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From:   "Guilherme G. Piccoli" <gpiccoli@igalia.com>
To:     akpm@linux-foundation.org, bhe@redhat.com, pmladek@suse.com,
        kexec@lists.infradead.org
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        paulmck@kernel.org, peterz@infradead.org, rostedt@goodmis.org,
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        tglx@linutronix.de, vgoyal@redhat.com, vkuznets@redhat.com,
        will@kernel.org, "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Sergei Shtylyov <sergei.shtylyov@gmail.com>
Subject: [PATCH v2 08/13] tracing: Improve panic/die notifiers
Date:   Tue, 19 Jul 2022 16:53:21 -0300
Message-Id: <20220719195325.402745-9-gpiccoli@igalia.com>
In-Reply-To: <20220719195325.402745-1-gpiccoli@igalia.com>
References: <20220719195325.402745-1-gpiccoli@igalia.com>
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Currently the tracing dump_on_oops feature is implemented
through separate notifiers, one for die/oops and the other
for panic - given they have the same functionality, let's
unify them.

Also improve the function comment and change the priority of
the notifier to make it execute earlier, avoiding showing useless
trace data (like the callback names for the other notifiers);
finally, we also removed an unnecessary header inclusion.

Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

---

V2:
- Different approach; instead of using IDs to distinguish die and
panic events, rely on address comparison like other notifiers do
and as per Petr's suggestion;

- Removed ACK from Steven since the code changed.

 kernel/trace/trace.c | 55 ++++++++++++++++++++++----------------------
 1 file changed, 27 insertions(+), 28 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b8dd54627075..2a436b645c70 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -19,7 +19,6 @@
 #include <linux/kallsyms.h>
 #include <linux/security.h>
 #include <linux/seq_file.h>
-#include <linux/notifier.h>
 #include <linux/irqflags.h>
 #include <linux/debugfs.h>
 #include <linux/tracefs.h>
@@ -9777,40 +9776,40 @@ static __init int tracer_init_tracefs(void)
 
 fs_initcall(tracer_init_tracefs);
 
-static int trace_panic_handler(struct notifier_block *this,
-			       unsigned long event, void *unused)
-{
-	if (ftrace_dump_on_oops)
-		ftrace_dump(ftrace_dump_on_oops);
-	return NOTIFY_OK;
-}
+static int trace_die_panic_handler(struct notifier_block *self,
+				unsigned long ev, void *unused);
 
 static struct notifier_block trace_panic_notifier = {
-	.notifier_call  = trace_panic_handler,
-	.next           = NULL,
-	.priority       = 150   /* priority: INT_MAX >= x >= 0 */
+	.notifier_call = trace_die_panic_handler,
+	.priority = INT_MAX - 1,
 };
 
-static int trace_die_handler(struct notifier_block *self,
-			     unsigned long val,
-			     void *data)
-{
-	switch (val) {
-	case DIE_OOPS:
-		if (ftrace_dump_on_oops)
-			ftrace_dump(ftrace_dump_on_oops);
-		break;
-	default:
-		break;
-	}
-	return NOTIFY_OK;
-}
-
 static struct notifier_block trace_die_notifier = {
-	.notifier_call = trace_die_handler,
-	.priority = 200
+	.notifier_call = trace_die_panic_handler,
+	.priority = INT_MAX - 1,
 };
 
+/*
+ * The idea is to execute the following die/panic callback early, in order
+ * to avoid showing irrelevant information in the trace (like other panic
+ * notifier functions); we are the 2nd to run, after hung_task/rcu_stall
+ * warnings get disabled (to prevent potential log flooding).
+ */
+static int trace_die_panic_handler(struct notifier_block *self,
+				unsigned long ev, void *unused)
+{
+	if (!ftrace_dump_on_oops)
+		goto out;
+
+	if (self == &trace_die_notifier && ev != DIE_OOPS)
+		goto out;
+
+	ftrace_dump(ftrace_dump_on_oops);
+
+out:
+	return NOTIFY_DONE;
+}
+
 /*
  * printk is set to max of 1024, we really don't need it that big.
  * Nothing should be printing 1000 characters anyway.
-- 
2.37.1