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SubjectRe: [RFC PATCH 3/X] ptrace: introduce the empty "struct ptrace_task"
>  /**
> + * tracehook_init_task - initialize the new child
> + * @child: new child task


> + * @clone_flags: %CLONE_* flags from clone/fork/vfork system call
> + * @trace: return value from tracehook_prepare_clone()
> + *
> + * This is called immediately after dup_task_struct().

* It must clear/reset any tracing state so that tracehook_free_task()
* will work safely if the task creation fails. If the task creation
* succeeds, a tracehook_finish_clone() call will follow with locks
* held, before @task starts or is accessible to anyone else.

> +int alloc_ptrace_task(struct task_struct *tsk)

This deserves a short comment about the context it's called from,
and when it is or isn't called at all.

> + if (cmpxchg(&tsk->ptrace_task, NULL, ptrace_task) != NULL)
> + kfree(ptrace_task);

I don't see cmpxchg() used very often at all in generic kernel code. I
wonder how good a choice it is across every arch. Is there a reason not to
use e.g. task_lock() to mediate installing a new ->ptrace_task pointer?
That seems like a more conservative route.


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