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    Subject[PATCH 2/2] softlockup: check all tasks in hung_task
    Instead of checking only hung_task_check_count tasks, all tasks are checked.
    hung_task_check_count is still used to put an upper bound on the critical
    section. Every hung_task_check_count checks, the critical section is
    refreshed. Keeping the critical section small minimizes time preemption is
    disabled and keeps rcu grace periods small.

    To prevent following a stale pointer, get_task_struct is called on g and t.
    To verify that g and t have not been unhashed while outside the critical
    section, the task states are checked.

    The design was proposed by Frédéric Weisbecker.

    Frédéric Weisbecker ( wrote:
    > Instead of having this arbitrary limit of tasks, why not just
    > lurk the need_resched() and then schedule if it needs too.
    > I know that sounds a bit racy, because you will have to release the
    > tasklist_lock and
    > a lot of things can happen in the task list until you become resched.
    > But you can do a get_task_struct() on g and t before your thread is
    > going to sleep and then put them
    > when it is awaken.
    > Perhaps some tasks will disappear or be appended in the list before g
    > and t, but that doesn't really matter:
    > if they disappear, they didn't lockup, and if they were appended, they
    > are not enough cold to be analyzed :-)
    > This way you can drop the arbitrary limit of task number given by the user....
    > Frederic.

    Signed-off-by: Mandeep Singh Baines <>
    kernel/hung_task.c | 28 ++++++++++++++++++++++++++--
    1 files changed, 26 insertions(+), 2 deletions(-)

    diff --git a/kernel/hung_task.c b/kernel/hung_task.c
    index a841db3..1c8c9f9 100644
    --- a/kernel/hung_task.c
    +++ b/kernel/hung_task.c
    @@ -109,6 +109,25 @@ static void check_hung_task(struct task_struct *t, unsigned long now,
    panic("hung_task: blocked tasks");

    + /*
    + * To avoid extending the RCU grace period for an unbounded amount of time,
    + * periodically exit the critical section and enter a new one.
    + *
    + * For preemptible RCU it is sufficient to call rcu_read_unlock in order
    + * exit the grace period. For classic RCU, a reschedule is required.
    + */
    +static void check_hung_rcu_refresh(struct task_struct *g, struct task_struct *t)
    + get_task_struct(g);
    + get_task_struct(t);
    + rcu_read_unlock();
    + if (need_resched())
    + schedule();
    + rcu_read_lock();
    + put_task_struct(t);
    + put_task_struct(g);
    * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
    * a really long time (120 seconds). If that happens, print out
    @@ -129,8 +148,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)

    do_each_thread(g, t) {
    - if (!--max_count)
    - goto unlock;
    + if (sysctl_hung_task_check_count && !(max_count--)) {
    + max_count = sysctl_hung_task_check_count;
    + check_hung_rcu_refresh(g, t);
    + /* Exit if t or g was unhashed during refresh. */
    + if (t->state == TASK_DEAD || g->state == TASK_DEAD)
    + goto unlock;
    + }
    /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
    if (t->state == TASK_UNINTERRUPTIBLE)
    check_hung_task(t, now, timeout);
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      Last update: 2009-01-30 21:53    [W:0.026 / U:5.176 seconds]
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