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 <rostedt@goodmis.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

4 files changed, 103 insertions, 1 deletions

diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index d39bb5cce883..ca71db69da07 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -23,6 +23,7 @@ config PSTORE_FTRACE
         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
diff --git a/fs/pstore/ftrace.c b/fs/pstore/ftrace.c
index a130d484b7d3..2d57e1ac0115 100644
--- a/fs/pstore/ftrace.c
+++ b/fs/pstore/ftrace.c
@@ -17,19 +17,113 @@
 #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);
 }
diff --git a/fs/pstore/internal.h b/fs/pstore/internal.h
index 0d0d3b7d5f12..4847f588b7d5 100644
--- a/fs/pstore/internal.h
+++ b/fs/pstore/internal.h
@@ -39,6 +39,12 @@ pstore_ftrace_decode_cpu(struct pstore_ftrace_record *rec)
 #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);
diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 29996e8793a7..6c23eab7f76c 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -236,6 +236,7 @@ int pstore_register(struct pstore_info *psi)
 
         kmsg_dump_register(&pstore_dumper);
         pstore_register_console();
+        pstore_register_ftrace();
 
         if (pstore_update_ms >= 0) {
                 pstore_timer.expires = jiffies +