linux-kernel - [PATCH RT 1/6] net: Properly annotate the try-lock for the seqlock

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Date:   Fri, 06 Nov 2020 21:06:37 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        Mike Galbraith <efault@....de>, stable-rt@...r.kernel.org
Subject: [PATCH RT 1/6] net: Properly annotate the try-lock for the seqlock

5.4.74-rt42-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

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

In patch
   ("net/Qdisc: use a seqlock instead seqcount")

the seqcount has been replaced with a seqlock to allow to reader to
boost the preempted writer.
The try_write_seqlock() acquired the lock with a try-lock but the
seqcount annotation was "lock".

Opencode write_seqcount_t_begin() and use the try-lock annotation for
lockdep.

Reported-by: Mike Galbraith <efault@....de>
Cc: stable-rt@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 include/linux/seqlock.h   |  9 ---------
 include/net/sch_generic.h | 10 +++++++++-
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index e5207897c33e..f390293974ea 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -489,15 +489,6 @@ static inline void write_seqlock(seqlock_t *sl)
 	__raw_write_seqcount_begin(&sl->seqcount);
 }
 
-static inline int try_write_seqlock(seqlock_t *sl)
-{
-	if (spin_trylock(&sl->lock)) {
-		__raw_write_seqcount_begin(&sl->seqcount);
-		return 1;
-	}
-	return 0;
-}
-
 static inline void write_sequnlock(seqlock_t *sl)
 {
 	__raw_write_seqcount_end(&sl->seqcount);
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index e6afb4b9cede..112d2dca8b08 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -168,8 +168,16 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 		return false;
 	}
 #ifdef CONFIG_PREEMPT_RT
-	if (try_write_seqlock(&qdisc->running))
+	if (spin_trylock(&qdisc->running.lock)) {
+		seqcount_t *s = &qdisc->running.seqcount;
+		/*
+		 * Variant of write_seqcount_t_begin() telling lockdep that a
+		 * trylock was attempted.
+		 */
+		__raw_write_seqcount_begin(s);
+		seqcount_acquire(&s->dep_map, 0, 1, _RET_IP_);
 		return true;
+	}
 	return false;
 #else
 	/* Variant of write_seqcount_begin() telling lockdep a trylock
-- 
2.28.0