Date: Sun, 2 Nov 2014 16:54:46 +0100 From: Attilio Rao <attilio@freebsd.org> To: Konstantin Belousov <kib@freebsd.org> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org> Subject: Re: svn commit: r273966 - in head: share/man/man9 sys/kern sys/sys Message-ID: <CAJ-FndB5%2BhhHprTiV1ODJoE9RNZLtOe2eQDi3MEpgPKYbM3LAw@mail.gmail.com> In-Reply-To: <201411021310.sA2DAWmD003298@svn.freebsd.org> References: <201411021310.sA2DAWmD003298@svn.freebsd.org>
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On Sun, Nov 2, 2014 at 2:10 PM, Konstantin Belousov <kib@freebsd.org> wrote: > Author: kib > Date: Sun Nov 2 13:10:31 2014 > New Revision: 273966 > URL: https://svnweb.freebsd.org/changeset/base/273966 > > Log: > Fix two issues with lockmgr(9) LK_CAN_SHARE() test, which determines > whether the shared request for already shared-locked lock could be > granted. Both problems result in the exclusive locker starvation. > > The concurrent exclusive request is indicated by either > LK_EXCLUSIVE_WAITERS or LK_EXCLUSIVE_SPINNERS flags. The reverse > condition, i.e. no exclusive waiters, must check that both flags are > cleared. > > Add a flag LK_NODDLKTREAT for shared lock request to indicate that > current thread guarantees that it does not own the lock in shared > mode. This turns back the exclusive lock starvation avoidance code; > see man page update for detailed description. > > Use LK_NODDLKTREAT when doing lookup(9). The right way to implement this (selectively disabling writer starvation avoidance) must be on a lock-basis. So you need a new flag to pass to lockinit(). This is to support it "globaly". Then, you can pass it on a lockmgr() instance basis (like FreeBSD also passes, for example, LK_NOWITNESS). You can use it during lookup() calls. Maybe you will need to propagate it via the vn_lock() interface. The patch will be bigger but much cleaner and more correct than what is in head now. Attilio -- Peace can only be achieved by understanding - A. Einstein
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