linux-kernel - [PATCH RT 07/12] locking/seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin

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Message-Id: <20231018194833.651674-8-joseph.salisbury@canonical.com>
Date:   Wed, 18 Oct 2023 15:48:28 -0400
From:   Joseph Salisbury <joseph.salisbury@...onical.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <daniel.wagner@...e.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Clark Williams <williams@...hat.com>,
        Pavel Machek <pavel@...x.de>,
        Joseph Salisbury <joseph.salisbury@...onical.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH RT 07/12] locking/seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

v5.15.133-rt70-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


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@...ove.SAKURA.ne.jp>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20230920104627._DTHgPyA@linutronix.de

Closes: https://lore.kernel.org/20230621130641.-5iueY1I@linutronix.de
(cherry picked from commit 41b43b6c6e30a832c790b010a06772e793bca193)
Signed-off-by: Joseph Salisbury <joseph.salisbury@...onical.com>
---
 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 37ded6b8fee6..2c5d0102315d 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -516,8 +516,8 @@ 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);
 }
 
 /**
-- 
2.34.1