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

From: Michal Hocko <mhocko@suse.com>
To: Petr Mladek <pmladek@suse.com>
Cc: John Ogness <john.ogness@linutronix.de>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] printk: Enough to disable preemption in printk deferred context
Date: Wed, 19 Apr 2023 14:14:15 +0200	[thread overview]
Message-ID: <ZD/bF6Tev9ujlkBY@dhcp22.suse.cz> (raw)
In-Reply-To: <ZD_Yka6MJ_-HOKpj@alley>

On Wed 19-04-23 14:03:29, Petr Mladek wrote:
> On Wed 2023-04-19 11:11:52, John Ogness wrote:
> > On 2023-04-19, Petr Mladek <pmladek@suse.com> wrote:
> > > it is safe to interrupt one writer now. The preemption still
> > > has to be disabled because the deferred context is CPU specific.
> > 
> > Really it is enough to disable migration.
> 
> True. But it gets too far to my taste. As you describe below.
> It affects all printk's on the CPU.
> 
> Sigh, even the enabled intrrupts might be questionable. For example,
> when the iterrupt is from a watchdog and want's to report a stall.
> 
> > We need to keep an eye on the usage of this function. By allowing
> > interrupts and preemption, it means that other printk's on that CPU will
> > also be deferred if the context interrupted within the deferred block.
> 
> A solution would be to make this more clear in the comment.
> Something like:
> 
> /*
>  * The printk_deferred_enter/exit macros are available only as a hack.
>  * They define a per-CPU context where all printk console printing is
>  * deferred because it might cause a deadlock otherwise.
>  *
>  * The API user is responsible for calling the corresponding enter/exit
>  * pair on the same CPU. It is highly recommended to use them only in
>  * a context with interrupts disabled. Otherwise, other unrelated
>  * printk() calls might be deferred when they interrupt/preempt
>  * the deferred code section.
>  */

This looks better but I would argue that as a potential user of those I
would appreciate less internal implementation details and more
instructions on how/when to use it. What about something like this?

/*
 * The printk_deferred_enter/exit macros are available only as a hack
 * for code paths which are prone to printk related deadlocks. That
 * might be caused by locking context around printk which can be reused
 * directly or indirectly by lower level printk infrastructure.
 * 
 * Any new use of these MUST be consulted with printk maintainers as the
 * use might have some unexpected side effects on the printk
 * infrastructure.
 *
 * enter/exit pair must be called from the same CPU without any
 * preemption in between.
 */
-- 
Michal Hocko
SUSE Labs