Date: Wed, 02 Jan 2002 17:30:02 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: Bernd Walter <ticso@cicely9.cicely.de> Cc: Peter Jeremy <peter.jeremy@alcatel.com.au>, Michal Mertl <mime@traveller.cz>, Bruce Evans <bde@zeta.org.au>, Mike Smith <msmith@FreeBSD.ORG>, Bernd Walter <ticso@cicely8.cicely.de>, arch@FreeBSD.ORG, Matthew Dillon <dillon@apollo.backplane.com> Subject: Re: When to use atomic_ functions? (was: 64 bit counters) Message-ID: <XFMail.020102173002.jhb@FreeBSD.org> In-Reply-To: <20020103003214.GC53199@cicely9.cicely.de>
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On 03-Jan-02 Bernd Walter wrote: > On Wed, Jan 02, 2002 at 04:02:14PM -0800, Matthew Dillon wrote: >> :Look at PCPU_GET/PCPU_SET. Note that since an interrupt can preempt you >> :and >> :push you off onto another CPU, you have to use a critical section while >> :updating per-CPU variables. If desired, some kind of free area could be >> :stuck >> :in struct pcpu (or more likely, struct pcpu would hold a pointer to the >> :area) >> :that could be galloc/gfree'd or some such. >> : >> :-- >> : >> :John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ >> :"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ >> >> Maybe we are going about this all wrong. If a particular interface >> counter can only be modified from the device interrupt, or only be >> modified while holding the appropriate mutex, do we need any locking >> at all? > > You need to hold the mutex while writing and reading. > If you hold the mutex only while writing another CPU might still use > old cached values. Yes. > The same goes for device interrrupt as it doesn't enshure inter CPU > consistency. Yes. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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