Ocfs2 devel - Jun 2023 - [PATCH] fs: ocfs: fix potential deadlock on &qs->qs_lock

As &qs->qs_lock is also acquired by the timer o2net_idle_timer()
which executes under softirq context, code executing under process
context should disable irq before acquiring the lock, otherwise
deadlock could happen if the process context hold the lock then
preempt by the timer.

Possible deadlock scenario:
o2quo_make_decision (workqueue)
    -> spin_lock(&qs->qs_lock);
        <timer interrupt>
        -> o2net_idle_timer
        -> o2quo_conn_err
        -> spin_lock(&qs->qs_lock); (deadlock here)

This flaw was found using an experimental static analysis tool we are
developing for irq-related deadlock.

The tentative patch fix the potential deadlock by spin_lock_irqsave().

Signed-off-by: Chengfeng Ye <dg573847474 at gmail.com>
---
 fs/ocfs2/cluster/quorum.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/ocfs2/cluster/quorum.c b/fs/ocfs2/cluster/quorum.c
index 189c111bc371..f14313c3e27e 100644
--- a/fs/ocfs2/cluster/quorum.c
+++ b/fs/ocfs2/cluster/quorum.c
@@ -92,8 +92,9 @@ static void o2quo_make_decision(struct work_struct *work)
 	int quorum;
 	int lowest_hb, lowest_reachable = 0, fence = 0;
 	struct o2quo_state *qs = &o2quo_state;
+	unsigned long flags;
 
-	spin_lock(&qs->qs_lock);
+	spin_lock_irqsave(&qs->qs_lock, flags);
 
 	lowest_hb = find_first_bit(qs->qs_hb_bm, O2NM_MAX_NODES);
 	if (lowest_hb != O2NM_MAX_NODES)
@@ -146,14 +147,14 @@ static void o2quo_make_decision(struct work_struct *work)
 
 out:
 	if (fence) {
-		spin_unlock(&qs->qs_lock);
+		spin_unlock_irqrestore(&qs->qs_lock, flags);
 		o2quo_fence_self();
 	} else {
 		mlog(ML_NOTICE, "not fencing this node, heartbeating: %d, "
 			"connected: %d, lowest: %d (%sreachable)\n",
 			qs->qs_heartbeating, qs->qs_connected, lowest_hb,
 			lowest_reachable ? "" : "un");
-		spin_unlock(&qs->qs_lock);
+		spin_unlock_irqrestore(&qs->qs_lock, flags);
 
 	}
 
-- 
2.17.1