Linux 5.5 compat: blkg_tryget()

Commit torvalds/linux@9e8d42a0f accidentally
converted the static inline function blkg_tryget() to GPL-only for
kernels built with CONFIG_PREEMPT_RCU=y and CONFIG_BLK_CGROUP=y.

Resolve the build issue by providing our own equivalent functionality
when needed which uses rcu_read_lock_sched() internally as before.

Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#9745
Closes openzfs#10072
Signed-off-by: Pavel Snajdr <[email protected]>

module/os/linux/zfs/vdev_disk.c

@@ -530,6 +530,36 @@ vdev_submit_bio_impl(struct bio *bio)
 
 #ifdef HAVE_BIO_SET_DEV
 #if defined(CONFIG_BLK_CGROUP) && defined(HAVE_BIO_SET_DEV_GPL_ONLY)
+/*
+ * The Linux 5.5 kernel updated percpu_ref_tryget() which is inlined by
+ * blkg_tryget() to use rcu_read_lock() instead of rcu_read_lock_sched().
+ * As a side effect the function was converted to GPL-only.  Define our
+ * own version when needed which uses rcu_read_lock_sched().
+ */
+#if defined(HAVE_BLKG_TRYGET_GPL_ONLY)
+static inline bool
+vdev_blkg_tryget(struct blkcg_gq *blkg)
+{
+	struct percpu_ref *ref = &blkg->refcnt;
+	unsigned long __percpu *count;
+	bool rc;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &count)) {
+		this_cpu_inc(*count);
+		rc = true;
+	} else {
+		rc = atomic_long_inc_not_zero(&ref->count);
+	}
+
+	rcu_read_unlock_sched();
+
+	return (rc);
+}
+#else
+#define	vdev_blkg_tryget(bg)	blkg_tryget(bg)
+#endif
 /*
  * The Linux 5.0 kernel updated the bio_set_dev() macro so it calls the
  * GPL-only bio_associate_blkg() symbol thus inadvertently converting
@@ -544,7 +574,7 @@ vdev_bio_associate_blkg(struct bio *bio)
 	ASSERT3P(q, !=, NULL);
 	ASSERT3P(bio->bi_blkg, ==, NULL);
 
-	if (blkg_tryget(q->root_blkg))
+	if (q->root_blkg && vdev_blkg_tryget(q->root_blkg))
 		bio->bi_blkg = q->root_blkg;
 }
 #define	bio_associate_blkg vdev_bio_associate_blkg