fs: sys_ringbuffer

Add new syscalls for generic ringbuffers that can be attached to
arbitrary (supporting) file descriptors.

A ringbuffer consists of:
 - a single page for head/tail pointers, size/mask, and other ancilliary
   metadata, described by 'struct ringbuffer_ptrs'
 - a data buffer, consisting of one or more pages mapped at
   'ringbuffer_ptrs.data_offset' above the address of 'ringbuffer_ptrs'

The data buffer is always a power of two size. Head and tail pointers
are u32 byte offsets, and they are stored unmasked (i.e., they use the
full 32 bit range) - they must be masked for reading.

- ringbuffer(int fd, int rw, u32 size, ulong *addr)

Create or get address of an existing ringbuffer for either reads or
writes, of at least size bytes, and attach it to the given file
descriptor; the address of the ringbuffer is returned via addr.

Since files can be shared between processes in different address spaces
a ringbuffer may be mapped into multiple address spaces via this
syscall.

- ringbuffer_wait(int fd, int rw)

Wait for space to be availaable (on a ringbuffer for writing), or data
to be available (on a ringbuffer for writing).

todo: add parameters for timeout, minimum amount of data/space to wait for

- ringbuffer_wakeup(int fd, int rw)

Required after writing to a previously empty ringbuffer, or reading from
a previously full ringbuffer to notify waiters on the other end

todo - investigate integrating with futexes?
todo - add extra fields to ringbuffer_ptrs for waiting on a minimum
amount of data/space, i.e. to signal when a wakeup is required

Kernel interfaces:
 - To indicate that ringbuffers are supported on a file, set
   FOP_RINGBUFFER_READ and/or FOP_RINGBUFFER_WRITE in your
   file_operations.
 - To read or write to a file's associated ringbuffers
   (file->f_ringbuffer), use ringbuffer_read() or ringbuffer_write().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

0 files changed, 0 insertions, 0 deletions