[PATCH 25/30] panic, printk: Add console flush parameter and convert panic_print to a notifier

["Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx>]

Currently the parameter "panic_print" relies in a function called
directly on panic path; one of the flags the users can set for
panic_print triggers a console replay mechanism, to show the
entire kernel log buffer (from the beginning) in a panic event.

Two problems with that: the dual nature of the panic_print
isn't really appropriate, the function was originally meant
to allow users dumping system information on panic events,
and was "overridden" to also force a console flush of the full
kernel log buffer. It also turns the code a bit more complex
and duplicate than it needs to be.

This patch proposes 2 changes: first, we decouple panic_print
from the console flushing mechanism, in the form of a new kernel
core parameter (panic_console_replay); we kept the functionality
on panic_print to avoid userspace breakage, although we comment
in both code and documentation that this panic_print usage is
deprecated.

We converted panic_print function to a panic notifier too, adding
it on the panic informational notifier list, executed as the final
callback. This allows a more clear code and makes sense, as
panic_print_sys_info() is really a panic-time only function.
We also moved its code to kernel/printk.c, it seems to make more
sense given it's related to printing stuff.

Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         | 12 +++-
 Documentation/admin-guide/sysctl/kernel.rst   |  5 +-
 include/linux/console.h                       |  2 +
 include/linux/panic.h                         |  1 -
 include/linux/printk.h                        |  1 +
 kernel/panic.c                                | 51 +++------------
 kernel/printk/printk.c                        | 62 +++++++++++++++++++
 7 files changed, 87 insertions(+), 47 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 8d3524060ce3..c99da8b2b216 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3791,6 +3791,14 @@
                        timeout < 0: reboot immediately
                        Format: <timeout>
 
+       panic_console_replay
+                       [KNL] Force a kernel log replay in the console on
+                       panic event. Notice that there is already a flush
+                       mechanism for pending messages; this option is meant
+                       for users that wish to replay the *full* buffer.
+                       It deprecates the bit 5 setting on "panic_print",
+                       both having the same functionality.
+
        panic_notifiers_level=
                        [KNL] Set the panic notifiers execution order.
                        Format: <unsigned int>
@@ -3825,12 +3833,14 @@
                        bit 2: print timer info
                        bit 3: print locks info if CONFIG_LOCKDEP is on
                        bit 4: print ftrace buffer
-                       bit 5: print all printk messages in buffer
+                       bit 5: print all printk messages in buffer (DEPRECATED)
                        bit 6: print all CPUs backtrace (if available in the 
arch)
                        *Be aware* that this option may print a _lot_ of lines,
                        so there are risks of losing older messages in the log.
                        Use this option carefully, maybe worth to setup a
                        bigger log buffer with "log_buf_len" along with this.
+                       Also, notice that bit 5 was deprecated in favor of the
+                       parameter "panic_console_replay".
 
        panic_on_taint= Bitmask for conditionally calling panic() in add_taint()
                        Format: <hex>[,nousertaint]
diff --git a/Documentation/admin-guide/sysctl/kernel.rst 
b/Documentation/admin-guide/sysctl/kernel.rst
index 1144ea3229a3..17b293a0e566 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -763,10 +763,13 @@ bit 1  print system memory info
 bit 2  print timer info
 bit 3  print locks info if ``CONFIG_LOCKDEP`` is on
 bit 4  print ftrace buffer
-bit 5  print all printk messages in buffer
+bit 5  print all printk messages in buffer (DEPRECATED)
 bit 6  print all CPUs backtrace (if available in the arch)
 =====  ============================================
 
+Notice that bit 5 was deprecated in favor of kernel core parameter
+"panic_console_replay" (see kernel-parameters.txt documentation).
+
 So for example to print tasks and memory info on panic, user can::
 
   echo 3 > /proc/sys/kernel/panic_print
diff --git a/include/linux/console.h b/include/linux/console.h
index 7cd758a4f44e..351c14f623ad 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -169,6 +169,8 @@ enum con_flush_mode {
        CONSOLE_REPLAY_ALL,
 };
 
