Date:   Thu, 18 Feb 2021 10:29:46 -0300
From:   André Almeida <andrealmeid@...labora.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...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

Hi Gabriel,

Às 16:59 de 15/02/21, Gabriel Krisman Bertazi escreveu:
> André Almeida <andrealmeid@...labora.com> writes:
> 
>> +/**
>> + * struct futexv_head - List of futexes to be waited
>> + * @task:    Task to be awaken
>> + * @hint:    Was someone on this list awakened?
>> + * @objects: List of futexes
>> + */
>> +struct futexv_head {
>> +	struct task_struct *task;
>> +	bool hint;
>> +	struct futex_waiter objects[0];
>> +};
> 
> this structure is also used for a single futex.  maybe struct futex_waiter_head?

One could argue that a single futex is a futexv of one element, but I 
can see that futex_waiter_head makes more sense. Fixed.

>> +/**
>> + * struct futex_single_waiter - Wrapper for a futexv_head of one element
>> + * @futexv: Single futexv element
>> + * @waiter: Single waiter element
>> + */
>> +struct futex_single_waiter {
>> +	struct futexv_head futexv;
>> +	struct futex_waiter waiter;
>> +} __packed;
> 
> Is this struct necessary?  can't you just allocate the necessary space,
> i.e. a struct futexv_head with 1 futexv_head->object?

I don't feel that makes sense to use dynamic allocation for a fixed 
sized memory. Given that, using this struct was the way I found to have 
a futexv_head of a single element in a static allocation fashion.

>> +
>> +	key->offset = address % PAGE_SIZE;
>> +	address -= key->offset;
>> +	key->pointer = (u64)address;
>> +	key->index = (unsigned long)current->mm;
> 
> Why split the key in offset and pointer and waste 1/3 more space to
> store each key?
> 

We need three fields for storing the shared key in the current design, 
and given that the futex key currently lives inside struct futex_waiter, 
private and shared keys need to use the same amount of space. Even if I 
don't use offset for now, the next patch would expand the memory anyway. 
I see that the way I organized the patches made this confusing.

To avoid that we could allocate the key space in futex_wait and make 
futex key point there.

>> +
>> +	/* Generate hash key for this futex using uaddr and current->mm */
>> +	hash_key = jhash2((u32 *)key, sizeof(*key) / sizeof(u32), 0);
>> +
>> +	/* Since HASH_SIZE is 2^n, subtracting 1 makes a perfect bit mask */
>> +	return &futex_table[hash_key & (futex2_hashsize - 1)];
> 
> If someone inadvertely changes futex2_hashsize to something not 2^n this
> will silently break.  futex2_hashsize should be constant and you need
> a BUILD_BUG_ON().

Given that futex2_hashsize is calcutated at boot time, not sure what we 
could to about this, maybe BUG_ON()?

> 
>> +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;
> 
> If you are awakened, you don't need to waste time with trying to get the
> next key.
> 

Yes, and this is what this return is supposed to do. What I'm missing?

> 
>> +/**
>> + * futex_wait - Setup the timer (if there's one) and wait on a list of futexes
>> + * @futexv:     List of futexes
>> + * @nr_futexes: Length of futexv
>> + * @timo:	Timeout
>> + * @flags:	Timeout flags
>> + *
>> + * Return:
>> + * * 0 >= - Hint of which futex woke us
>> + * * 0 <  - Error code
>> + */
>> +static int futex_set_timer_and_wait(struct futexv_head *futexv,
>> +				    unsigned int nr_futexes,
>> +				    struct __kernel_timespec __user *timo,
>> +				    unsigned int flags)
>> +{
>> +	struct hrtimer_sleeper timeout;
>> +	int ret;
>> +
>> +	if (timo) {
>> +		ret = futex_setup_time(timo, &timeout, flags);
>> +		if (ret)
>> +			return ret;
>> +	}
>> +
>> +	ret = __futex_wait(futexv, nr_futexes, timo ? &timeout : NULL);
>> +
>> +	if (timo)
>> +		hrtimer_cancel(&timeout.timer);
>> +
>> +	return ret;
>> +}
> 
> I'm having a hard time understanding why this function exists.  part of
> the futex is set up outside of it, part inside.  Not sure if this isn't
> just part of sys_futex_wait.
> 

I wrote this function since setting the timer, waiting and canceling the 
timer is common for both wait and waitv, so this would avoid some code 
duplication. But I probably can just do the timer stuff inside __futex_wait.

> Thanks,
> 

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