linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Date:   Tue, 16 Jan 2018 11:19:03 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On Tue 2018-01-16 13:47:16, Sergey Senozhatsky wrote:
> if you don't mind, let me fix the thing that I broke.
> that would be responsible. I believe I also must say the following:
>   Tetsuo, many thanks for reporting the issues for song long, and
>   sorry that it took quite a while to revert that change.
> 
> 8<====
> 
> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Subject: [PATCH] printk: never set console_may_schedule in console_trylock()
> 
> This patch, basically, reverts commit 6b97a20d3a79 ("printk:
> set may_schedule for some of console_trylock() callers").
> That commit was a mistake, it introduced a big dependency
> on the scheduler, by enabling preemption under console_sem
> in printk()->console_unlock() path, which is rather too
> critical. The patch did not significantly reduce the
> possibilities of printk() lockups, but made it possible to
> stall printk(), as has been reported by Tetsuo Handa [1].
> 
> Another issues is that preemption under console_sem also
> messes up with Steven Rostedt's hand off scheme, by making
> it possible to sleep with console_sem both in console_unlock()
> and in vprintk_emit(), after acquiring the console_sem
> ownership (anywhere between printk_safe_exit_irqrestore() in
> console_trylock_spinning() and printk_safe_enter_irqsave()
> in console_unlock()). This makes hand off less likely and,
> at the same time, may result in a significant amount of
> pending logbuf messages. Preempted console_sem owner makes
> it impossible for other CPUs to emit logbuf messages, but
> does not make it impossible for other CPUs to append new
> messages to the logbuf.
> 
> Reinstate the old behavior and make printk() non-preemptible.
> Should any printk() lockup reports arrive they must be handled
> in a different way.
> 
> [1] https://marc.info/?l=linux-mm&m=145692016122716
> Fixes: 6b97a20d3a79 ("printk: set may_schedule for some of console_trylock() callers")
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

IMHO, this is a step in the right direction.

Reviewed-by: Petr Mladek <pmladek@...e.com>

I'll wait for Steven's review and push this into printk.git.
I'll also add your Acks for the other patches.

Thanks for the patch and the various observations.

Best Regards,
Petr