linux-kernel - Re: [PATCH] ring-buffer: Synchronize ring_buffer_subbuf_order_set() with rb_check

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Message-ID: <20241011130922.60cf66cf@gandalf.local.home>
Date: Fri, 11 Oct 2024 13:09:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Petr Pavlu <petr.pavlu@...e.com>
Subject: Re: [PATCH] ring-buffer: Synchronize ring_buffer_subbuf_order_set()
 with rb_check_pages()

On Fri, 11 Oct 2024 11:28:50 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The rb_check_pages() scans the ring buffer sub buffers to make sure they
> are valid after an update. But locks are released during the update to not
> hold preemption for too long.
> 
> The ring_buffer_subbuf_order_set() updates the counter used by
> rb_check_pages() without any locks. But it also updates the pages. Even
> though it is likely that the buffer->mutex is enough to protect this, but
> since rb_check_pages() uses the cpu_buffer->reader_lock for
> synchronization, take that lock as well when updating the pages and
> counter in ring_buffer_subbuf_order_set().
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

I rejected this patch as I believe the version Petr has is required.

-- Steve

> ---
> Note, this is based on top of:
> 
>   https://lore.kernel.org/linux-trace-kernel/20240715145141.5528-1-petr.pavlu@suse.com/
> 
>  kernel/trace/ring_buffer.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 696d422d5b35..0672df07b599 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6774,6 +6774,7 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  	}
>  
>  	for_each_buffer_cpu(buffer, cpu) {
> +		unsigned long flags;
>  
>  		if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  			continue;
> @@ -6800,11 +6801,15 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
>  						     struct buffer_page, list);
>  		list_del_init(&cpu_buffer->reader_page->list);
>  
> +		/* Synchronize with rb_check_pages() */
> +		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> +
>  		/* The cpu_buffer pages are a link list with no head */
>  		cpu_buffer->pages = cpu_buffer->new_pages.next;
>  		cpu_buffer->new_pages.next->prev = cpu_buffer->new_pages.prev;
>  		cpu_buffer->new_pages.prev->next = cpu_buffer->new_pages.next;
>  		cpu_buffer->cnt++;
> +		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
>  
>  		/* Clear the new_pages list */
>  		INIT_LIST_HEAD(&cpu_buffer->new_pages);