fscache: Implement cache-level access helpers
Add a pair of functions to pin/unpin a cache that we're wanting to do a
high-level access to (such as creating or removing a volume):
bool fscache_begin_cache_access(struct fscache_cache *cache,
enum fscache_access_trace why);
void fscache_end_cache_access(struct fscache_cache *cache,
enum fscache_access_trace why);
The way the access gate works/will work is:
(1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
then we return false to indicate access was not permitted.
(2) If the cache tests as live, then we increment the n_accesses count and
then recheck the liveness, ending the access if it ceased to be live.
(3) When we end the access, we decrement n_accesses and wake up the any
waiters if it reaches 0.
(4) Whilst the cache is caching, n_accesses is kept artificially
incremented to prevent wakeups from happening.
(5) When the cache is taken offline, the state is changed to prevent new
accesses, n_accesses is decremented and we wait for n_accesses to
become 0.
Note that some of this is implemented in a later patch.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819593239.215744.7537428720603638088.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906893368.143852.14164004598465617981.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967093977.1823006.6967886507023056409.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021499995.640689.18286203753480287850.stgit@warthog.procyon.org.uk/ # v4
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 71c8977..be29816 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -23,6 +23,8 @@
#ifdef CONFIG_PROC_FS
extern const struct seq_operations fscache_caches_seq_ops;
#endif
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
struct fscache_cache *fscache_lookup_cache(const char *name, bool is_cache);
void fscache_put_cache(struct fscache_cache *cache, enum fscache_cache_trace where);