fscache: Implement volume-level access helpers

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 way the access gate on the volume 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 volume's n_accesses
      count and then recheck the cache liveness, ending the access if it
      ceased to be live.

  (3) When we end the access, we decrement the volume's n_accesses and wake
      up the any waiters if it reaches 0.

  (4) Whilst the cache is caching, the volume's 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, the volume's n_accesses is decremented and we wait for it to
      become 0.

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/163819594158.215744.8285859817391683254.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906894315.143852.5454793807544710479.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967095028.1823006.9173132503876627466.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021501546.640689.9631510472149608443.stgit@warthog.procyon.org.uk/ # v4

3 files changed, 88 insertions, 0 deletions

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index be29816b37ef..91a4ea08ec0b 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -130,6 +130,9 @@ 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,
+				 struct fscache_cookie *cookie,
+				 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 876f4bee5840..6cab5d99ba4c 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 630894fefd02..20497f0f10bb 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -33,6 +33,90 @@ 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,
+					  struct fscache_cookie *cookie,
+					  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, cookie ? cookie->debug_id : 0,
+				    refcount_read(&volume->ref),
+				    n_accesses, why);
+}
+
+/**
+ * fscache_begin_volume_access - Pin a cache so a volume can be accessed
+ * @volume: The volume cookie
+ * @cookie: A datafile cookie for a tracing reference (or NULL)
+ * @why: An indication of the circumstances of the access for tracing
+ *
+ * Attempt to pin the cache to prevent it from going away whilst we're
+ * accessing a volume and returns true if successful.  This works as follows:
+ *
+ *  (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 volume's n_accesses
+ *      count and then recheck the cache liveness, ending the access if it
+ *      ceased to be live.
+ *
+ *  (3) When we end the access, we decrement the volume's n_accesses and wake
+ *      up the any waiters if it reaches 0.
+ *
+ *  (4) Whilst the cache is caching, the volume's 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, the volume's n_accesses is decremented and we wait for it to
+ *      become 0.
+ *
+ * The datafile @cookie and the @why indicator are merely provided for tracing
+ * purposes.
+ */
+bool fscache_begin_volume_access(struct fscache_volume *volume,
+				 struct fscache_cookie *cookie,
+				 enum fscache_access_trace why)
+{
+	if (!fscache_cache_is_live(volume->cache))
+		return false;
+	__fscache_begin_volume_access(volume, cookie, why);
+	if (!fscache_cache_is_live(volume->cache)) {
+		fscache_end_volume_access(volume, cookie, fscache_access_unlive);
+		return false;
+	}
+	return true;
+}
+
+/**
+ * fscache_end_volume_access - Unpin a cache at the end of an access.
+ * @volume: The volume cookie
+ * @cookie: A datafile cookie for a tracing reference (or NULL)
+ * @why: An indication of the circumstances of the access for tracing
+ *
+ * Unpin a cache volume after we've accessed it.  The datafile @cookie and the
+ * @why indicator are merely provided for tracing purposes.
+ */
+void fscache_end_volume_access(struct fscache_volume *volume,
+			       struct fscache_cookie *cookie,
+			       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, cookie ? cookie->debug_id : 0,
+				    refcount_read(&volume->ref),
+				    n_accesses, why);
+	if (n_accesses == 0)
+		wake_up_var(&volume->n_accesses);
+}
+EXPORT_SYMBOL(fscache_end_volume_access);
+
 static bool fscache_volume_same(const struct fscache_volume *a,
 				const struct fscache_volume *b)
 {