[RFC RT v5.10] [rt] repair usage of raw_v6_hashinfo.lock in raw_seq_start()

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

Avoid a possible circular locking dependency by taking the softirq_ctrl.lock
before taking raw_v6_hashinfo.lock in raw_seq_start(), keeping locking order
consistent.

Lockdep splat:

[  254.623863] ======================================================
[  254.623864] WARNING: possible circular locking dependency detected
[  254.623865] 5.10.65-rt53+ #1 Tainted: G S      W I      
[  254.623866] ------------------------------------------------------
[  254.623866] read_all/3963 is trying to acquire lock:
[  254.623867] ffffa05cde218220 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip+0x116/0x2f0
[  254.623876] 
               but task is already holding lock:
[  254.623877] ffffffffb68e93e0 (raw_v6_hashinfo.lock){++.+}-{0:0}, at: raw_seq_start+0x25/0x60
[  254.623883] 
               which lock already depends on the new lock.
...
[  254.623974]  Possible unsafe locking scenario:

[  254.623975]        CPU0                    CPU1
[  254.623975]        ----                    ----
[  254.623975]   lock(raw_v6_hashinfo.lock);
[  254.623976]                                lock((softirq_ctrl.lock).lock);
[  254.623977]                                lock(raw_v6_hashinfo.lock);
[  254.623978]   lock((softirq_ctrl.lock).lock);
[  254.623978] 
                *** DEADLOCK ***

[  254.623979] 3 locks held by read_all/3963:
[  254.623980]  #0: ffffa055d0c35990 (&p->lock){+.+.}-{0:0}, at: seq_read_iter+0x56/0x420
[  254.623983]  #1: ffffffffb68e93e0 (raw_v6_hashinfo.lock){++.+}-{0:0}, at: raw_seq_start+0x25/0x60
[  254.623986]  #2: ffffffffb64c8fa0 (rcu_read_lock){....}-{1:2}, at: rt_read_lock+0x7d/0x1e0


Reported-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---
 net/ipv4/raw.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
index a0188eb2689f..584d08679e3a 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -1051,7 +1051,7 @@ void *raw_seq_start(struct seq_file *seq, loff_t *pos)
 {
 	struct raw_hashinfo *h = PDE_DATA(file_inode(seq->file));
 
-	read_lock(&h->lock);
+	read_lock_bh(&h->lock);
 	return *pos ? raw_get_idx(seq, *pos - 1) : SEQ_START_TOKEN;
 }
 EXPORT_SYMBOL_GPL(raw_seq_start);
@@ -1073,7 +1073,7 @@ void raw_seq_stop(struct seq_file *seq, void *v)
 {
 	struct raw_hashinfo *h = PDE_DATA(file_inode(seq->file));
 
-	read_unlock(&h->lock);
+	read_unlock_bh(&h->lock);
 }
 EXPORT_SYMBOL_GPL(raw_seq_stop);