Re: [PATCH v2] ring-buffer: Limit time with disabled interrupts in rb_check_pages()

[Steven Rostedt <rostedt@goodmis.org>]

On Wed,  3 Jul 2024 09:53:14 +0200
Petr Pavlu <petr.pavlu@suse.com> wrote:

> The function rb_check_pages() validates the integrity of a specified
> per-CPU tracing ring buffer. It does so by traversing the underlying
> linked list and checking its next and prev links.
> 
> To guarantee that the list isn't modified during the check, a caller
> typically needs to take cpu_buffer->reader_lock. This prevents the check
> from running concurrently, for example, with a potential reader which
> can make the list temporarily inconsistent when swapping its old reader
> page into the buffer.
> 
> A problem with this approach is that the time when interrupts are
> disabled is non-deterministic, dependent on the ring buffer size. This
> particularly affects PREEMPT_RT because the reader_lock is a raw
> spinlock which doesn't become sleepable on PREEMPT_RT kernels.
> 
> Modify the check so it still attempts to traverse the entire list, but
> gives up the reader_lock between checking individual pages. Introduce
> for this purpose a new variable ring_buffer_per_cpu.pages_era which is

I'm dumb. What's an "era"?

-- Steve

> bumped any time the list is modified. The value is used by
> rb_check_pages() to detect such a change and restart the check.
> 
> Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>