+extern bool panic_console_replay;
+
 extern int add_preferred_console(char *name, int idx, char *options);
 extern void register_console(struct console *);
 extern int unregister_console(struct console *);
diff --git a/include/linux/panic.h b/include/linux/panic.h
index f5844908a089..34175d0188d0 100644
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -22,7 +22,6 @@ extern unsigned int sysctl_oops_all_cpu_backtrace;
 #endif /* CONFIG_SMP */
 
 extern int panic_timeout;
-extern unsigned long panic_print;
 extern int panic_on_oops;
 extern int panic_on_unrecovered_nmi;
 extern int panic_on_io_nmi;
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 1522df223c0f..aee2e8ebd541 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -13,6 +13,7 @@
 extern const char linux_banner[];
 extern const char linux_proc_banner[];
 
+extern unsigned long panic_print;
 extern int oops_in_progress;   /* If set, an oops, panic(), BUG() or die() is 
in progress */
 
 #define PRINTK_MAX_SINGLE_HEADER_LEN 2
diff --git a/kernel/panic.c b/kernel/panic.c
index b7c055d4f87f..ff257bd8f81b 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -68,15 +68,6 @@ unsigned int panic_notifiers_level = 2;
 /* DEPRECATED in favor of panic_notifiers_level */
 bool crash_kexec_post_notifiers;
 
-#define PANIC_PRINT_TASK_INFO          0x00000001
-#define PANIC_PRINT_MEM_INFO           0x00000002
-#define PANIC_PRINT_TIMER_INFO         0x00000004
-#define PANIC_PRINT_LOCK_INFO          0x00000008
-#define PANIC_PRINT_FTRACE_INFO                0x00000010
-#define PANIC_PRINT_ALL_PRINTK_MSG     0x00000020
-#define PANIC_PRINT_ALL_CPU_BT         0x00000040
-unsigned long panic_print;
-
 ATOMIC_NOTIFIER_HEAD(panic_hypervisor_list);
 EXPORT_SYMBOL(panic_hypervisor_list);
 
@@ -168,33 +159,6 @@ void nmi_panic(struct pt_regs *regs, const char *msg)
 }
 EXPORT_SYMBOL(nmi_panic);
 
-static void panic_print_sys_info(bool console_flush)
-{
-       if (console_flush) {
-               if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
-                       console_flush_on_panic(CONSOLE_REPLAY_ALL);
-               return;
-       }
-
-       if (panic_print & PANIC_PRINT_ALL_CPU_BT)
-               trigger_all_cpu_backtrace();
-
-       if (panic_print & PANIC_PRINT_TASK_INFO)
-               show_state();
-
-       if (panic_print & PANIC_PRINT_MEM_INFO)
-               show_mem(0, NULL);
-
-       if (panic_print & PANIC_PRINT_TIMER_INFO)
-               sysrq_timer_list_show();
-
-       if (panic_print & PANIC_PRINT_LOCK_INFO)
-               debug_show_all_locks();
-
-       if (panic_print & PANIC_PRINT_FTRACE_INFO)
-               ftrace_dump(DUMP_ALL);
-}
-
 /*
  * Helper that accumulates all console flushing routines executed on panic.
  */
@@ -218,7 +182,11 @@ static void console_flushing(void)
        debug_locks_off();
        console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 
-       panic_print_sys_info(true);
+       /* In case users wish to replay the full log buffer... */
+       if (panic_console_replay) {
+               pr_warn("Replaying the log buffer from the beginning\n");
+               console_flush_on_panic(CONSOLE_REPLAY_ALL);
+       }
 }
 
 #define PN_HYPERVISOR_BIT      0
@@ -431,10 +399,8 @@ void panic(const char *fmt, ...)
        crash_smp_send_stop();
        panic_notifier_hypervisor_once(buf);
 
-       if (panic_notifier_info_once(buf)) {
-               panic_print_sys_info(false);
+       if (panic_notifier_info_once(buf))
                kmsg_dump(KMSG_DUMP_PANIC);
-       }
 
        panic_notifier_pre_reboot_once(buf);
 
