btrfs: fix assertion failure and blocking during nowait buffered write

When doing a nowait buffered write we can trigger the following assertion:

[11138.437027] assertion failed: !path->nowait, in fs/btrfs/ctree.c:4658
[11138.438251] ------------[ cut here ]------------
[11138.438254] kernel BUG at fs/btrfs/messages.c:259!
[11138.438762] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
[11138.439450] CPU: 4 PID: 1091021 Comm: fsstress Not tainted 6.1.0-rc4-btrfs-next-128 #1
[11138.440611] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[11138.442553] RIP: 0010:btrfs_assertfail+0x19/0x1b [btrfs]
[11138.443583] Code: 5b 41 5a 41 (...)
[11138.446437] RSP: 0018:ffffbaf0cf05b840 EFLAGS: 00010246
[11138.447235] RAX: 0000000000000039 RBX: ffffbaf0cf05b938 RCX: 0000000000000000
[11138.448303] RDX: 0000000000000000 RSI: ffffffffb2ef59f6 RDI: 00000000ffffffff
[11138.449370] RBP: ffff9165f581eb68 R08: 00000000ffffffff R09: 0000000000000001
[11138.450493] R10: ffff9167a88421f8 R11: 0000000000000000 R12: ffff9164981b1000
[11138.451661] R13: 000000008c8f1000 R14: ffff9164991d4000 R15: ffff9164981b1000
[11138.452225] FS:  00007f1438a66440(0000) GS:ffff9167ad600000(0000) knlGS:0000000000000000
[11138.452949] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[11138.453394] CR2: 00007f1438a64000 CR3: 0000000100c36002 CR4: 0000000000370ee0
[11138.454057] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[11138.454879] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[11138.455779] Call Trace:
[11138.456211]  <TASK>
[11138.456598]  btrfs_next_old_leaf.cold+0x18/0x1d [btrfs]
[11138.457827]  ? kmem_cache_alloc+0x18d/0x2a0
[11138.458516]  btrfs_lookup_csums_range+0x149/0x4d0 [btrfs]
[11138.459407]  csum_exist_in_range+0x56/0x110 [btrfs]
[11138.460271]  can_nocow_file_extent+0x27c/0x310 [btrfs]
[11138.461155]  can_nocow_extent+0x1ec/0x2e0 [btrfs]
[11138.461672]  btrfs_check_nocow_lock+0x114/0x1c0 [btrfs]
[11138.462951]  btrfs_buffered_write+0x44c/0x8e0 [btrfs]
[11138.463482]  btrfs_do_write_iter+0x42b/0x5f0 [btrfs]
[11138.463982]  ? lock_release+0x153/0x4a0
[11138.464347]  io_write+0x11b/0x570
[11138.464660]  ? lock_release+0x153/0x4a0
[11138.465213]  ? lock_is_held_type+0xe8/0x140
[11138.466003]  io_issue_sqe+0x63/0x4a0
[11138.466339]  io_submit_sqes+0x238/0x770
[11138.466741]  __do_sys_io_uring_enter+0x37b/0xb10
[11138.467206]  ? lock_is_held_type+0xe8/0x140
[11138.467879]  ? syscall_enter_from_user_mode+0x1d/0x50
[11138.468688]  do_syscall_64+0x38/0x90
[11138.469265]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[11138.470017] RIP: 0033:0x7f1438c539e6

This is because to check if we can NOCOW, we check that if we can NOCOW
into an extent (it's prealloc extent or the inode has NOCOW attribute),
and then check if there are csums for the extent's range in the csum tree.
The search may leave us beyond the last slot of a leaf, and then when
we call btrfs_next_leaf() we end up at btrfs_next_old_leaf() with a
time_seq of 0.

This triggers a failure of the first assertion at btrfs_next_old_leaf(),
since we have a nowait path. With assertions disabled, we simply don't
respect the NOWAIT semantics, allowing the write to block on locks or
blocking on IO for reading an extent buffer from disk.

Fix this by:

1) Triggering the assertion only if time_seq is not 0, which means that
   search is being done by a tree mod log user, and in the buffered and
   direct IO write paths we don't use the tree mod log;

2) Implementing NOWAIT semantics at btrfs_next_old_leaf(). Any failure to
   lock an extent buffer should return immediately and not retry the
   search, as well as if we need to do IO to read an extent buffer from
   disk.

Fixes: c922b016f3 ("btrfs: assert nowait mode is not used for some btree search functions")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -4663,7 +4663,12 @@ int btrfs_next_old_leaf(struct btrfs_root *root, struct btrfs_path *path,
 	int ret;
 	int i;
 
-	ASSERT(!path->nowait);
+	/*
+	 * The nowait semantics are used only for write paths, where we don't
+	 * use the tree mod log and sequence numbers.
+	 */
+	if (time_seq)
+		ASSERT(!path->nowait);
 
 	nritems = btrfs_header_nritems(path->nodes[0]);
 	if (nritems == 0)
@@ -4683,7 +4688,14 @@ again:
 		if (path->need_commit_sem) {
 			path->need_commit_sem = 0;
 			need_commit_sem = true;
-			down_read(&fs_info->commit_root_sem);
+			if (path->nowait) {
+				if (!down_read_trylock(&fs_info->commit_root_sem)) {
+					ret = -EAGAIN;
+					goto done;
+				}
+			} else {
+				down_read(&fs_info->commit_root_sem);
+			}
 		}
 		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
 	}
@@ -4759,7 +4771,7 @@ again:
 		next = c;
 		ret = read_block_for_search(root, path, &next, level,
 					    slot, &key);
-		if (ret == -EAGAIN)
+		if (ret == -EAGAIN && !path->nowait)
 			goto again;
 
 		if (ret < 0) {
@@ -4769,6 +4781,10 @@ again:
 
 		if (!path->skip_locking) {
 			ret = btrfs_try_tree_read_lock(next);
+			if (!ret && path->nowait) {
+				ret = -EAGAIN;
+				goto done;
+			}
 			if (!ret && time_seq) {
 				/*
 				 * If we don't get the lock, we may be racing
@@ -4799,7 +4815,7 @@ again:
 
 		ret = read_block_for_search(root, path, &next, level,
 					    0, &key);
-		if (ret == -EAGAIN)
+		if (ret == -EAGAIN && !path->nowait)
 			goto again;
 
 		if (ret < 0) {
@@ -4807,8 +4823,16 @@ again:
 			goto done;
 		}
 
-		if (!path->skip_locking)
-			btrfs_tree_read_lock(next);
+		if (!path->skip_locking) {
+			if (path->nowait) {
+				if (!btrfs_try_tree_read_lock(next)) {
+					ret = -EAGAIN;
+					goto done;
+				}
+			} else {
+				btrfs_tree_read_lock(next);
+			}
+		}
 	}
 	ret = 0;
 done: