[PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()

[Sebastian Andrzej Siewior]

It was brought up by Tetsuo that the following sequence
   write_seqlock_irqsave()
   printk_deferred_enter()

could lead to a deadlock if the lockdep annotation within
write_seqlock_irqsave() triggers. The problem is that the sequence
counter is incremented before the lockdep annotation is performed. The
lockdep splat would then attempt to invoke printk() but the reader side,
of the same seqcount, could have a tty_port::lock acquired waiting for
the sequence number to become even again.

The other lockdep annotations come before the actual locking because "we
want to see the locking error before it happens". There is no reason why
seqcount should be different here.

Do the lockdep annotation first then perform the locking operation (the
sequence increment).

Fixes: 1ca7d67cf5d5a ("seqcount: Add lockdep functionality to seqcount/seqlock structures")
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/20230621130641.-5iueY1I@xxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 include/linux/seqlock.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 3926e90279477..d778af83c8f36 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -512,8 +512,8 @@ do {									\
 
 static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
 {
-	do_raw_write_seqcount_begin(s);
 	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+	do_raw_write_seqcount_begin(s);
 }
 
 /**
-- 
2.40.1