Hi Gabriel,
Às 16:59 de 15/02/21, Gabriel Krisman Bertazi escreveu:
André Almeida <andrealmeid@xxxxxxxxxxxxx> 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,
