On Wed, 12 Aug 2020 14:53:22 -0600
dann frazier <dann.frazier@xxxxxxxxxxxxx> wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
>
> commit 7ef282e05132d56b6f6b71e3873f317664bea78b upstream
>
> If a process has the trace_pipe open on a trace_array, the current tracer
> for that trace array should not be changed. This was original enforced by a
> global lock, but when instances were introduced, it was moved to the
> current_trace. But this structure is shared by all instances, and a
> trace_pipe is for a single instance. There's no reason that a process that
> has trace_pipe open on one instance should prevent another instance from
> changing its current tracer. Move the reference counter to the trace_array
> instead.
>
> This is marked as "Fixes" but is more of a clean up than a true fix.
> Backport if you want, but its not critical.
>
A note to stable maintainers. I originally thought this was just a
clean up, but it was then found that it actually does fix a bug.
(See below)
-- Steve
> Fixes: cf6ab6d9143b1 ("tracing: Add ref count to tracer for when they are being read by pipe")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 5.4.x
> [Resolved conflict in __remove_instance()]
> Signed-off-by: dann frazier <dann.frazier@xxxxxxxxxxxxx>
> ---
> This addresses an issue we've seen with users trying to change
> current_tracer when they happen to have rasdaemon installed.
> rasdaemon uses the trace_pipe interface at runtime, which therefore
> blocks changing the current tracer. But of course, unless
> you know about rasdaemon internals, it isn't exactly an obvious
> failure mode.
>
> Conflict in __remove_instance() was due to a change in reference counter
> semantics introduced later in v5.5 via commit 288797871473
> "tracing: Adding new functions for kernel access to Ftrace instances".
> Resolved by leaving the first test in the if statement as it was in v5.4.
>
> kernel/trace/trace.c | 12 ++++++------
> kernel/trace/trace.h | 2 +-
> 2 files changed, 7 insertions(+), 7 deletions(-)
>
