Hi Gabriel,
Às 16:59 de 15/02/21, Gabriel Krisman Bertazi escreveu:
André Almeida andrealmeid@collabora.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,