bcachefs: Assorted preemption fixes

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

fs/bcachefs/alloc_background.c

@@ -291,8 +291,10 @@ int bch2_alloc_read(struct bch_fs *c, struct list_head *journal_replay_list)
 				bch2_alloc_read_key(c, bkey_i_to_s_c(k));
 	}
 
+	percpu_down_write(&c->mark_lock);
 	for_each_member_device(ca, c, i)
 		bch2_dev_usage_from_buckets(c, ca);
+	percpu_up_write(&c->mark_lock);
 
 	mutex_lock(&c->bucket_clock[READ].lock);
 	for_each_member_device(ca, c, i) {

fs/bcachefs/btree_gc.c

@@ -354,8 +354,6 @@ void bch2_mark_dev_superblock(struct bch_fs *c, struct bch_dev *ca,
 	if (c) {
 		lockdep_assert_held(&c->sb_lock);
 		percpu_down_read(&c->mark_lock);
-	} else {
-		preempt_disable();
 	}
 
 	for (i = 0; i < layout->nr_superblocks; i++) {
@@ -377,11 +375,8 @@ void bch2_mark_dev_superblock(struct bch_fs *c, struct bch_dev *ca,
 					  gc_phase(GC_PHASE_SB), flags);
 	}
 
-	if (c) {
+	if (c)
 		percpu_up_read(&c->mark_lock);
-	} else {
-		preempt_enable();
-	}
 }
 
 static void bch2_mark_superblocks(struct bch_fs *c)

fs/bcachefs/buckets.c

@@ -393,14 +393,19 @@ void bch2_dev_usage_from_buckets(struct bch_fs *c, struct bch_dev *ca)
 	struct bucket_array *buckets;
 	struct bucket *g;
 
-	percpu_down_read(&c->mark_lock);
+	/*
+	 * This is only called during startup, before there's any multithreaded
+	 * access to c->usage:
+	 */
+	preempt_disable();
 	fs_usage = this_cpu_ptr(c->usage[0]);
+	preempt_enable();
+
 	buckets = bucket_array(ca);
 
 	for_each_bucket(g, buckets)
 		if (g->mark.data_type)
 			bch2_dev_usage_update(c, ca, fs_usage, old, g->mark, false);
-	percpu_up_read(&c->mark_lock);
 }
 
 #define bucket_data_cmpxchg(c, ca, fs_usage, g, new, expr)	\
@@ -513,8 +518,12 @@ void bch2_mark_alloc_bucket(struct bch_fs *c, struct bch_dev *ca,
 			    size_t b, bool owned_by_allocator,
 			    struct gc_pos pos, unsigned flags)
 {
+	preempt_disable();
+
 	do_mark_fn(__bch2_mark_alloc_bucket, c, pos, flags,
 		   ca, b, owned_by_allocator);
+
+	preempt_enable();
 }
 
 static int bch2_mark_alloc(struct bch_fs *c, struct bkey_s_c k,

fs/bcachefs/journal.c

@@ -835,8 +835,6 @@ static int __bch2_set_nr_journal_buckets(struct bch_dev *ca, unsigned nr,
 		if (c) {
 			percpu_down_read(&c->mark_lock);
 			spin_lock(&c->journal.lock);
-		} else {
-			preempt_disable();
 		}
 
 		pos = ja->nr ? (ja->cur_idx + 1) % ja->nr : 0;
@@ -866,8 +864,6 @@ static int __bch2_set_nr_journal_buckets(struct bch_dev *ca, unsigned nr,
 		if (c) {
 			spin_unlock(&c->journal.lock);
 			percpu_up_read(&c->mark_lock);
-		} else {
-			preempt_enable();
 		}
 
 		if (!new_fs)