linux-kernel - Re: [PATCH v2] ring-buffer: Limit time with disabled interrupts in rb_check

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Message-ID: <20240703174410.099e8784@rorschach.local.home>
Date: Wed, 3 Jul 2024 17:44:10 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Petr Pavlu <petr.pavlu@...e.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ring-buffer: Limit time with disabled interrupts in
 rb_check_pages()

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