[PATCH v2 13/53] fscache: Implement volume-level access helpers

[David Howells]

Add a pair of helper functions to manage access to a volume, pinning the
volume in place for the duration to prevent cache withdrawal from removing
it:

	bool fscache_begin_volume_access(struct fscache_volume *volume,
					 enum fscache_access_trace why);
	void fscache_end_volume_access(struct fscache_volume *volume,
				       enum fscache_access_trace why);

The first is intended for internal use only, but the second will be used by
the cache backend also.

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

 fs/fscache/internal.h          |    2 ++
 fs/fscache/main.c              |    1 +
 fs/fscache/volume.c            |   47 ++++++++++++++++++++++++++++++++++++++++
 include/linux/fscache-cache.h  |    3 +++
 include/trace/events/fscache.h |   30 ++++++++++++++++++++++++++
 5 files changed, 83 insertions(+)

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 5546c24d18d8..18f9a408a4f4 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -107,6 +107,8 @@ struct fscache_volume *fscache_get_volume(struct fscache_volume *volume,
 					  enum fscache_volume_trace where);
 void fscache_put_volume(struct fscache_volume *volume,
 			enum fscache_volume_trace where);
+bool fscache_begin_volume_access(struct fscache_volume *volume,
+				 enum fscache_access_trace why);
 void fscache_create_volume(struct fscache_volume *volume, bool wait);
 
 
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index e1f14b29cff4..6a024c45eb0b 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -22,6 +22,7 @@ MODULE_PARM_DESC(fscache_debug,
 		 "FS-Cache debugging mask");
 
 EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
+EXPORT_TRACEPOINT_SYMBOL(fscache_access_volume);
 
 struct workqueue_struct *fscache_wq;
 EXPORT_SYMBOL(fscache_wq);
diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index 924851888f18..d539d6ec2664 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -33,6 +33,53 @@ static void fscache_see_volume(struct fscache_volume *volume,
 	trace_fscache_volume(volume->debug_id, ref, where);
 }
 
+/*
+ * Pin the cache behind a volume so that we can access it.
+ */
+static void __fscache_begin_volume_access(struct fscache_volume *volume,
+					  enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	n_accesses = atomic_inc_return(&volume->n_accesses);
+	smp_mb__after_atomic();
+	trace_fscache_access_volume(volume->debug_id, refcount_read(&volume->ref),
+				    n_accesses, why);
+}
+
+/*
+ * If the cache behind a volume is live, pin it so that we can access it.
+ */
+bool fscache_begin_volume_access(struct fscache_volume *volume,
+				 enum fscache_access_trace why)
+{
+	if (!fscache_cache_is_live(volume->cache))
+		return false;
+	__fscache_begin_volume_access(volume, why);
+	if (!fscache_cache_is_live(volume->cache)) {
+		fscache_end_volume_access(volume, fscache_access_unlive);
+		return false;
+	}
+	return true;
+}
+
+/*
+ * Mark the end of an access on a volume.
+ */
+void fscache_end_volume_access(struct fscache_volume *volume,
+			       enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	smp_mb__before_atomic();
+	n_accesses = atomic_dec_return(&volume->n_accesses);
+	trace_fscache_access_volume(volume->debug_id, refcount_read(&volume->ref),
+				    n_accesses, why);
+	if (n_accesses == 0)
+		wake_up_var(&volume->n_accesses);
+}
+EXPORT_SYMBOL(fscache_end_volume_access);
+
 static long fscache_compare_volume(const struct fscache_volume *a,
 				   const struct fscache_volume *b)
 {
diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index e075cca1a30d..7db4dda74951 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -77,6 +77,9 @@ extern struct fscache_cache *fscache_acquire_cache(const char *name);
 extern void fscache_put_cache(struct fscache_cache *cache,
 			      enum fscache_cache_trace where);
 
+extern void fscache_end_volume_access(struct fscache_volume *volume,
+				      enum fscache_access_trace why);
+
 extern struct fscache_cookie *fscache_get_cookie(struct fscache_cookie *cookie,
 						 enum fscache_cookie_trace where);
 extern void fscache_put_cookie(struct fscache_cookie *cookie,
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index d09d7e3ac86c..4b35fe9cd906 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -40,6 +40,7 @@ enum fscache_volume_trace {
 	fscache_volume_put_relinquish,
 	fscache_volume_see_create_work,
 	fscache_volume_see_hash_wake,
+	fscache_volume_wait_create_work,
 };
 
 enum fscache_cookie_trace {
@@ -239,6 +240,35 @@ TRACE_EVENT(fscache_access_cache,
 		      __entry->n_accesses)
 	    );
 
+TRACE_EVENT(fscache_access_volume,
+	    TP_PROTO(unsigned int volume_debug_id,
+		     int ref,
+		     int n_accesses,
+		     enum fscache_access_trace why),
+
+	    TP_ARGS(volume_debug_id, ref, n_accesses, why),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		volume		)
+		    __field(int,			ref		)
+		    __field(int,			n_accesses	)
+		    __field(enum fscache_access_trace,	why		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->volume	= volume_debug_id;
+		    __entry->ref	= ref;
+		    __entry->n_accesses	= n_accesses;
+		    __entry->why	= why;
+			   ),
+
+	    TP_printk("V=%08x %s r=%d a=%d",
+		      __entry->volume,
+		      __print_symbolic(__entry->why, fscache_access_traces),
+		      __entry->ref,
+		      __entry->n_accesses)
+	    );
+
 TRACE_EVENT(fscache_acquire,
 	    TP_PROTO(struct fscache_cookie *cookie),