linux-kernel - Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

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Date:   Fri, 28 Jan 2022 02:31:24 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tglx@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org, x86@...nel.org,
        pjt@...gle.com, posk@...gle.com, avagin@...gle.com,
        jannh@...gle.com, tdelisle@...terloo.ca, mark.rutland@....com,
        posk@...k.io, Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:

Another iterator of the patch, some nits below.

[...]

> +/*
> + * Pinning a page inhibits rmap based unmap for Anon pages. Doing a load
> + * through the user mapping ensures the user mapping exists.
> + */
> +#define umcg_pin_and_load(_self, _pagep, _member)				\
> +({										\
> +	__label__ __out;							\
> +	int __ret = -EFAULT;							\
> +										\
> +	if (pin_user_pages_fast((unsigned long)(_self), 1, 0, &(_pagep)) != 1)	\
> +		goto __out;							\
> +										\
> +	if (!PageAnon(_pagep) ||						\
> +	    get_user(_member, &(_self)->_member)) {				\

Here should be 'get_user(_member, &(_self)->##_member))'
if I am not wrong.

> +		unpin_user_page(_pagep);					\
> +		goto __out;							\
> +	}									\
> +	__ret = 0;								\
> +__out:	__ret;									\
> +})

[...]

> +
> +/*
> + * Enqueue @tsk on it's server's runnable list
> + *
> + * Must be called in umcg_pin_pages() context, relies on tsk->umcg_server.
> + *
> + * cmpxchg based single linked list add such that list integrity is never
> + * violated.  Userspace *MUST* remove it from the list before changing ->state.
> + * As such, we must change state to RUNNABLE before enqueue.
> + *
> + * Returns:
> + *   0: success
> + *   -EFAULT
> + */
> +static int umcg_enqueue_runnable(struct task_struct *tsk)
> +{
> +	struct umcg_task __user *server = tsk->umcg_server_task;
> +	struct umcg_task __user *self = tsk->umcg_task;
> +	u64 first_ptr, *head = &server->runnable_workers_ptr;
> +	u64 self_ptr = (unsigned long)self;

Why not 'u64 self_ptr = (u64)self;' ?

> +	/*
> +	 * umcg_pin_pages() did access_ok() on both pointers, use self here
> +	 * only because __user_access_begin() isn't available in generic code.
> +	 */
> +	if (!user_access_begin(self, sizeof(*self)))
> +		return -EFAULT;
> +
> +	unsafe_get_user(first_ptr, head, Efault);
> +	do {
> +		unsafe_put_user(first_ptr, &self->runnable_workers_ptr, Efault);
> +	} while (!unsafe_try_cmpxchg_user(head, &first_ptr, self_ptr, Efault));
> +
> +	user_access_end();
> +	return 0;
> +
> +Efault:
> +	user_access_end();
> +	return -EFAULT;
> +}

[...]

> +/*
> + * Handles ::next_tid as per sys_umcg_wait().
> + *
> + * ::next_tid		- return
> + * -----------------------------
> + * 0			- 0 (success)
> + *
> + * tid			- -ESRCH (no such task, or not of this UMCG)
> + *			- -EAGAIN (next::state != RUNNABLE)
> + *			- 0 (success, ::next_tid |= RUNNING)
> + *
> + * tid|RUNNING		- -EAGAIN (next::state != RUNNING)
> + *			- 0 (success)
> + *
> + * Returns:
> + *  0: success
> + *  -EFAULT
> + *  -ESRCH
> + *  -EAGAIN
> + */
> +static int umcg_wake_next(struct task_struct *tsk, struct umcg_task __user *self)

@tsk is not used in function.

> +{
> +	struct umcg_task __user *next_task;
> +	struct task_struct *next;
> +	u32 next_tid, state;
> +	int ret;
> +
> +	if (get_user(next_tid, &self->next_tid))
> +		return -EFAULT;
> +
> +	if (!next_tid)
> +		return 0;
> +
> +	next = umcg_get_task(next_tid);
> +	if (!next)
> +		return -ESRCH;
> +
> +	next_task = READ_ONCE(next->umcg_task);
> +
> +	if (next_tid & UMCG_TID_RUNNING) {
> +		ret = -EFAULT;
> +		if (get_user(state, &next_task->state))
> +			goto put_next;
> +
> +		ret = 0;
> +		if ((state & UMCG_TASK_MASK) != UMCG_TASK_RUNNING)
> +			ret = -EAGAIN;
> +
> +	} else {
> +		ret = umcg_wake_task(next, next_task);
> +		if (ret)
> +			goto put_next;
> +
> +		ret = -EFAULT;
> +		if (put_user(next_tid | UMCG_TID_RUNNING, &self->next_tid))
> +			goto put_next;
> +
> +		/*
> +		 * If this is a worker doing sys_umcg_wait() switching to
> +		 * another worker, userspace has the responsibility to update
> +		 * server::next_tid.
> +		 */
> +
> +		ret = 0;
> +	}
> +
> +put_next:
> +	put_task_struct(next);
> +	return ret;
> +}
> +