Merge tag 'trace-v5.14-rc4-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rosted...

Merge tag 'trace-v5.14-rc4-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull tracing fixes from Steven Rostedt:
 "Fix tracepoint race between static_call and callback data

  As callbacks to a tracepoint are paired with the data that is passed
  in when the callback is registered to the tracepoint, it must have
  that data passed to the callback when the tracepoint is triggered,
  else bad things will happen. To keep the two together, they are both
  assigned to a tracepoint structure and added to an array. The
  tracepoint call site will dereference the structure (via RCU) and call
  the callback in that structure along with the data in that structure.
  This keeps the callback and data tightly coupled.

  Because of the overhead that retpolines have on tracepoint callbacks,
  if there's only one callback attached to a tracepoint (a common case),
  then it is called via a static call (code modified to do a direct call
  instead of an indirect call). But to implement this, the data had to
  be decoupled from the callback, as now the callback is implemented via
  a direct call from the static call and not an indirect call from the
  dereferenced structure.

  Note, the static call only calls a callback used when there's a single
  callback attached to the tracepoint. If more than one callback is
  attached to the same tracepoint, then the static call will call an
  iterator function that goes back to dereferencing the structure
  keeping the callback and its data tightly coupled again.

  Issues can arise when going from 0 callbacks to one, as the static
  call is assigned to the callback, and it must take care that the data
  passed to it is loaded before the static call calls the callback.
  Going from 1 to 2 callbacks is not an issue, as long as the static
  call is updated to the iterator before the tracepoint structure array
  is updated via RCU. Going from 2 to more or back down to 2 is not an
  issue as the iterator can handle all theses cases. But going from 2 to
  1, care must be taken as the static call is now calling a callback and
  the data that is loaded must be the data for that callback.

  Care was taken to ensure the callback and data would be in-sync, but
  after a bug was reported, it became clear that not enough was done to
  make sure that was the case. These changes address this.

  The first change is to compare the old and new data instead of the old
  and new callback, as it's the data that can corrupt the callback, even
  if the callback is the same (something getting freed).

  The next change is to convert these transitions into states, to make
  it easier to know when a synchronization is needed, and to perform
  those synchronizations. The problem with this patch is that it slows
  down disabling all events from under a second, to making it take over
  10 seconds to do the same work. But that is addressed in the final
  patch.

  The final patch uses the RCU state functions to keep track of the RCU
  state between the transitions, and only needs to perform the
  synchronization if an RCU synchronization hasn't been done already.
  This brings the performance of disabling all events back to its
  original value. That's because no synchronization is required between
  disabling tracepoints but is required when enabling a tracepoint after
  its been disabled. If an RCU synchronization happens after the
  tracepoint is disabled, and before it is re-enabled, there's no need
  to do the synchronization again.

  Both the second and third patch have subtle complexities that they are
  separated into two patches. But because the second patch causes such a
  regression in performance, the third patch adds a "Fixes" tag to the
  second patch, such that the two must be backported together and not
  just the second patch"

* tag 'trace-v5.14-rc4-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  tracepoint: Use rcu get state and cond sync for static call updates
  tracepoint: Fix static call function vs data state mismatch
  tracepoint: static call: Compare data on transition from 2->1 callees