[PATCHSET] Improve io_uring cancellations

[Jens Axboe <axboe@kernel.dk>]

We have a few issues currently:

- While the wqe->lock protects the stability of the worker->cur_work
  pointer, it does NOT stabilize the actual work item. For cancelling
  specific work, io_uring needs to dereference it to see if it matches.

- While we know the worker structure won't go away under the RCU read
  lock, the worker might exit. It's not safe to wake up or signal this
  process without ensuring it's still alive.

- We're not consistent in comparing worker->cur_work and sending a
  signal to cancel it.

These two patches fix the above issues. The first is a prep patch that
adds referencing of work items, which makes it safe to dereference
->cur_work if we ensure that ->cur_work itself is stable. The other
patch reworks how we ensure that tasks are alive for signaling, and
that we have a consistent view of ->cur_work while sending a signal.

We add a new lock for ->cur_work, as we cannot safely use wqe->lock
for this. See comment in patch 2 on signalfd usage.

 fs/io-wq.c    | 91 +++++++++++++++++++++++++++++++++++++--------------
 fs/io-wq.h    |  7 +++-
 fs/io_uring.c | 17 +++++++++-
 3 files changed, 89 insertions(+), 26 deletions(-)

-- 
Jens Axboe