Date: Thu, 16 Jun 2005 06:47:05 -0700 From: Alfred Perlstein <alfred@freebsd.org> To: Jeff Roberson <jroberson@chesapeake.net> Cc: cvs-src@FreeBSD.org, Jeff Roberson <jeff@FreeBSD.org>, src-committers@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern vfs_subr.c Message-ID: <20050616134705.GT87251@elvis.mu.org> In-Reply-To: <20050616004203.N66638@mail.chesapeake.net> References: <200506160441.j5G4fg0l078419@repoman.freebsd.org> <20050616004203.N66638@mail.chesapeake.net>
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* Jeff Roberson <jroberson@chesapeake.net> [050615 21:47] wrote: > On Thu, 16 Jun 2005, Jeff Roberson wrote: > > > jeff 2005-06-16 04:41:42 UTC > > This fixes duplicate frees from the vnode zone, crashes in vnlrureclaim > and vlru_free() and a few other related problems. Basically, I've > converted the holdcnt to be a proper ref counting system, while use count > maintains the traditional vfs use count semantics, which are not really > compatbile with structure life-time issues. It looks like a big change > but it really makes things much simpler. Briefly, the rules are this: > > 1) vget/grab a use count if you're a normal consumer of the vnode API. > That goes for syscalls, mmaping, nfs, etc. > > 2) vhold() if you want to prevent a vnode from being recycled while you > hold a reference to it. The name cache uses this, as does the vm when > pages reference the vnode's vm object. This is also used for vlrureclaim, > vflush, vtryrecyle, etc. which want to make sure the vnode is not recycled > while it has a local reference. > > I'll probably add a comment to vnode.h to describe this in more detail. Please do, last I remember, vhold and vget are the same (gaining a ref), except vget can return the vnode locked. This is a bit confusing. > > > > > FreeBSD src repository > > > > Modified files: > > sys/kern vfs_subr.c > > Log: > > - Change holdcnt use around vnode recycling. We now always keep a holdcnt > > ref while we're calling vgone(). This prevents transient refs from > > re-adding us to the free list. Previously, a vfree() triggered via > > vinvalbuf() getting rid of all of a vnode's pages could place a partially > > destructed vnode on the free list where vtryrecycle() could find it. The > > first call to vtryrecycle would hang up on the vnode lock, but when it > > failed it would place a now dead vnode onto the free list, and another > > call to vtryrecycle() would free an already free vnode. There were many > > complications of having a zero ref count while freeing which can now go > > away. > > - Change vdropl() to release the interlock before returning. All callers > > now respect this, so vdropl() directly frees VI_DOOMED vnodes once the > > last ref is dropped. This means that we'll never have VI_DOOMED vnodes > > on the free list. > > - Seperate v_incr_usecount() into v_incr_usecount(), v_decr_usecount() and > > v_decr_useonly(). The incr/decr split is so that incr usecount can > > return with the interlock still held while decr drops the interlock so > > it can call vdropl() which will potentially free the vnode. The calling > > function can't drop the lock of an already free'd node. v_decr_useonly() > > drops a usecount without droping the hold count. This is done so the > > usecount reaches zero in vput() before we recycle, however the holdcount > > is still 1 which prevents any new references from placing the vnode > > back on the free list. > > - Fix vnlrureclaim() to vhold the vnode since it doesn't do a vget(). We > > wouldn't want vnlrureclaim() to bump the usecount since this has > > different semantics. Also change vnlrureclaim() to do a NOWAIT on the > > vn_lock. When this function runs we're usually in a desperate situation > > and we wouldn't want to wait for any specific vnode to be released. > > - Fix a bunch of misc comments to reflect the new behavior. > > - Add vhold() and vdrop() to vflush() for the same reasons that we do in > > vlrureclaim(). Previously we held no reference and a vnode could have > > been freed while we were waiting on the lock. > > - Get rid of vlruvp() and vfreehead(). Neither are used. vlruvp() should > > really be rethought before it's reintroduced. > > - vgonel() always returns with the vnode locked now and never puts the > > vnode back on a free list. The vnode will be freed as soon as the last > > reference is released. > > > > Sponsored by: Isilon Systems, Inc. > > Debugging help from: Kris Kennaway, Peter Holm > > Approved by: re (blanket vfs) > > > > Revision Changes Path > > 1.632 +198 -202 src/sys/kern/vfs_subr.c > > -- - Alfred Perlstein - email: bright@mu.org cell: 408-480-4684
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