@@ -442,10 +408,8 @@ void panic(const char *fmt, ...)
 
        panic_notifier_hypervisor_once(buf);
 
-       if (panic_notifier_info_once(buf)) {
-               panic_print_sys_info(false);
+       if (panic_notifier_info_once(buf))
                kmsg_dump(KMSG_DUMP_PANIC);
-       }
 
        panic_notifier_pre_reboot_once(buf);
 
@@ -814,7 +778,6 @@ EXPORT_SYMBOL(__stack_chk_fail);
 #endif
 
 core_param(panic, panic_timeout, int, 0644);
-core_param(panic_print, panic_print, ulong, 0644);
 core_param(pause_on_oops, pause_on_oops, int, 0644);
 core_param(panic_on_warn, panic_on_warn, int, 0644);
 core_param(panic_notifiers_level, panic_notifiers_level, uint, 0644);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e3a1c429fbbc..ad91d4c04246 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -35,6 +35,7 @@
 #include <linux/memblock.h>
 #include <linux/syscalls.h>
 #include <linux/crash_core.h>
+#include <linux/panic_notifier.h>
 #include <linux/ratelimit.h>
 #include <linux/kmsg_dump.h>
 #include <linux/syslog.h>
@@ -3234,6 +3235,61 @@ void __init console_init(void)
        }
 }
 
+#define PANIC_PRINT_TASK_INFO          0x00000001
+#define PANIC_PRINT_MEM_INFO           0x00000002
+#define PANIC_PRINT_TIMER_INFO         0x00000004
+#define PANIC_PRINT_LOCK_INFO          0x00000008
+#define PANIC_PRINT_FTRACE_INFO                0x00000010
+
+/* DEPRECATED - please use "panic_console_replay" */
+#define PANIC_PRINT_ALL_PRINTK_MSG     0x00000020
+
+#define PANIC_PRINT_ALL_CPU_BT         0x00000040
+
+unsigned long panic_print;
+bool panic_console_replay;
+
+static int panic_print_sys_info(struct notifier_block *self,
+                               unsigned long ev, void *unused)
+{
+       if (panic_print & PANIC_PRINT_ALL_CPU_BT)
+               trigger_all_cpu_backtrace();
+
+       if (panic_print & PANIC_PRINT_TASK_INFO)
+               show_state();
+
+       if (panic_print & PANIC_PRINT_MEM_INFO)
+               show_mem(0, NULL);
+
+       if (panic_print & PANIC_PRINT_TIMER_INFO)
+               sysrq_timer_list_show();
+
+       if (panic_print & PANIC_PRINT_LOCK_INFO)
+               debug_show_all_locks();
+
+       if (panic_print & PANIC_PRINT_FTRACE_INFO)
+               ftrace_dump(DUMP_ALL);
+
+       /*
+        * This is legacy/deprecated feature from panic_print,
+        * the console force flushing. We have now the parameter
+        * "panic_console_replay", but we need to keep the
+        * retro-compatibility with the old stuff...
+        */
+       if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
+               panic_console_replay = true;
+
+       return NOTIFY_DONE;
+}
+
+static struct notifier_block panic_print_nb = {
+       .notifier_call = panic_print_sys_info,
+       .priority = INT_MIN, /* defer to run as late as possible */
+};
+
+core_param(panic_print, panic_print, ulong, 0644);
+core_param(panic_console_replay, panic_console_replay, bool, 0644);
+
 /*
  * Some boot consoles access data that is in the init section and which will
  * be discarded after the initcalls have been run. To make sure that no code
@@ -3253,6 +3309,12 @@ static int __init printk_late_init(void)
        struct console *con;
        int ret;
 
+       /*
+        * Register the panic notifier to print user information
+        * in case the user have that set.
+        */
+       atomic_notifier_chain_register(&panic_info_list, &panic_print_nb);
+
        for_each_console(con) {
                if (!(con->flags & CON_BOOT))
                        continue;
-- 
2.36.0