linux-kernel - Re: [syzbot] WARNING: locking bug in umh

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Date:   Fri, 3 Feb 2023 19:48:35 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     linux-kernel@...r.kernel.org, mcgrof@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+6cd18e123583550cf469@...kaller.appspotmail.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [syzbot] WARNING: locking bug in umh_complete

On 2023/02/03 19:22, Tetsuo Handa wrote:
> Yes, this bug is caused by commit f5d39b020809 ("freezer,sched: Rewrite core freezer
> logic"), for that commit for unknown reason omits wait_for_completion(&done) call
> when wait_for_completion_state(&done, state) returned -ERESTARTSYS.
> 
> Peter, is it safe to restore wait_for_completion(&done) call?
> 

Something like this?

diff --git a/kernel/umh.c b/kernel/umh.c
index 850631518665..97230edb1849 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -441,8 +441,8 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	if (wait & UMH_KILLABLE)
 		state |= TASK_KILLABLE;
 
-	if (wait & UMH_FREEZABLE)
-		state |= TASK_FREEZABLE;
+	//if (wait & UMH_FREEZABLE)
+	//	state |= TASK_FREEZABLE;
 
 	retval = wait_for_completion_state(&done, state);
 	if (!retval)
@@ -452,7 +452,9 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 		/* umh_complete() will see NULL and free sub_info */
 		if (xchg(&sub_info->complete, NULL))
 			goto unlock;
+		/* fallthrough, umh_complete() was already called */
 	}
+	wait_for_completion(&done);
 
 wait_done:
 	retval = sub_info->retval;

How does TASK_FREEZABLE affect here? Since call_usermodehelper_exec() is a function
for starting and waiting for termination of a userspace process (which is subjected
to freezing), the caller of call_usermodehelper_exec() can't wait for the termination
of that userspace process if that process was frozen, and wait_for_completion()
blocks forever?