bcachefs: bch2_dev_tryget()

Most uses of bch2_dev_bkey_exists() are going away, where we assume that
because a key references a device the device most exist - instead, we'll
be explicitly checking if the device exists and getting a reference to
it.

This adds the new helpers.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/sb-members.c

@@ -9,6 +9,11 @@
 #include "sb-members.h"
 #include "super-io.h"
 
+void bch2_dev_missing(struct bch_fs *c, unsigned dev)
+{
+	bch2_fs_inconsistent(c, "pointer to nonexistent device %u", dev);
+}
+
 #define x(t, n, ...) [n] = #t,
 static const char * const bch2_iops_measurements[] = {
 	BCH_IOPS_MEASUREMENTS()

fs/bcachefs/sb-members.h

@@ -217,6 +217,26 @@ static inline struct bch_dev *bch2_dev_rcu(struct bch_fs *c, unsigned dev)
 		: NULL;
 }
 
+static inline struct bch_dev *bch2_dev_tryget_noerror(struct bch_fs *c, unsigned dev)
+{
+	rcu_read_lock();
+	struct bch_dev *ca = bch2_dev_rcu(c, dev);
+	if (ca)
+		bch2_dev_get(ca);
+	rcu_read_unlock();
+	return ca;
+}
+
+void bch2_dev_missing(struct bch_fs *, unsigned);
+
+static inline struct bch_dev *bch2_dev_tryget(struct bch_fs *c, unsigned dev)
+{
+	struct bch_dev *ca = bch2_dev_tryget_noerror(c, dev);
+	if (!ca)
+		bch2_dev_missing(c, dev);
+	return ca;
+}
+
 /* XXX kill, move to struct bch_fs */
 static inline struct bch_devs_mask bch2_online_devs(struct bch_fs *c)
 {