[tip: locking/urgent] locking/seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     41b43b6c6e30a832c790b010a06772e793bca193
Gitweb:        https://git.kernel.org/tip/41b43b6c6e30a832c790b010a06772e793bca193
Author:        Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate:    Wed, 20 Sep 2023 12:46:27 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 21 Sep 2023 08:37:44 +02:00

locking/seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()

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>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230920104627._DTHgPyA@xxxxxxxxxxxxx

Closes: https://lore.kernel.org/20230621130641.-5iueY1I@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 987a59d..e9bd2f6 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);
 }
 
 /**