Date: Wed, 18 Mar 2015 12:44:42 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mateusz Guzik <mjguzik@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: atomic v_usecount and v_holdcnt Message-ID: <20150318104442.GS2379@kib.kiev.ua> In-Reply-To: <20150317014412.GA10819@dft-labs.eu> References: <20141122002812.GA32289@dft-labs.eu> <20141122092527.GT17068@kib.kiev.ua> <20141122211147.GA23623@dft-labs.eu> <20141124095251.GH17068@kib.kiev.ua> <20150314225226.GA15302@dft-labs.eu> <20150316094643.GZ2379@kib.kiev.ua> <20150317014412.GA10819@dft-labs.eu>
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On Tue, Mar 17, 2015 at 02:44:12AM +0100, Mateusz Guzik wrote: > On Mon, Mar 16, 2015 at 11:46:43AM +0200, Konstantin Belousov wrote: > > On Sat, Mar 14, 2015 at 11:52:26PM +0100, Mateusz Guzik wrote: > > > On Mon, Nov 24, 2014 at 11:52:52AM +0200, Konstantin Belousov wrote: > > > > On Sat, Nov 22, 2014 at 10:11:47PM +0100, Mateusz Guzik wrote: > > > > > On Sat, Nov 22, 2014 at 11:25:27AM +0200, Konstantin Belousov wrote: > > > > > > On Sat, Nov 22, 2014 at 01:28:12AM +0100, Mateusz Guzik wrote: > > > > > > > The idea is that we don't need an interlock as long as we don't > > > > > > > transition either counter 1->0 or 0->1. > > > > > > I already said that something along the lines of the patch should work. > > > > > > In fact, you need vnode lock when hold count changes between 0 and 1, > > > > > > and probably the same for use count. > > > > > > > > > > > > > > > > I don't see why this would be required (not that I'm an VFS expert). > > > > > vnode recycling seems to be protected with the interlock. > > > > > > > > > > In fact I would argue that if this is really needed, current code is > > > > > buggy. > > > > Yes, it is already (somewhat) buggy. > > > > > > > > Most need of the lock is for the case of counts coming from 1 to 0. > > > > The reason is the handling of the active vnode list, which is used > > > > for limiting the amount of vnode list walking in syncer. When hold > > > > count is decremented to 0, vnode is removed from the active list. > > > > When use count is decremented to 0, vnode is supposedly inactivated, > > > > and vinactive() cleans the cached pages belonging to vnode. In other > > > > words, VI_OWEINACT for dirty vnode is sort of bug. > > > > > > > > > > Modified the patch to no longer have the usecount + interlock dropped + > > > VI_OWEINACT set window. > > > > > > Extended 0->1 hold count + vnode not locked window remains. I can fix > > > that if it is really necessary by having _vhold return with interlock > > > held if it did such transition. > > > > In v_upgrade_usecount(), you call v_incr_devcount() without without interlock > > held. What prevents the devfs vnode from being recycled, in particular, > > from invalidation of v_rdev pointer ? > > > > Right, that was buggy. Fixed in the patch below. Why non-atomicity of updates to several counters is safe ? This at least requires an explanation in the comment, I mean holdcnt/usecnt pair. Assume the thread increased the v_usecount, but did not managed to acquire dev_mtx. Another thread performs vrele() and progressed to v_decr_devcount(). It decreases the si_usecount, which might allow yet another thread to see the si_usecount as too low and start unwanted action. I think that the tests for VCHR must be done at the very start of the functions, and devfs vnodes must hold vnode interlock unconditionally. > > > I think that refcount_acquire_if_greater() KPI is excessive. You always > > calls acquire with val == 0, and release with val == 1. > > > > Yea i noted in my prevoius e-mail it should be changed (see below). > > I replaced them with refcount_acquire_if_not_zero and > refcount_release_if_not_last. I dislike the length of the names. Can you propose something shorter ? The type for the local variable old in both functions should be u_int. > > > WRT to _refcount_release_lock, why is lock_object->lc_lock/lc_unlock KPI > > cannot be used ? This allows to make refcount_release_lock() a function > > instead of gcc extension macros. Not to mention that the macro is unused. > > These were supposed to be used by other code, forgot to remove it from > the patch I sent here. > > We can discuss this in another thread. > > Striclty speaking we could use it here for vnode interlock, but I did > not want to get around VI_LOCK macro (which right now is just a > mtx_lock, but this may change). > > Updated patch is below: Do not introduce ASSERT_VI_LOCK, the name difference between ASSERT_VI_LOCKED and ASSERT_VI_LOCK is only in the broken grammar. I do not see anything wrong with explicit if() statements where needed, in all four places. In vputx(), wrap the long line (if (refcount_release() || VI_DOINGINACT)).
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