linux-kernel - Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

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Date:   Tue, 16 Feb 2021 10:35:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     André Almeida <andrealmeid@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, krisman@...labora.com,
        pgriffais@...vesoftware.com, z.figura12@...il.com,
        joel@...lfernandes.org, malteskarupke@...tmail.fm,
        linux-api@...r.kernel.org, fweimer@...hat.com,
        libc-alpha@...rceware.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, acme@...nel.org, corbet@....net
Subject: Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

On Mon, Feb 15, 2021 at 12:23:52PM -0300, André Almeida wrote:

> +static int futex_dequeue_multiple(struct futexv_head *futexv, unsigned int nr)
> +{
> +	int i, ret = -1;
> +
> +	for (i = 0; i < nr; i++) {
> +		spin_lock(&futexv->objects[i].bucket->lock);
> +		if (!list_empty_careful(&futexv->objects[i].list)) {
> +			list_del_init_careful(&futexv->objects[i].list);
> +			bucket_dec_waiters(futexv->objects[i].bucket);

What's with the careful? AFAICT all sites have that bucket->lock.

> +		} else {
> +			ret = i;
> +		}
> +		spin_unlock(&futexv->objects[i].bucket->lock);
> +	}
> +
> +	return ret;
> +}

> +static int futex_enqueue(struct futexv_head *futexv, unsigned int nr_futexes,
> +			 int *awakened)
> +{
> +	int i, ret;
> +	u32 uval, *uaddr, val;
> +	struct futex_bucket *bucket;
> +
> +retry:
> +	set_current_state(TASK_INTERRUPTIBLE);
> +
> +	for (i = 0; i < nr_futexes; i++) {
> +		uaddr = (u32 * __user)futexv->objects[i].uaddr;
> +		val = (u32)futexv->objects[i].val;
> +
> +		bucket = futexv->objects[i].bucket;
> +
> +		bucket_inc_waiters(bucket);
> +		spin_lock(&bucket->lock);
> +
> +		ret = futex_get_user(&uval, uaddr);
> +
> +		if (unlikely(ret)) {
> +			spin_unlock(&bucket->lock);
> +
> +			bucket_dec_waiters(bucket);
> +			__set_current_state(TASK_RUNNING);
> +			*awakened = futex_dequeue_multiple(futexv, i);
> +
> +			if (__get_user(uval, uaddr))
> +				return -EFAULT;
> +
> +			if (*awakened >= 0)
> +				return 1;
> +
> +			goto retry;
> +		}
> +
> +		if (uval != val) {
> +			spin_unlock(&bucket->lock);
> +
> +			bucket_dec_waiters(bucket);
> +			__set_current_state(TASK_RUNNING);
> +			*awakened = futex_dequeue_multiple(futexv, i);
> +
> +			if (*awakened >= 0)
> +				return 1;
> +
> +			return -EAGAIN;
> +		}
> +
> +		list_add_tail(&futexv->objects[i].list, &bucket->list);

and here

> +		spin_unlock(&bucket->lock);
> +	}
> +
> +	return 0;
> +}
> +

> +static void futex_mark_wake(struct futex_waiter *waiter,
> +			    struct futex_bucket *bucket,
> +			    struct wake_q_head *wake_q)
> +{
> +	struct task_struct *task;
> +	struct futexv_head *parent = futex_get_parent((uintptr_t)waiter,
> +						      waiter->index);
> +

	lockdep_assert_held(&bucket->lock);

> +	parent->hint = true;
> +	task = parent->task;
> +	get_task_struct(task);
> +	list_del_init_careful(&waiter->list);

and here

> +	wake_q_add_safe(wake_q, task);
> +	bucket_dec_waiters(bucket);
> +}