> +struct unwind_work;
> +
> +typedef void (*unwind_callback_t)(struct unwind_work *work, struct unwind_stacktrace *trace, u64 timestamp);
> +
> +struct unwind_work {
> + struct list_head list;
Does this really need to be a list? Single linked list like
callback_head not good enough?
> + unwind_callback_t func;
> +};
> +
> #ifdef CONFIG_UNWIND_USER
>
> void unwind_task_init(struct task_struct *task);
> @@ -12,10 +22,15 @@ void unwind_task_free(struct task_struct *task);
>
> int unwind_deferred_trace(struct unwind_stacktrace *trace);
>
> +int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func);
> +int unwind_deferred_request(struct unwind_work *work, u64 *timestamp);
> +void unwind_deferred_cancel(struct unwind_work *work);
> +
> static __always_inline void unwind_exit_to_user_mode(void)
> {
> if (unlikely(current->unwind_info.cache))
> current->unwind_info.cache->nr_entries = 0;
> + current->unwind_info.timestamp = 0;
Surely clearing that timestamp is only relevant when there is a cache
around? Better to not add this unconditional write to the exit path.
> }
>
> #else /* !CONFIG_UNWIND_USER */
> @@ -24,6 +39,9 @@ static inline void unwind_task_init(struct task_struct *task) {}
> static inline void unwind_task_free(struct task_struct *task) {}
>
> static inline int unwind_deferred_trace(struct unwind_stacktrace *trace) { return -ENOSYS; }
> +static inline int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func) { return -ENOSYS; }
> +static inline int unwind_deferred_request(struct unwind_work *work, u64 *timestamp) { return -ENOSYS; }
> +static inline void unwind_deferred_cancel(struct unwind_work *work) {}
>
> static inline void unwind_exit_to_user_mode(void) {}
>
> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> index db5b54b18828..5df264cf81ad 100644
> --- a/include/linux/unwind_deferred_types.h
> +++ b/include/linux/unwind_deferred_types.h
> @@ -9,6 +9,9 @@ struct unwind_cache {
>
> struct unwind_task_info {
> struct unwind_cache *cache;
> + struct callback_head work;
> + u64 timestamp;
> + int pending;
> };
>
> #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> index e3913781c8c6..b76c704ddc6d 100644
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -2,13 +2,35 @@
> /*
> * Deferred user space unwinding
> */
> +#include <linux/sched/task_stack.h>
> +#include <linux/unwind_deferred.h>
> +#include <linux/sched/clock.h>
> +#include <linux/task_work.h>
> #include <linux/kernel.h>
> #include <linux/sched.h>
> #include <linux/slab.h>
> -#include <linux/unwind_deferred.h>
> +#include <linux/mm.h>
>
> #define UNWIND_MAX_ENTRIES 512
>
> +/* Guards adding to and reading the list of callbacks */
> +static DEFINE_MUTEX(callback_mutex);
> +static LIST_HEAD(callbacks);
Global state.. smells like failure.
> +/*
> + * Read the task context timestamp, if this is the first caller then
> + * it will set the timestamp.
> + */
> +static u64 get_timestamp(struct unwind_task_info *info)
> +{
> + lockdep_assert_irqs_disabled();
> +
> + if (!info->timestamp)
> + info->timestamp = local_clock();
> +
> + return info->timestamp;
> +}
> +
> /**
> * unwind_deferred_trace - Produce a user stacktrace in faultable context
> * @trace: The descriptor that will store the user stacktrace
> @@ -59,11 +81,117 @@ int unwind_deferred_trace(struct unwind_stacktrace *trace)
> return 0;
> }
>
> +static void unwind_deferred_task_work(struct callback_head *head)
> +{
> + struct unwind_task_info *info = container_of(head, struct unwind_task_info, work);
> + struct unwind_stacktrace trace;
> + struct unwind_work *work;
> + u64 timestamp;
> +
> + if (WARN_ON_ONCE(!info->pending))
> + return;
> +
> + /* Allow work to come in again */
> + WRITE_ONCE(info->pending, 0);
> +
> + /*
> + * From here on out, the callback must always be called, even if it's
> + * just an empty trace.
> + */
> + trace.nr = 0;
> + trace.entries = NULL;
> +
> + unwind_deferred_trace(&trace);
> +
> + timestamp = info->timestamp;
> +
> + guard(mutex)(&callback_mutex);
> + list_for_each_entry(work, &callbacks, list) {
> + work->func(work, &trace, timestamp);
> + }
So now you're globally serializing all return-to-user instances. How is
that not a problem?
> +}
