On 7/24/2015 10:05, NeilBrown wrote: > On Mon, 13 Jul 2015 05:45:53 +0100 Al Viro <viro@xxxxxxxxxxxxxxxxxx> > wrote: > >> On Mon, Jul 13, 2015 at 02:20:59PM +1000, NeilBrown wrote: >> >>> Actually, with that change to pin_kill, this side of things becomes >>> really easy. >>> All expXXX_pin_kill needs to do is call your new cache_delete_entry. >>> If that doesn't cause the entry to be put, then something else has a >>> temporary reference which will be put soon. In any case, pin_kill() >>> will wait long enough, but not indefinitely. >>> No need for kref_get_unless_zero() or any of that. >> >> No. You are seriously misunderstanding what ->kill() is for and what the >> existing instances are doing. Again, there is no promise whatsoever that >> the object containing fs_pin instance will *survive* past ->kill(). >> At all. >> >> RTFS, please. What is sorely missing in this recurring patchset is a clear >> description of lifetime rules and ordering (who waits for whom and how long). >> For all the objects involved. > > Good point. Let me try. > > Entries in the sunrpc 'cache' each contain some 'key' fields and some > 'content' fields. > > The key fields are set by the .init() method when the entry is > created, which can happen in a call to sunrpc_cache_lookup() or to > sunrpc_cache_update(). > > The content fields are set by the .update() method when a value is > provided for the cache entry. This happens in sunrpc_cache_update(); > > A cache entry can be not-valid, negative, or valid. > It starts non-valid when sunrpc_cache_lookup() fails to find the search > key and so creates a new entry (and sets up the key with .init). > It then transitions to either negative or valid. > This can happen through sunrpc_cache_update() or through an error when > instigating an up-call, in which case it goes to negative. > Once it is negative or valid, it stays that way until it is released. > If sunrpc_cache_update is called on an entry that is not not-valid, > then a new entry is created and the old one is marked as expired. > A cache search will find the new one before the old. > > The vfsmount object is involved in two separate caches. > It is part of the content of svc_expkey and part of the key of > svc_export. > > An svc_expkey entry is only ever held transiently. It is held while an > update is being processed, and it is held briefly while mapping a > filehandle to a mnt+dentry. > Firstly part of the filehandle is used to acccess the svc_expkey cache > to get the vfsmnt. Then that vfsmnt plus the client identification is > looked up in the svc_export cache to find the export options. Then the > svc_expkey cache entry is released. > > So it is only held during a lookup of another cache. This can take an > arbitrarily long time as the lookup can go to rpc.mountd in user-space. > > > The svc_export cache entry can be held for the duration of a single NFS > request. It is stored in the 'struct svc_fh' file handle structure > which is release at the end of handling the request. > > The vfsmnt and dentry are only "used" to validate the filehandle and > then while that filehandle is still active. > > > To avoid having unmount hang while nfsd is performing an upcall to > mountd, we need to legitimize the vfsmnt in the svc_expkey. If that > fails, exp_find_key() can fail and we would never perform the lookup on > svc_export. > > If it succeeds, then the legitimacy can be handed over to the svc_export > cache entry, which could then continue to own it, or could hand it on > to the svc_fh. > > The latter is *probably* cleanest. > i.e. an svc_fh should always own a reference to exp->ex_path.mnt, and > fh_put must put it. I don't agree adding new argument (eg, fh_vfsmnt) in svc_fh. With it, should nfsd using fh_vfsmnt always, never using exp->ex_path.mnt outside of export.c/export.h ? If choose fh_vfsmnt, so many codes need be updated, especially functions. If exp->ex_path.mnt, the new argument fh_vfsmnt seems redundant. Thanks for your work. It reminders a new method, 1. There are only one outlet from each cache, exp_find_key() for expkey, exp_get_by_name() for export. 2. Any fsid to export or filehandle to export will call the function. 3. exp_get()/exp_put() increase/decrease the reference of export. Like the fh_vfsmnt (not same), call legitimize_mntget() in the only outlet function exp_find_key()/exp_get_by_name(), if fail return STALE, otherwise, any valid expkey/export from the cache is validated (Have get the reference of vfsmnt). Add mntget() in exp_get() and mntput() in exp_put(), because the export passed to exp_get/exp_put are returned from exp_find_key/exp_get_by_name. > > exp_find_key needs to legitimize ek->ek_path.mnt, so a successful > return from exp_find implies an active refernece to ->ex_path.mnt. > If exp_find fails, it needs to mnt_put(ek->ek_path.mnt). Yes, it's great. > All callers of exp_find need to mnt_put(exp->ex_path.mnt) when they > decide not to use the exp, and must otherwise store it in an svc_fh. > > With this, pin_kill() should only need to wait for exp_find_key() to > discover that it cannot legitimize the mount, or for expkey_path() to > replace the key via sunrpc_cache_update(), or maybe for cache_clean() > to discard an old entry. > > Hopefully that makes it all clear. Yes, thanks again. With my method, for expkey cache, 1. At first, a fsid is passed to exp_find_key, and lookup a cache in svc_expkey_lookup, if success, ekey->ek_path is pined to mount. 2. Then call legitimize_mntget getting a reference of vfsmnt before return from exp_find_key. 3. Any calling exp_find_key with valid cache must put the vfsmnt. for export cache, 1. At first, a path (returned from exp_find_key) with validate vfsmnt is passed to exp_get_by_name, if success, exp->ex_path is pined to mount. 2. Then call legitimize_mntget getting a reference of vfsmnt before return from exp_get_by_name. 3. Any calling exp_get_by_name with valid cache must put the vfsmnt by exp_put(); 4. Any using the exp returned from exp_get_by_name must call exp_get(), will increase the reference of vfsmnt. So, a. After getting the reference in 2, any umount of filesystem will get -EBUSY. b. After put all reference after 4, or before get the reference in 2, any umount of filesystem will call pin_kill, and delete the cache directly, also unpin the vfsmount. c. Between 1 and 2, have get the reference of exp/key cache, with invalidate vfsmnt. As you said, umount of filesystem only wait exp_find_key/exp_get_by_name put the reference of cache when legitimize_mntget fail. A new update of this patch site will be push later. thanks, Kinglong Mee -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
