[PATCH v2 12/53] fscache: Implement cache-level access helpers

[David Howells <dhowells@xxxxxxxxxx>]

Add a pair of helper functions to pin/unpin a cache that we're wanting to
do a high-level access to (such as creating or removing a volume).

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: linux-cachefs@xxxxxxxxxx
---

 fs/fscache/cache.c             |   37 ++++++++++++++++++++++++++++++++++++
 fs/fscache/internal.h          |    2 ++
 fs/fscache/main.c              |    2 ++
 include/trace/events/fscache.h |   41 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 82 insertions(+)

diff --git a/fs/fscache/cache.c b/fs/fscache/cache.c
index da08e29ee5b9..3cde698c5015 100644
--- a/fs/fscache/cache.c
+++ b/fs/fscache/cache.c
@@ -171,6 +171,43 @@ void fscache_put_cache(struct fscache_cache *cache,
 }
 EXPORT_SYMBOL(fscache_put_cache);
 
+/*
+ * Get an increment on a cache's access counter if the cache is live to prevent
+ * it from going away whilst we're accessing it.
+ */
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	if (!fscache_cache_is_live(cache))
+		return false;
+
+	n_accesses = atomic_inc_return(&cache->n_accesses);
+	smp_mb__after_atomic(); /* Reread live flag after n_accesses */
+	trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+				   n_accesses, why);
+	if (!fscache_cache_is_live(cache)) {
+		fscache_end_cache_access(cache, fscache_access_unlive);
+		return false;
+	}
+	return true;
+}
+
+/*
+ * Drop an increment on a cache's access counter.
+ */
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	smp_mb__before_atomic();
+	n_accesses = atomic_dec_return(&cache->n_accesses);
+	trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+				   n_accesses, why);
+	if (n_accesses == 0)
+		wake_up_var(&cache->n_accesses);
+}
+
 #ifdef CONFIG_PROC_FS
 static const char fscache_cache_states[NR__FSCACHE_CACHE_STATE] = "-PAEW";
 
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 9daacd7de9ea..5546c24d18d8 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);
 
 /*
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index fad9c1933987..e1f14b29cff4 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -21,6 +21,8 @@ module_param_named(debug, fscache_debug, uint,
 MODULE_PARM_DESC(fscache_debug,
 		 "FS-Cache debugging mask");
 
+EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
+
 struct workqueue_struct *fscache_wq;
 EXPORT_SYMBOL(fscache_wq);
 
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index cfa4c153c72b..d09d7e3ac86c 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -59,6 +59,12 @@ enum fscache_cookie_trace {
 	fscache_cookie_see_work,
 };
 
+enum fscache_access_trace {
+	fscache_access_cache_pin,
+	fscache_access_cache_unpin,
+	fscache_access_unlive,
+};
+
 #endif
 
 /*
@@ -101,6 +107,11 @@ enum fscache_cookie_trace {
 	EM(fscache_cookie_see_withdraw,		"-   x-wth")		\
 	E_(fscache_cookie_see_work,		"-   work ")
 
+#define fscache_access_traces		\
+	EM(fscache_access_cache_pin,		"PIN   cache  ")	\
+	EM(fscache_access_cache_unpin,		"UNPIN cache  ")	\
+	E_(fscache_access_unlive,		"END   unlive ")
+
 /*
  * Export enum symbols via userspace.
  */
@@ -112,6 +123,7 @@ enum fscache_cookie_trace {
 fscache_cache_traces;
 fscache_volume_traces;
 fscache_cookie_traces;
+fscache_access_traces;
 
 /*
  * Now redefine the EM() and E_() macros to map the enums to the strings that
@@ -198,6 +210,35 @@ TRACE_EVENT(fscache_cookie,
 		      __entry->ref)
 	    );
 
+TRACE_EVENT(fscache_access_cache,
+	    TP_PROTO(unsigned int cache_debug_id,
+		     int ref,
+		     int n_accesses,
+		     enum fscache_access_trace why),
+
+	    TP_ARGS(cache_debug_id, ref, n_accesses, why),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		cache		)
+		    __field(int,			ref		)
+		    __field(int,			n_accesses	)
+		    __field(enum fscache_access_trace,	why		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->cache	= cache_debug_id;
+		    __entry->ref	= ref;
+		    __entry->n_accesses	= n_accesses;
+		    __entry->why	= why;
+			   ),
+
+	    TP_printk("C=%08x %s r=%d a=%d",
+		      __entry->cache,
+		      __print_symbolic(__entry->why, fscache_access_traces),
+		      __entry->ref,
+		      __entry->n_accesses)
+	    );
+
 TRACE_EVENT(fscache_acquire,
 	    TP_PROTO(struct fscache_cookie *cookie),