pstore/ftrace: Convert to its own enable/disable debugfs knob

With this patch we no longer reuse function tracer infrastructure, now
we register our own tracer back-end via a debugfs knob.

It's a bit more code, but that is the only downside. On the bright side we
have:

- Ability to make persistent_ram module removable (when needed, we can
move ftrace_ops struct into a module). Note that persistent_ram is still
not removable for other reasons, but with this patch it's just one
thing less to worry about;

- Pstore part is more isolated from the generic function tracer. We tried
it already by registering our own tracer in available_tracers, but that
way we're loosing ability to see the traces while we record them to
pstore. This solution is somewhere in the middle: we only register
"internal ftracer" back-end, but not the "front-end";

- When there is only pstore tracing enabled, the kernel will only write
to the pstore buffer, omitting function tracer buffer (which, of course,
still can be enabled via 'echo function > current_tracer').

Suggested-by: Steven Rostedt <[e-mail]>
Signed-off-by: Anton Vorontsov <[e-mail]>

Documentation/ramoops.txt

···
 file. Here is an example of usage:
 
  # mount -t debugfs debugfs /sys/kernel/debug/
- # cd /sys/kernel/debug/tracing
- # echo function > current_tracer
- # echo 1 > options/func_pstore
+ # echo 1 > /sys/kernel/debug/pstore/record_ftrace
  # reboot -f
  [...]
  # mount -t pstore pstore /mnt/

fs/pstore/Kconfig

···
 	bool "Persistent function tracer"
 	depends on PSTORE
 	depends on FUNCTION_TRACER
+	depends on DEBUG_FS
 	help
 	  With this option kernel traces function calls into a persistent
 	  ram buffer that can be decoded and dumped after reboot through

fs/pstore/ftrace.c

···
 #include <linux/percpu.h>
 #include <linux/smp.h>
 #include <linux/atomic.h>
+#include <linux/types.h>
+#include <linux/mutex.h>
+#include <linux/ftrace.h>
+#include <linux/fs.h>
+#include <linux/debugfs.h>
+#include <linux/err.h>
+#include <linux/cache.h>
 #include <asm/barrier.h>
 #include "internal.h"
 
-void notrace pstore_ftrace_call(unsigned long ip, unsigned long parent_ip)
+static void notrace pstore_ftrace_call(unsigned long ip,
+				       unsigned long parent_ip)
 {
+	unsigned long flags;
 	struct pstore_ftrace_record rec = {};
 
 	if (unlikely(oops_in_progress))
 		return;
+
+	local_irq_save(flags);
 
 	rec.ip = ip;
 	rec.parent_ip = parent_ip;
 	pstore_ftrace_encode_cpu(&rec, raw_smp_processor_id());
 	psinfo->write_buf(PSTORE_TYPE_FTRACE, 0, NULL, 0, (void *)&rec,
 			  sizeof(rec), psinfo);
+
+	local_irq_restore(flags);
+}
+
+static struct ftrace_ops pstore_ftrace_ops __read_mostly = {
+	.func	= pstore_ftrace_call,
+};
+
+static DEFINE_MUTEX(pstore_ftrace_lock);
+static bool pstore_ftrace_enabled;
+
+static ssize_t pstore_ftrace_knob_write(struct file *f, const char __user *buf,
+					size_t count, loff_t *ppos)
+{
+	u8 on;
+	ssize_t ret;
+
+	ret = kstrtou8_from_user(buf, count, 2, &on);
+	if (ret)
+		return ret;
+
+	mutex_lock(&pstore_ftrace_lock);
+
+	if (!on ^ pstore_ftrace_enabled)
+		goto out;
+
+	if (on)
+		ret = register_ftrace_function(&pstore_ftrace_ops);
+	else
+		ret = unregister_ftrace_function(&pstore_ftrace_ops);
+	if (ret) {
+		pr_err("%s: unable to %sregister ftrace ops: %zd\n",
+		       __func__, on ? "" : "un", ret);
+		goto err;
+	}
+
+	pstore_ftrace_enabled = on;
+out:
+	ret = count;
+err:
+	mutex_unlock(&pstore_ftrace_lock);
+
+	return ret;
+}
+
+static ssize_t pstore_ftrace_knob_read(struct file *f, char __user *buf,
+				       size_t count, loff_t *ppos)
+{
+	char val[] = { '0' + pstore_ftrace_enabled, '\n' };
+
+	return simple_read_from_buffer(buf, count, ppos, val, sizeof(val));
+}
+
+static const struct file_operations pstore_knob_fops = {
+	.open	= simple_open,
+	.read	= pstore_ftrace_knob_read,
+	.write	= pstore_ftrace_knob_write,
+};
+
+void pstore_register_ftrace(void)
+{
+	struct dentry *dir;
+	struct dentry *file;
+
+	if (!psinfo->write_buf)
+		return;
+
+	dir = debugfs_create_dir("pstore", NULL);
+	if (!dir) {
+		pr_err("%s: unable to create pstore directory\n", __func__);
+		return;
+	}
+
+	file = debugfs_create_file("record_ftrace", 0600, dir, NULL,
+				   &pstore_knob_fops);
+	if (!file) {
+		pr_err("%s: unable to create record_ftrace file\n", __func__);
+		goto err_file;
+	}
+
+	return;
+err_file:
+	debugfs_remove(dir);
 }

