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Message-ID: <0f8a0451e547f318febcbe36823e1f9914cc684b.camel@sipsolutions.net>
Subject: Re: Kernel deadlock in 6.7.5 + hacks, maybe debugfs related.
From: Johannes Berg <johannes@sipsolutions.net>
To: Ben Greear <greearb@candelatech.com>, linux-wireless
	 <linux-wireless@vger.kernel.org>
Date: Tue, 27 Feb 2024 14:47:57 +0100
In-Reply-To: <95d56b9a-4b91-476f-bab3-7af0f28ff469@candelatech.com>
References: <1c9fa9e5-09f1-0522-fdbc-dbcef4d255ca@candelatech.com>
	 <bdc7c51e3b6afaa7ac79efa15c6d1d41e9e918b5.camel@sipsolutions.net>
	 <95d56b9a-4b91-476f-bab3-7af0f28ff469@candelatech.com>
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> Feb 26 06:01:45 ct523c-0b0b kernel: task:ip              state:D stack:0 =
    pid:28125 tgid:28125 ppid:3604   flags:0x00004002
> Feb 26 06:01:45 ct523c-0b0b kernel: Call Trace:
> Feb 26 06:01:45 ct523c-0b0b kernel:  <TASK>
> Feb 26 06:01:45 ct523c-0b0b kernel:  __schedule+0x42c/0xde0
> Feb 26 06:01:45 ct523c-0b0b kernel:  schedule+0x3c/0x120
> Feb 26 06:01:45 ct523c-0b0b kernel:  schedule_timeout+0x19c/0x1b0
> Feb 26 06:01:45 ct523c-0b0b kernel:  ? mark_held_locks+0x49/0x70
> Feb 26 06:01:45 ct523c-0b0b kernel:  __wait_for_common+0xba/0x1d0
> Feb 26 06:01:45 ct523c-0b0b kernel:  ? usleep_range_state+0xb0/0xb0
> Feb 26 06:01:45 ct523c-0b0b kernel:  remove_one+0x6b/0x100

Can you say where this remove_one+0x6b is?

I feel it's probably this:

       if (!refcount_dec_and_test(&fsd->active_users)) {
               wait_for_completion(&fsd->active_users_drained);

which ... looking at it, seems wrong?

_Completely_ untested:

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 034a617cb1a5..fb636478c54d 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -751,13 +751,19 @@ static void __debugfs_file_removed(struct dentry *den=
try)
 	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
 		return;
=20
-	/* if we hit zero, just wait for all to finish */
-	if (!refcount_dec_and_test(&fsd->active_users)) {
-		wait_for_completion(&fsd->active_users_drained);
-		return;
-	}
+	/*
+	 * Now that debugfs_file_get() no longer sees a valid entry,
+	 * decrement the refcount to remove the initial reference.
+	 */
+	refcount_dec(&fsd->active_users);
=20
-	/* if we didn't hit zero, try to cancel any we can */
+	/*
+	 * As long as it's not zero, try to cancel any cancellations,
+	 * new incoming ones will wake up the completion as we might
+	 * have raced: debugfs_file_get() had already been done, but
+	 * debugfs_enter_cancellation() hadn't, by the time we got
+	 * to this point here.
+	 */
 	while (refcount_read(&fsd->active_users)) {
 		struct debugfs_cancellation *c;
=20


johannes