On Fri, Oct 22, 2021 at 05:09:35PM +0200, Peter Zijlstra wrote:
> Recent patches to get_wchan() made it more robust by only doing the
> unwind when the task was blocked and serialized against wakeups.
>
> Extract this functionality as a simpler companion to task_call_func()
> named task_try_func() that really only cares about blocked tasks. Then
> employ this new function to implement the same robustness for
> ARCH_STACKWALK based stack_trace_save_tsk().
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
> include/linux/wait.h | 1
> kernel/sched/core.c | 62 ++++++++++++++++++++++++++++++++++++++++++++-------
> kernel/stacktrace.c | 13 ++++++----
> 3 files changed, 63 insertions(+), 13 deletions(-)
>
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -1162,5 +1162,6 @@ int autoremove_wake_function(struct wait
>
> typedef int (*task_call_f)(struct task_struct *p, void *arg);
> extern int task_call_func(struct task_struct *p, task_call_f func, void *arg);
> +extern int task_try_func(struct task_struct *p, task_call_f func, void *arg);
>
> #endif /* _LINUX_WAIT_H */
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1966,21 +1966,21 @@ bool sched_task_on_rq(struct task_struct
> return task_on_rq_queued(p);
> }
>
> +static int try_get_wchan(struct task_struct *p, void *arg)
> +{
> + unsigned long *wchan = arg;
> + *wchan = __get_wchan(p);
> + return 0;
> +}
> +
> unsigned long get_wchan(struct task_struct *p)
> {
> unsigned long ip = 0;
> - unsigned int state;
>
> if (!p || p == current)
> return 0;
>
> - /* Only get wchan if task is blocked and we can keep it that way. */
> - raw_spin_lock_irq(&p->pi_lock);
> - state = READ_ONCE(p->__state);
> - smp_rmb(); /* see try_to_wake_up() */
> - if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
> - ip = __get_wchan(p);
> - raw_spin_unlock_irq(&p->pi_lock);
> + task_try_func(p, try_get_wchan, &ip);
>
> return ip;
> }
> @@ -4184,6 +4184,52 @@ int task_call_func(struct task_struct *p
> return ret;
> }
>
> +/*
> + * task_try_func - Invoke a function on task in blocked state
> + * @p: Process for which the function is to be invoked
> + * @func: Function to invoke
> + * @arg: Argument to function
> + *
> + * Fix the task in a blocked state, when possible. And if so, invoke @func on it.
> + *
> + * Returns:
> + * -EBUSY or whatever @func returns
> + */
> +int task_try_func(struct task_struct *p, task_call_f func, void *arg)
> +{
> + unsigned long flags;
> + unsigned int state;
> + int ret = -EBUSY;
> +
> + raw_spin_lock_irqsave(&p->pi_lock, flags);
> +
> + state = READ_ONCE(p->__state);
> +
> + /*
> + * Ensure we load p->on_rq after p->__state, otherwise it would be
> + * possible to, falsely, observe p->on_rq == 0.
> + *
> + * See try_to_wake_up() for a longer comment.
> + */
> + smp_rmb();
> +
> + /*
> + * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> + * the task is blocked. Make sure to check @state since ttwu() can drop
> + * locks at the end, see ttwu_queue_wakelist().
> + */
> + if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq) {
> + /*
> + * The task is blocked and we're holding off wakeupsr. For any
> + * of the other task states, see task_call_func().
> + */
> + ret = func(p, arg);
> + }
> +
> + raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> + return ret;
> +}
> +
> /**
> * wake_up_process - Wake up a specific process
> * @p: The process to be woken up.
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -123,6 +123,13 @@ unsigned int stack_trace_save(unsigned l
> }
> EXPORT_SYMBOL_GPL(stack_trace_save);
>
> +static int try_arch_stack_walk_tsk(struct task_struct *tsk, void *arg)
> +{
> + stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
> + arch_stack_walk(consume_entry, arg, tsk, NULL);
> + return 0;
> +}
> +
> /**
> * stack_trace_save_tsk - Save a task stack trace into a storage array
> * @task: The task to examine
> @@ -135,7 +142,6 @@ EXPORT_SYMBOL_GPL(stack_trace_save);
> unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
> unsigned int size, unsigned int skipnr)
> {
> - stack_trace_consume_fn consume_entry = stack_trace_consume_entry_nosched;
> struct stacktrace_cookie c = {
> .store = store,
> .size = size,
> @@ -143,11 +149,8 @@ unsigned int stack_trace_save_tsk(struct
> .skip = skipnr + (current == tsk),
> };
>
> - if (!try_get_task_stack(tsk))
> - return 0;
> + task_try_func(tsk, try_arch_stack_walk_tsk, &c);
Pardon my thin understanding of the scheduler, but I assume this change
doesn't mean stack_trace_save_tsk() stops working for "current", right?
In trying to answer this for myself, I couldn't convince myself what value
current->__state have here. Is it one of TASK_(UN)INTERRUPTIBLE ?
Assuming this does actually remain callable for current:
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> - arch_stack_walk(consume_entry, &c, tsk, NULL);
> - put_task_stack(tsk);
> return c.len;
> }
>
>
>
--
Kees Cook
