Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 20, 2023 at 03:54:14PM -0500, Mathieu Desnoyers wrote:
> When invoked from system call enter/exit instrumentation, accessing
> user-space data is a common use-case for tracers. However, tracepoints
> currently disable preemption around iteration on the registered
> tracepoint probes and invocation of the probe callbacks, which prevents
> tracers from handling page faults.
> 
> Extend the tracepoint and trace event APIs to allow defining a faultable
> tracepoint which invokes its callback with preemption enabled.
> 
> Also extend the tracepoint API to allow tracers to request specific
> probes to be connected to those faultable tracepoints. When the
> TRACEPOINT_MAY_FAULT flag is provided on registration, the probe
> callback will be called with preemption enabled, and is allowed to take
> page faults. Faultable probes can only be registered on faultable
> tracepoints and non-faultable probes on non-faultable tracepoints.
> 
> The tasks trace rcu mechanism is used to synchronize read-side
> marshalling of the registered probes with respect to faultable probes
> unregistration and teardown.

What is trace-trace rcu and why is it needed here? What's wrong with
SRCU ?