af_unix: Set gc_in_progress to true in unix_gc().

[ Upstream commit d82ba05 ]

Igor Ushakov reported that unix_gc() could run with gc_in_progress
being false if the work is scheduled while running:

  Thread 1         Thread 2                     Thread 3
  --------         --------                     --------
                   unix_schedule_gc()           unix_schedule_gc()
                   `- if (!gc_in_progress)      `- if (!gc_in_progress)
                      |- gc_in_progress = true     |
                      `- queue_work()              |
  unix_gc() <----------------/                     |
  |                                                |- gc_in_progress = true
  ...                                              `- queue_work()
  |                                                       |
  `- gc_in_progress = false                               |
                                                          |
  unix_gc() <---------------------------------------------'
  |
  ... /* gc_in_progress == false */
  |
  `- gc_in_progress = false

unix_peek_fpl() relies on gc_in_progress not to confuse GC
by MSG_PEEK.

Let's set gc_in_progress to true in unix_gc().

Fixes: 8b90a9f ("af_unix: Run GC on only one CPU.")
Reported-by: Igor Ushakov <sysroot314@gmail.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260501073945.1884564-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Add setting gc_in_progress in __unix_gc(). Keep the existing
  set in unix_gc() for wait_for_unix_gc() over-limit throttling. ]
Signed-off-by: Igor Ushakov <sysroot314@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: q2ven

net/unix/garbage.c

@@ -606,6 +606,8 @@ static void __unix_gc(struct work_struct *work)
 	struct sk_buff_head hitlist;
 	struct sk_buff *skb;
 
+	WRITE_ONCE(gc_in_progress, true);
+
 	spin_lock(&unix_gc_lock);
 
 	if (unix_graph_state == UNIX_GRAPH_NOT_CYCLIC) {