Re: [syzbot] [kernel?] INFO: task hung in restrict_one_thread_callback

[Hillf Danton]

On Fri, Feb 20, 2026 at 03:11:21AM -0800
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    635c467cc14e Add linux-next specific files for 20260213
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1452f6e6580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=61690c38d1398936
> dashboard link: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16e41c02580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15813652580000

#syz test

--- x/security/landlock/tsync.c
+++ y/security/landlock/tsync.c
@@ -540,11 +540,8 @@ int landlock_restrict_sibling_threads(co
 		 * of for_each_thread().  We can reset it on each loop iteration because
 		 * all previous loop iterations are done with it already.
 		 *
-		 * num_preparing is initialized to 1 so that the counter can not go to 0
-		 * and mark the completion as done before all task works are registered.
-		 * We decrement it at the end of the loop body.
 		 */
-		atomic_set(&shared_ctx.num_preparing, 1);
+		atomic_set(&shared_ctx.num_preparing, 0);
 		reinit_completion(&shared_ctx.all_prepared);
 
 		/*
@@ -553,11 +550,7 @@ int landlock_restrict_sibling_threads(co
 		 */
 		found_more_threads = schedule_task_work(&works, &shared_ctx);
 
-		/*
-		 * Decrement num_preparing for current, to undo that we initialized it
-		 * to 1 a few lines above.
-		 */
-		if (atomic_dec_return(&shared_ctx.num_preparing) > 0) {
+		if (atomic_read(&shared_ctx.num_preparing) > 0) {
 			if (wait_for_completion_interruptible(
 				    &shared_ctx.all_prepared)) {
 				/* In case of interruption, we need to retry the system call. */
--