fs/pstore/internal.h

···
 #endif
 }
 
+#ifdef CONFIG_PSTORE_FTRACE
+extern void pstore_register_ftrace(void);
+#else
+static inline void pstore_register_ftrace(void) {}
+#endif
+
 extern struct pstore_info *psinfo;
 
 extern void	pstore_set_kmsg_bytes(int);

fs/pstore/platform.c

···
 
 	kmsg_dump_register(&pstore_dumper);
 	pstore_register_console();
+	pstore_register_ftrace();
 
 	if (pstore_update_ms >= 0) {
 		pstore_timer.expires = jiffies +

include/linux/pstore.h

···
 	void		*data;
 };
 
-
-#ifdef CONFIG_PSTORE_FTRACE
-extern void pstore_ftrace_call(unsigned long ip, unsigned long parent_ip);
-#else
-static inline void pstore_ftrace_call(unsigned long ip, unsigned long parent_ip)
-{ }
-#endif
-
 #ifdef CONFIG_PSTORE
 extern int pstore_register(struct pstore_info *);
 #else

kernel/trace/trace_functions.c

···
 #include <linux/debugfs.h>
 #include <linux/uaccess.h>
 #include <linux/ftrace.h>
-#include <linux/pstore.h>
 #include <linux/fs.h>
 
 #include "trace.h"
···
 	preempt_enable_notrace();
 }
 
-/* Our two options */
+/* Our option */
 enum {
 	TRACE_FUNC_OPT_STACK	= 0x1,
-	TRACE_FUNC_OPT_PSTORE	= 0x2,
 };
 
 static struct tracer_flags func_flags;
···
 	disabled = atomic_inc_return(&data->disabled);
 
 	if (likely(disabled == 1)) {
-		/*
-		 * So far tracing doesn't support multiple buffers, so
-		 * we make an explicit call for now.
-		 */
-		if (unlikely(func_flags.val & TRACE_FUNC_OPT_PSTORE))
-			pstore_ftrace_call(ip, parent_ip);
 		pc = preempt_count();
 		trace_function(tr, ip, parent_ip, flags, pc);
 	}
···
 #ifdef CONFIG_STACKTRACE
 	{ TRACER_OPT(func_stack_trace, TRACE_FUNC_OPT_STACK) },
 #endif
-#ifdef CONFIG_PSTORE_FTRACE
-	{ TRACER_OPT(func_pstore, TRACE_FUNC_OPT_PSTORE) },
-#endif
 	{ } /* Always set a last empty entry */
 };
 
···
 			register_ftrace_function(&trace_ops);
 		}
 
-		break;
-	case TRACE_FUNC_OPT_PSTORE:
 		break;
 	default:
 		return -EINVAL;