[PATCHSET wq/for-6.9] workqueue: Implement BH workqueue and convert several tasklet users

[Tejun Heo]

Hello,

The only generic interface to execute asynchronously in the BH context is
tasklet; however, it's marked deprecated and has some design flaws such as
the execution code accessing the tasklet item after the execution is
complete which can lead to subtle use-after-free in certain usage scenarios
and less-developed flush and cancel mechanisms.

Mikulas Patocka recently reported that dm-crypt and dm-crypt are affected by
the access-after-completion issue and suggested adding TASKLET_STATE_ONESHOT
flag which selectively removes post-completion access while significantly
limiting how the tasklet can be used in the following thread:

 http://lkml.kernel.org/r/82b964f0-c2c8-a2c6-5b1f-f3145dc2c8e5@xxxxxxxxxx

Linus didn't like the approach and suggested extending workqueue to support
execution from atomic context:

 http://lkml.kernel.org/r/CAHk-=wjDW53w4-YcSmgKC5RruiRLHmJ1sXeYdp_ZgVoBw=5byA@xxxxxxxxxxxxxx

As suggested, this patchset implements BH workqueues which are like regular
workqueues but executes work items in the BH (softirq) context and converts
several tasklet users.

- The name bh is used instead of the suggested atomic as it's more in line
  with widely used execution context interface - local_bh_enable/disable()
  and friends.

- The system default BH workqueues - system_bh_wq and system_bh_highpri_wq -
  are provided. As queue-wide flushing doesn't exist in tasklet, all
  existing tasklet users should be able to use the system BH workqueues
  without creating their own.

- BH workqueues currently use tasklet to run the work items to avoid
  priority inversions involving tasklet_hi and WQ_BH | WQ_HIGHPRI. Once all
  tasklet users are converted, tasklet code can be removed and BH workqueues
  can take over its softirqs.

This patchset is on top of wq/for-6.9 (aae17ebb53c ("workqueue: Avoid using
isolated cpus' timers on queue_delayed_work")) and contains the following
eight patches.

 0001-workqueue-Update-lock-debugging-code.patch
 0002-workqueue-Factor-out-init_cpu_worker_pool.patch
 0003-workqueue-Implement-BH-workqueues-to-eventually-repl.patch
 0004-backtracetest-Convert-from-tasklet-to-BH-workqueue.patch
 0005-usb-core-hcd-Convert-from-tasklet-to-BH-workqueue.patch
 0006-net-tcp-tsq-Convert-from-tasklet-to-BH-workqueue.patch
 0007-dm-crypt-Convert-from-tasklet-to-BH-workqueue.patch
 0008-dm-verity-Convert-from-tasklet-to-BH-workqueue.patch

0001-0003 prepare and implement BH workqueues.

0004-0008 convert some tasklet users to BH workqueue. The conversions are
relatively straightforward but are in descending order of confidence.

The patchset is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git wq-bh-v1

diffstat follows. Thanks.

 Documentation/core-api/workqueue.rst |   29 ++++-
 drivers/md/dm-crypt.c                |   36 -------
 drivers/md/dm-verity-target.c        |    8 -
 drivers/md/dm-verity.h               |    2
 drivers/usb/core/hcd.c               |   23 ++--
 include/linux/usb/hcd.h              |    2
 include/linux/workqueue.h            |    9 +
 include/net/tcp.h                    |    2
 kernel/backtracetest.c               |   18 +--
 kernel/workqueue.c                   |  312 ++++++++++++++++++++++++++++++++++++++++++++++++--------------
 kernel/workqueue_internal.h          |    3
 net/ipv4/tcp.c                       |    2
 net/ipv4/tcp_output.c                |   36 +++----
 tools/workqueue/wq_dump.py           |   11 +-
 14 files changed, 335 insertions(+), 158 deletions(-)

--
tejun