linux-kernel - [PATCH v4 1/1] blktrace: switch trace spinlock to a raw spinlock

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Message-Id: <20211220130357.8790-2-wander@redhat.com>
Date:   Mon, 20 Dec 2021 10:03:57 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-block@...r.kernel.org (open list:BLOCK LAYER)
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Wander Lairson Costa <wander@...hat.com>
Subject: [PATCH v4 1/1] blktrace: switch trace spinlock to a raw spinlock

The running_trace_lock protects running_trace_list and is acquired
within the tracepoint which implies disabled preemption. The spinlock_t
typed lock can not be acquired with disabled preemption on PREEMPT_RT
because it becomes a sleeping lock.
The runtime of the tracepoint depends on the number of entries in
running_trace_list and has no limit. The blk-tracer is considered debug
code and higher latencies here are okay.

Make running_trace_lock a raw_spinlock_t.

Signed-off-by: Wander Lairson Costa <wander@...hat.com>
---
 kernel/trace/blktrace.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index c221e4c3f625..0ce1fadce078 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -34,7 +34,7 @@ static struct trace_array *blk_tr;
 static bool blk_tracer_enabled __read_mostly;
 
 static LIST_HEAD(running_trace_list);
-static __cacheline_aligned_in_smp DEFINE_SPINLOCK(running_trace_lock);
+static __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(running_trace_lock);
 
 /* Select an alternative, minimalistic output than the original one */
 #define TRACE_BLK_OPT_CLASSIC	0x1
@@ -121,12 +121,12 @@ static void trace_note_tsk(struct task_struct *tsk)
 	struct blk_trace *bt;
 
 	tsk->btrace_seq = blktrace_seq;
-	spin_lock_irqsave(&running_trace_lock, flags);
+	raw_spin_lock_irqsave(&running_trace_lock, flags);
 	list_for_each_entry(bt, &running_trace_list, running_list) {
 		trace_note(bt, tsk->pid, BLK_TN_PROCESS, tsk->comm,
 			   sizeof(tsk->comm), 0);
 	}
-	spin_unlock_irqrestore(&running_trace_lock, flags);
+	raw_spin_unlock_irqrestore(&running_trace_lock, flags);
 }
 
 static void trace_note_time(struct blk_trace *bt)
@@ -666,9 +666,9 @@ static int __blk_trace_startstop(struct request_queue *q, int start)
 			blktrace_seq++;
 			smp_mb();
 			bt->trace_state = Blktrace_running;
-			spin_lock_irq(&running_trace_lock);
+			raw_spin_lock_irq(&running_trace_lock);
 			list_add(&bt->running_list, &running_trace_list);
-			spin_unlock_irq(&running_trace_lock);
+			raw_spin_unlock_irq(&running_trace_lock);
 
 			trace_note_time(bt);
 			ret = 0;
@@ -676,9 +676,9 @@ static int __blk_trace_startstop(struct request_queue *q, int start)
 	} else {
 		if (bt->trace_state == Blktrace_running) {
 			bt->trace_state = Blktrace_stopped;
-			spin_lock_irq(&running_trace_lock);
+			raw_spin_lock_irq(&running_trace_lock);
 			list_del_init(&bt->running_list);
-			spin_unlock_irq(&running_trace_lock);
+			raw_spin_unlock_irq(&running_trace_lock);
 			relay_flush(bt->rchan);
 			ret = 0;
 		}
-- 
2.27.0