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SubjectRe: [RFC PATCH 03/21] pipe: Use head and tail pointers for the ring, not cursor and length
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On 15/10/2019 23.48, David Howells wrote:
> Convert pipes to use head and tail pointers for the buffer ring rather than
> pointer and length as the latter requires two atomic ops to update (or a
> combined op) whereas the former only requires one.
>
> (1) The head pointer is the point at which production occurs and points to
> the slot in which the next buffer will be placed. This is equivalent
> to pipe->curbuf + pipe->nrbufs.
>
> The head pointer belongs to the write-side.
>
> (2) The tail pointer is the point at which consumption occurs. It points
> to the next slot to be consumed. This is equivalent to pipe->curbuf.
>
> The tail pointer belongs to the read-side.
>
> (3) head and tail are allowed to run to UINT_MAX and wrap naturally. They
> are only masked off when the array is being accessed, e.g.:
>
> pipe->bufs[head & mask]
>
> This means that it is not necessary to have a dead slot in the ring as
> head == tail isn't ambiguous.
>
> (4) The ring is empty if "head == tail".
>
> (5) The occupancy of the ring is "head - tail".
>
> (6) The number of free slots in the ring is "(tail + pipe->ring_size) -
> head".

Seems an odd way of writing pipe->ring_size - (head - tail) ; i.e.
obviously #free slots is #size minus #occupancy.

> (7) The ring is full if "head >= (tail + pipe->ring_size)", which can also
> be written as "head - tail >= pipe->ring_size".
>

No it cannot, it _must_ be written in the latter form. Assuming
sizeof(int)==1 for simplicity, consider ring_size = 16, tail = 240.
Regardless whether head is 240, 241, ..., 255, 0, tail + ring_size wraps
to 0, so the former expression states the ring is full in all cases.

Better spell out somewhere that while head and tail are free-running, at
any point in time they satisfy the invariant head - tail <= pipe_size
(and also 0 <= head - tail, but that's a tautology for unsigned
ints...). Then it's a matter of taste if one wants to write "full" as
head-tail == pipe_size or head-tail >= pipe_size.

> Also split pipe->buffers into pipe->ring_size (which indicates the size of the
> ring) and pipe->max_usage (which restricts the amount of ring that write() is
> allowed to fill). This allows for a pipe that is both writable by the kernel
> notification facility and by userspace, allowing plenty of ring space for
> notifications to be added whilst preventing userspace from being able to use
> up too much buffer space.

That seems like something that should be added in a separate patch -
adding ->max_usage and switching appropriate users of ->ring_size over,
so it's more clear where you're using one or the other.

> @@ -1949,8 +1950,12 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>
> pipe_lock(pipe);
>
> - bufs = kvmalloc_array(pipe->nrbufs, sizeof(struct pipe_buffer),
> - GFP_KERNEL);
> + head = pipe->head;
> + tail = pipe->tail;
> + mask = pipe->ring_size - 1;
> + count = head - tail;
> +
> + bufs = kvmalloc_array(count, sizeof(struct pipe_buffer), GFP_KERNEL);
> if (!bufs) {
> pipe_unlock(pipe);
> return -ENOMEM;
> @@ -1958,8 +1963,8 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
>
> nbuf = 0;
> rem = 0;
> - for (idx = 0; idx < pipe->nrbufs && rem < len; idx++)
> - rem += pipe->bufs[(pipe->curbuf + idx) & (pipe->buffers - 1)].len;
> + for (idx = tail; idx < head && rem < len; idx++)
> + rem += pipe->bufs[idx & mask].len;
>
> ret = -EINVAL;
> if (rem < len)
> @@ -1970,16 +1975,16 @@ static ssize_t fuse_dev_splice_write(struct pipe_inode_info *pipe,
> struct pipe_buffer *ibuf;
> struct pipe_buffer *obuf;
>
> - BUG_ON(nbuf >= pipe->buffers);
> - BUG_ON(!pipe->nrbufs);
> - ibuf = &pipe->bufs[pipe->curbuf];
> + BUG_ON(nbuf >= pipe->ring_size);
> + BUG_ON(tail == head);
> + ibuf = &pipe->bufs[tail];

I don't see where tail gets masked between tail = pipe->tail; above and
here, but I may be missing it. In any case, how about seeding head and
tail with something like 1<<20 when creating the pipe so bugs like that
are hit more quickly.

> @@ -515,17 +525,19 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
> static long pipe_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> {
> struct pipe_inode_info *pipe = filp->private_data;
> - int count, buf, nrbufs;
> + int count, head, tail, mask;
>
> switch (cmd) {
> case FIONREAD:
> __pipe_lock(pipe);
> count = 0;
> - buf = pipe->curbuf;
> - nrbufs = pipe->nrbufs;
> - while (--nrbufs >= 0) {
> - count += pipe->bufs[buf].len;
> - buf = (buf+1) & (pipe->buffers - 1);
> + head = pipe->head;
> + tail = pipe->tail;
> + mask = pipe->ring_size - 1;
> +
> + while (tail < head) {
> + count += pipe->bufs[tail & mask].len;
> + tail++;
> }

This is broken if head has wrapped but tail has not. It has to be "while
(head - tail)" or perhaps just "while (tail != head)" or something along
those lines.

> @@ -1086,17 +1104,21 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
> }
>
> /*
> - * We can shrink the pipe, if arg >= pipe->nrbufs. Since we don't
> - * expect a lot of shrink+grow operations, just free and allocate
> - * again like we would do for growing. If the pipe currently
> + * We can shrink the pipe, if arg is greater than the ring occupancy.
> + * Since we don't expect a lot of shrink+grow operations, just free and
> + * allocate again like we would do for growing. If the pipe currently
> * contains more buffers than arg, then return busy.
> */
> - if (nr_pages < pipe->nrbufs) {
> + mask = pipe->ring_size - 1;
> + head = pipe->head & mask;
> + tail = pipe->tail & mask;
> + n = pipe->head - pipe->tail;

I think it's confusing to "premask" head and tail here. Can you either
drop that (pipe_set_size should hardly be a hot path?), or perhaps call
them something else to avoid a future reader seeing an unmasked
bufs[head] and thinking that's a bug?

> @@ -1254,9 +1290,10 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
> struct page **pages, size_t maxsize, unsigned maxpages,
> size_t *start)
> {
> + unsigned int p_tail;
> + unsigned int i_head;
> unsigned npages;
> size_t capacity;
> - int idx;
>
> if (!maxsize)
> return 0;
> @@ -1264,12 +1301,15 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
> if (!sanity(i))
> return -EFAULT;
>
> - data_start(i, &idx, start);
> - /* some of this one + all after this one */
> - npages = ((i->pipe->curbuf - idx - 1) & (i->pipe->buffers - 1)) + 1;
> - capacity = min(npages,maxpages) * PAGE_SIZE - *start;
> + data_start(i, &i_head, start);
> + p_tail = i->pipe->tail;
> + /* Amount of free space: some of this one + all after this one */
> + npages = (p_tail + i->pipe->ring_size) - i_head;

Hm, it's not clear that this is equivalent to the old computation. Since
it seems repeated in a few places, could it be factored to a little
helper (before this patch) and the "some of this one + all after this
one" comment perhaps expanded to explain what is going on?

Rasmus

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