Re: proposed smp_rendezvous change Message-ID: <4DD26256.2070008@FreeBSD.org> In-Reply-To: <4DD22BD9.6070504@FreeBSD.org> References: <4DCD357D.6000109@FreeBSD.org> <201105161421.27665.jhb@freebsd.org> <4DD17AB3.1070606@FreeBSD.org> <201105161609.21898.jhb@freebsd.org> <4DD22BD9.6070504@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On 5/17/11 4:03 AM, Andriy Gapon wrote: > on 16/05/2011 23:09 John Baldwin said the following: >> is probably just cut and pasted to match the other uses of values in >> the smp_rv_waiters[] array. >> >> (atomic_add_acq_int() could spin on architectures where it is implemented >> using compare-and-swap (e.g. sparc64) or locked-load conditional-store (e.g. >> Alpha).) > > > When you say "not strictly necessary", do you mean "not necessary"? > If you do not mean that, then when could it be (non-strictly) necessary? :) > > Couldn't [Shouldn't] the whole: > >>>> /* Ensure we have up-to-date values. */ >>>> atomic_add_acq_int(&smp_rv_waiters[0], 1); >>>> while (smp_rv_waiters[0]< smp_rv_ncpus) >>>> cpu_spinwait(); > > be just replaced with: > > rmb(); > > Or a proper MI function that does just a read memory barrier, if rmb() is not that. No, you could replace it with: atomic_add_acq_int(&smp_rv_waiters[0], 1); The key being that atomic_add_acq_int() will block (either in hardware or software) until it can safely perform the atomic operation. That means waiting until the write to set smp_rv_waiters[0] to 0 by the rendezvous initiator is visible to the current CPU. On some platforms a write by one CPU may not post instantly to other CPUs (e.g. it may sit in a store buffer). That is fine so long as an attempt to update that value atomically (using cas or a conditional-store, etc.) fails. For those platforms, the atomic(9) API is required to spin until it succeeds. This is why the mtx code spins if it can't set MTX_CONTESTED for example. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?4DD26256.2070008>