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: 8b90a9f819dc ("af_unix: Run GC on only one CPU.")
Reported-by: Igor Ushakov <redacted>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/unix/garbage.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index a7967a345827..0783555e2526 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c

@@ -607,6 +607,8 @@ 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) {

@@ -649,10 +651,8 @@ void unix_schedule_gc(struct user_struct *user)
 	    READ_ONCE(user->unix_inflight) < UNIX_INFLIGHT_SANE_USER)
 		return;
 
-	if (!READ_ONCE(gc_in_progress)) {
-		WRITE_ONCE(gc_in_progress, true);
+	if (!READ_ONCE(gc_in_progress))
 		queue_work(system_dfl_wq, &unix_gc_work);
-	}
 
 	if (user && READ_ONCE(unix_graph_cyclic_sccs))
 		flush_work(&unix_gc_work);

-- 
2.54.0.545.g6539524ca2-goog