linux-kernel - Re: [PATCH] printk: Enough to disable preemption in printk deferred context

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Date:   Wed, 19 Apr 2023 10:23:33 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: Enough to disable preemption in printk deferred
 context

On Wed 19-04-23 09:42:10, Petr Mladek wrote:
> The comment above printk_deferred_enter()/exit() definition claims
> that it can be used only when interrupts are disabled.
> 
> It was required by the original printk_safe_log_store() implementation.
> The code provided lockless synchronization between a single writer and
> a single reader. The interrupt and the normal context shared the same
> buffer.
> 
> The commit 93d102f094be ("printk: remove safe buffers") removed
> these temporary buffers. Instead, the messages are stored directly into
> the new global lockless buffer which supports multiple parallel writers.
> 
> As a result, it is safe to interrupt one writer now. The preemption still
> has to be disabled because the deferred context is CPU specific.

Thanks for the clarification and explanation.

> Fixes: 93d102f094be ("printk: remove safe buffers")

Is this a fix though? I would expect some users to be changed from irq
to preempt to disabling to be considered a fix.

> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  include/linux/printk.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 8ef499ab3c1e..915a321b491e 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -161,7 +161,7 @@ extern void __printk_safe_enter(void);
>  extern void __printk_safe_exit(void);
>  /*
>   * The printk_deferred_enter/exit macros are available only as a hack for
> - * some code paths that need to defer all printk console printing. Interrupts
> + * some code paths that need to defer all printk console printing. Preemption
>   * must be disabled for the deferred duration.
>   */
>  #define printk_deferred_enter __printk_safe_enter
> -- 
> 2.35.3

-- 
Michal Hocko
SUSE Labs