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    SubjectRe: [PATCH] leds: heartbeat: fix bug on panic
    On Wed, Jul 4, 2012 at 3:11 PM, Alexander Holler <holler@ahsoftware.de> wrote:
    > Am 04.07.2012 09:05, schrieb Bryan Wu:
    >
    >> On Tue, Jul 3, 2012 at 2:35 PM, Alexander Holler <holler@ahsoftware.de>
    >> wrote:
    >>>
    >>> With commit 49dca5aebfdeadd4bf27b6cb4c60392147dc35a4 I introduced
    >>> a bug (visible if CONFIG_PROVE_RCU is enabled) which occures when a panic
    >>> has happened:
    >>>
    >>> [ 1526.520230] ===============================
    >>> [ 1526.520230] [ INFO: suspicious RCU usage. ]
    >>> [ 1526.520230] 3.5.0-rc1+ #12 Not tainted
    >>> [ 1526.520230] -------------------------------
    >>> [ 1526.520230] /c/kernel-tests/mm/include/linux/rcupdate.h:436 Illegal
    >>> context switch in RCU read-side critical section!
    >>> [ 1526.520230]
    >>> [ 1526.520230] other info that might help us debug this:
    >>> [ 1526.520230]
    >>> [ 1526.520230]
    >>> [ 1526.520230] rcu_scheduler_active = 1, debug_locks = 0
    >>> [ 1526.520230] 3 locks held by net.agent/3279:
    >>> [ 1526.520230] #0: (&mm->mmap_sem){++++++}, at: [<ffffffff82f85962>]
    >>> do_page_fault+0x193/0x390
    >>> [ 1526.520230] #1: (panic_lock){+.+...}, at: [<ffffffff82ed2830>]
    >>> panic+0x37/0x1d3
    >>> [ 1526.520230] #2: (rcu_read_lock){.+.+..}, at: [<ffffffff810b9b28>]
    >>> rcu_lock_acquire+0x0/0x29
    >>> [ 1526.520230]
    >>> [ 1526.520230] stack backtrace:
    >>> [ 1526.520230] Pid: 3279, comm: net.agent Not tainted 3.5.0-rc1+ #12
    >>> [ 1526.520230] Call Trace:
    >>> [ 1526.520230] [<ffffffff810e1570>] lockdep_rcu_suspicious+0x109/0x112
    >>> [ 1526.520230] [<ffffffff810bfe3a>] rcu_preempt_sleep_check+0x45/0x47
    >>> [ 1526.520230] [<ffffffff810bfe5a>] __might_sleep+0x1e/0x19a
    >>> [ 1526.520230] [<ffffffff82f8010e>] down_write+0x26/0x81
    >>> [ 1526.520230] [<ffffffff8276a966>] led_trigger_unregister+0x1f/0x9c
    >>> [ 1526.520230] [<ffffffff8276def5>] heartbeat_reboot_notifier+0x15/0x19
    >>> [ 1526.520230] [<ffffffff82f85bf5>] notifier_call_chain+0x96/0xcd
    >>> [ 1526.520230] [<ffffffff82f85cba>]
    >>> __atomic_notifier_call_chain+0x8e/0xff
    >>> [ 1526.520230] [<ffffffff81094b7c>] ? kmsg_dump+0x37/0x1eb
    >>> [ 1526.520230] [<ffffffff82f85d3f>] atomic_notifier_call_chain+0x14/0x16
    >>> [ 1526.520230] [<ffffffff82ed28e1>] panic+0xe8/0x1d3
    >>> [ 1526.520230] [<ffffffff811473e2>] out_of_memory+0x15d/0x1d3
    >>>
    >>> So in case of a panic, now just turn of the LED. Other approaches like
    >>> scheduling a work to unregister the trigger aren't working because there
    >>> isn't much which still runs after a panic occured (except timers).
    >>>
    >>> Signed-off-by: Alexander Holler <holler@ahsoftware.de>
    >>> ---
    >>> drivers/leds/ledtrig-heartbeat.c | 16 +++++++++++++++-
    >>> 1 files changed, 15 insertions(+), 1 deletions(-)
    >>>
    >>> diff --git a/drivers/leds/ledtrig-heartbeat.c
    >>> b/drivers/leds/ledtrig-heartbeat.c
    >>> index 41dc76d..a019fbb 100644
    >>> --- a/drivers/leds/ledtrig-heartbeat.c
    >>> +++ b/drivers/leds/ledtrig-heartbeat.c
    >>> @@ -21,6 +21,8 @@
    >>> #include <linux/reboot.h>
    >>> #include "leds.h"
    >>>
    >>> +static int panic_heartbeats;
    >>> +
    >>> struct heartbeat_trig_data {
    >>> unsigned int phase;
    >>> unsigned int period;
    >>> @@ -34,6 +36,11 @@ static void led_heartbeat_function(unsigned long data)
    >>> unsigned long brightness = LED_OFF;
    >>> unsigned long delay = 0;
    >>>
    >>> + if (unlikely(panic_heartbeats)) {
    >>> + led_set_brightness(led_cdev, LED_OFF);
    >>> + return;
    >>> + }
    >>> +
    >>> /* acts like an actual heart beat -- ie thump-thump-pause... */
    >>> switch (heartbeat_data->phase) {
    >>> case 0:
    >>> @@ -111,12 +118,19 @@ static int heartbeat_reboot_notifier(struct
    >>> notifier_block *nb,
    >>> return NOTIFY_DONE;
    >>> }
    >>>
    >>> +static int heartbeat_panic_notifier(struct notifier_block *nb,
    >>> + unsigned long code, void *unused)
    >>> +{
    >>> + panic_heartbeats = 1;
    >>
    >>
    >> Can we just set LED as OFF and delete the timer here? because timer is
    >> also useless after a kernel panic.
    >> So we don't need this global static variable here.
    >
    >
    > No, the necessary information (heartbeat_trig_data) isn't available here.
    >

    Yeah, looks like there is no way to pass heartbeat_trig_data
    information to the notifier call function.
    Anyway, I will apply this patch to my for-next branch.

    Thanks,
    --
    Bryan Wu <bryan.wu@canonical.com>
    Kernel Developer +86.186-168-78255 Mobile
    Canonical Ltd. www.canonical.com
    Ubuntu - Linux for human beings | www.ubuntu.com


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