[PATCH v2 0/3] tracing/user_events: Share state with existing threads

[Jeff Barnes <jeffbarnes@linux.microsoft.com>]

A thread created before the first user_events registration can share an
mm with another thread while retaining a NULL user_event_mm. If it later
registers an event, it can create independent registration state instead
of observing the state already associated with the shared mm. This allows
pre-existing threads in one address space to register different events
using the same enable address and bit.

Patch 1 fixes a locking issue found while reviewing this path. Duplicate
enabler lookup performs a plain traversal of the enabler list, while
asynchronous fault cleanup can remove entries under event_mutex. Serialize
the traversal with that cleanup.

Patch 2 finds the already-published user_event_mm for the current mm and
attaches the pre-existing thread to it. The global list lock serializes
lookup and attachment with the final task-count decrement and list removal.

Patch 3 adds a regression test covering two pre-existing threads that
attempt to register different events at the same enable address and bit.

Tested on 7.3.0-rc6 with virtme-ng:

- user_events ftrace_test: 6/6 passed
- user_events dyn_test: 4/4 passed
- user_events perf_test: 2/2 passed
- user_events abi_test: 7/7 passed

The user.preexisting_threads_same_address test passed.

Changes since v1:

- Split the duplicate-enabler locking correction into a preparatory patch.
- Protect duplicate-enabler traversal with event_mutex.
- Document and assert the group reg_mutex contract.
- Simplify existing-thread attachment around user_event_mms_lock.
- Serialize the final tasks decrement and list removal with lookup and
  attachment.
- Keep the regression selftest as the final patch.

Jeff Barnes (3):
  tracing/user_events: Serialize duplicate enabler lookup with event
    removal
  tracing/user_events: Share event state with existing threads
  selftests/user_events: Test registration from existing threads

 kernel/trace/trace_events_user.c              |  56 ++++++--
 .../testing/selftests/user_events/abi_test.c  | 125 ++++++++++++++++++
 2 files changed, 171 insertions(+), 10 deletions(-)


base-commit: a90ee4305c4a5df72c11b31dacfdc76e00fcf78a
-- 
2.43.0