Merge tag 'trace-v6.7-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull tracing fixes from Steven Rostedt:

 - Snapshot buffer issues:

   1. When instances started allowing latency tracers, it uses a
      snapshot buffer (another buffer that is not written to but swapped
      with the main buffer that is). The snapshot buffer needs to be the
      same size as the main buffer. But when the snapshot buffers were
      added to instances, the code to make the snapshot equal to the
      main buffer still was only doing it for the main buffer and not
      the instances.

   2. Need to stop the current tracer when resizing the buffers.
      Otherwise there can be a race if the tracer decides to make a
      snapshot between resizing the main buffer and the snapshot buffer.

   3. When a tracer is "stopped" in disables both the main buffer and
      the snapshot buffer. This needs to be done for instances and not
      only the main buffer, now that instances also have a snapshot
      buffer.

 - Buffered event for filtering issues:

   When filtering is enabled, because events can be dropped often, it is
   quicker to copy the event into a temp buffer and write that into the
   main buffer if it is not filtered or just drop the event if it is,
   than to write the event into the ring buffer and then try to discard
   it. This temp buffer is allocated and needs special synchronization
   to do so. But there were some issues with that:

   1. When disabling the filter and freeing the buffer, a call to all
      CPUs is required to stop each per_cpu usage. But the code called
      smp_call_function_many() which does not include the current CPU.
      If the task is migrated to another CPU when it enables the CPUs
      via smp_call_function_many(), it will not enable the one it is
      currently on and this causes issues later on. Use
      on_each_cpu_mask() instead, which includes the current CPU.

    2.When the allocation of the buffered event fails, it can give a
      warning. But the buffered event is just an optimization (it's
      still OK to write to the ring buffer and free it). Do not WARN in
      this case.

    3.The freeing of the buffer event requires synchronization. First a
      counter is decremented to zero so that no new uses of it will
      happen. Then it sets the buffered event to NULL, and finally it
      frees the buffered event. There's a synchronize_rcu() between the
      counter decrement and the setting the variable to NULL, but only a
      smp_wmb() between that and the freeing of the buffer. It is
      theoretically possible that a user missed seeing the decrement,
      but will use the buffer after it is free. Another
      synchronize_rcu() is needed in place of that smp_wmb().

 - ring buffer timestamps on 32 bit machines

   The ring buffer timestamp on 32 bit machines has to break the 64 bit
   number into multiple values as cmpxchg is required on it, and a 64
   bit cmpxchg on 32 bit architectures is very slow. The code use to
   just use two 32 bit values and make it a 60 bit timestamp where the
   other 4 bits were used as counters for synchronization. It later came
   known that the timestamp on 32 bit still need all 64 bits in some
   cases. So 3 words were created to handle the 64 bits. But issues
   arised with this:

    1. The synchronization logic still only compared the counter with
       the first two, but not with the third number, so the
       synchronization could fail unknowingly.

    2. A check on discard of an event could race if an event happened
       between the discard and updating one of the counters. The counter
       needs to be updated (forcing an absolute timestamp and not to use
       a delta) before the actual discard happens.

* tag 'trace-v6.7-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Test last update in 32bit version of __rb_time_read()
  ring-buffer: Force absolute timestamp on discard of event
  tracing: Fix a possible race when disabling buffered events
  tracing: Fix a warning when allocating buffered events fails
  tracing: Fix incomplete locking when disabling buffered events
  tracing: Disable snapshot buffer when stopping instance tracers
  tracing: Stop current tracer when resizing buffer
  tracing: Always update snapshot buffer size

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