linux-kernel - Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of console

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Date:	Mon, 18 Jan 2016 17:17:44 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.com>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH -next 2/2] printk: set may_schedule for some of
 console_trylock callers

On Sun 2016-01-17 23:23:32, Sergey Senozhatsky wrote:
>  console_unlock() allows to cond_resched() if its caller has
> set `console_may_schedule' to 1 (this functionality present
> since commit 'printk: do cond_resched() between lines while
> outputting to consoles').
> 
> The rules are:
> -- console_lock() always sets `console_may_schedule' to 1
> -- console_trylock() always sets `console_may_schedule' to 0
> 
> However, console_trylock() callers (among them is printk()) are
> not necessarily executing in atomic contexts, and some of them
> can cond_resched() in console_unlock(). So console_trylock()
> can set `console_may_schedule' to 0 only if cond_resched() is
> invalid in the current context, and set it to 1 otherwise.
> 
> The patch also drops explicit preempt_disable()/preempt_enable()
> calls in vprintk_emit().

I do not see any explanation why it is safe to remove these
calls in this patch. If they were required only because of the
can_use_console() call, it would make sense to move this change
to the previous patch. The previous patch moved the
can_use_console() to locations protected by lockbuf_lock that
have disabled preemption because of the lock.


> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
> V2: rcu_preempt_depth() on preempt RCU kernels
> 
>  kernel/printk/printk.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index ae641d7..9ac05e0 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1777,20 +1777,12 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	if (!in_sched) {
>  		lockdep_off();
>  		/*
> -		 * Disable preemption to avoid being preempted while holding
> -		 * console_sem which would prevent anyone from printing to
> -		 * console
> -		 */
> -		preempt_disable();
> -
> -		/*
>  		 * Try to acquire and then immediately release the console
>  		 * semaphore.  The release will print out buffers and wake up
>  		 * /dev/kmsg and syslog() users.
>  		 */
>  		if (console_trylock_for_printk())
>  			console_unlock();
> -		preempt_enable();
>  		lockdep_on();
>  	}
>  
> @@ -2123,7 +2115,10 @@ int console_trylock(void)
>  		return 0;
>  	}
>  	console_locked = 1;
> -	console_may_schedule = 0;
> +	console_may_schedule = !(oops_in_progress ||
> +			irqs_disabled() ||
> +			in_atomic() ||
> +			rcu_preempt_depth());

Is it safe to call cond_resched() when the CPU is not online
and preemption is enabled, please? Your previous patch
suggests that this situation might happen. I guess that
it might break the CPU initialization code.

Best Regards,
Petr


>  	return 1;
>  }
>  EXPORT_SYMBOL(console_trylock);
> -- 
> 2.7.0
>