Re: [BUG] tracing: Too many tries to read user space

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 10 Jul 2026 12:22:31 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> However, this seems a bit strange that we only checks the CPU-wide context
> switching in the loop. Instead, can we introduce a per-cpu sequence counter
> to per-cpu buffer, and check it? 

I originally tried this but found a situation that it fails:

  tbuf->sequence = 0;

	Task 1				Task 2
	------				------

   tbuf->sequence++;
   seq = tbuf->sequence; (seq = 1)

   preempt_enable();

   [schedule] ---------------------->

				  tbuf->sequence++;
				  seq = tbuf->sequence; (seq = 2);

				  preempt_enable();

				  copy_from_user(buffer);

	     <--------------------[schedule]

   copy_from_user(buffer);

   *** BUFFER NOW CORRUPTED ***

   [schedule] ---------------------->

				  preempt_disable();

				} while (tubf->sequence != seq); // tbuf->sequence == seq !!!!


This is why we use a CPU wide counter.

-- Steve