Re: [PATCH v2 3/8] tracing: Have syscall trace events read user space string

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 23, 2025 at 09:05:00AM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@goodmis.org>
> 
> As of commit 654ced4a1377 ("tracing: Introduce tracepoint_is_faultable()")
> system call trace events allow faulting in user space memory. Have some of
> the system call trace events take advantage of this.
> 
> Introduce a way to read strings that are nul terminated into the trace
> event. The way this is accomplished is by creating a per CPU temporary
> buffer that is used to read unsafe user memory.
> 
> When a syscall trace event needs to read user memory, it reads the per CPU
> schedule switch counter. It then disables migration and enables
> preemption, copies the user space memory into this buffer, then disables
> preemption again. It reads the per CPU schedule switch counter again and
> if it matches it considers the buffer is valid.  Otherwise it needs to try
> again. This is similar to how seqcount works, but uses the per CPU context
> switch counter as the sequence counter.

And you can't just allocate memory and not bother with the
migrate_disable() and retry stuff because?