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Message-ID: <2236FBA76BA1254E88B949DDB74E612B802D08DA@IRSMSX102.ger.corp.intel.com> Date: Wed, 27 Dec 2017 12:10:15 +0000 From: "Reshetova, Elena" <elena.reshetova@...el.com> To: "J. Bruce Fields" <bfields@...ldses.org> CC: "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "jlayton@...nel.org" <jlayton@...nel.org>, "trond.myklebust@...marydata.com" <trond.myklebust@...marydata.com>, "anna.schumaker@...app.com" <anna.schumaker@...app.com>, "peterz@...radead.org" <peterz@...radead.org>, "keescook@...omium.org" <keescook@...omium.org> Subject: RE: [PATCH 1/4] lockd: convert nlm_host.h_count from atomic_t to refcount_t > On Fri, Dec 22, 2017 at 09:25:53AM -0500, J. Bruce Fields wrote: > > On Fri, Dec 22, 2017 at 09:29:15AM +0000, Reshetova, Elena wrote: > > > > > > On Wed, Nov 29, 2017 at 01:15:43PM +0200, Elena Reshetova wrote: > > > > atomic_t variables are currently used to implement reference > > > > counters with the following properties: > > > > - counter is initialized to 1 using atomic_set() > > > > - a resource is freed upon counter reaching zero > > > > - once counter reaches zero, its further > > > > increments aren't allowed > > > > - counter schema uses basic atomic operations > > > > (set, inc, inc_not_zero, dec_and_test, etc.) > > > > > > >Whoops, I forgot that this doesn't apply to h_count. > > > > > > >Well, it's confusing, because h_count is actually used in two different > > > >ways: depending on whether a nlm_host represents a client or server, it > > > >may have the above properties or not. > > > > > > > > > So, what happens when it is not having the above properties? Is the object > > > being reused or? > > > > The object isn't destroyed when the counter hits zero--zero is just > > taken as a hint to some garbage collection algorithm that it would be OK > > to destroy it. So decrementing to or incrementing from zero is OK. > > In more detail: the nlm_host objects that are used on the NFS server to > represent NFS clients are put by nlmsvc_release_host, and then may > eventually be freed by nlm_gc_hosts. > > The nlm_host objects that are used on the NFS client to represent NFS > servers are put (and freed when h_count goes to zero) by > nlmclnt_release_host. > > In both cases reference are taken by nlm_get_host. It would be possible > to replace nlm_get_host by two different functions if that would help. > Most callers are obviously only client-side or server-side. The only > exception is next_host_state. It could be passed a pointer to the "get" > function it should use. > > After that we might actually just want to define separate client and > server structs like: > > struct nlm_clnt_host { > struct nlm_host ch_host; > refcount_t ch_count; > ... > } > > struct nlm_srv_host { > struct nlm_host sh_host; > refcount_t sh_count; > ... > } > > rather than have a single h_count which is used in two confusingly > different ways. There are also some other nlm_host fields that really > only make sense for client or server. This sounds reasonable for me, but obviously it is a bigger change and I might not have enough knowledge on NFS to make it correctly. In any case, even for the current server case, when freeing might not happen and object gets re-used later on, is it possible to simply re-initialize the object (and its reference counter) properly before reusing? I think this is the only thing that is needed from the correct refcounting POV in this case, so instead of using refcount_inc() on reused object, you would explicitly do refcount_set(counter, 1) when reuse happens. Best Regards, Elena > > --b.
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