linux-kernel - Re: [PATCH v5 4/4] printk: use the lockless ringbuffer

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Date:   Sat, 18 Jul 2020 14:10:53 +0200
From:   Marco Elver <elver@...gle.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/4] printk: use the lockless ringbuffer

On Thu, Jul 09, 2020 at 03:29PM +0206, John Ogness wrote:
> Replace the existing ringbuffer usage and implementation with
> lockless ringbuffer usage. Even though the new ringbuffer does not
> require locking, all existing locking is left in place. Therefore,
> this change is purely replacing the underlining ringbuffer.
> 
> Changes that exist due to the ringbuffer replacement:
> 
> - The VMCOREINFO has been updated for the new structures.
> 
> - Dictionary data is now stored in a separate data buffer from the
>   human-readable messages. The dictionary data buffer is set to the
>   same size as the message buffer. Therefore, the total required
>   memory for both dictionary and message data is
>   2 * (2 ^ CONFIG_LOG_BUF_SHIFT) for the initial static buffers and
>   2 * log_buf_len (the kernel parameter) for the dynamic buffers.
> 
> - Record meta-data is now stored in a separate array of descriptors.
>   This is an additional 72 * (2 ^ (CONFIG_LOG_BUF_SHIFT - 5)) bytes
>   for the static array and 72 * (log_buf_len >> 5) bytes for the
>   dynamic array.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>

It seems this causes a regression observed at least with newline-only
printks. I noticed this during -next testing because various debugging
tools (K*SAN, lockdep, etc.) use e.g. pr_{err,warn,info}("\n") to format
reports.

Without wanting to wait for a report from one of these debugging tools,
a simple reproducer is below. Without this patch, the expected newline
is printed.

Thanks,
-- Marco

------ >8 ------

--- a/init/main.c
+++ b/init/main.c
@@ -1039,6 +1039,10 @@ asmlinkage __visible void __init start_kernel(void)
 	sfi_init_late();
 	kcsan_init();
 
+	pr_info("EXPECT BLANK LINE --vv\n");
+	pr_info("\n");
+	pr_info("EXPECT BLANK LINE --^^\n");
+
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();