linux-kernel - Re: [PATCH] printk: clear console_may

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Message-ID: <20160112135059.GA670@swordfish>
Date:	Tue, 12 Jan 2016 22:50:59 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, lkp@...org,
	LKML <linux-kernel@...r.kernel.org>,
	Kyle McMartin <kyle@...nel.org>,
	Dave Jones <davej@...emonkey.org.uk>, Jan Kara <jack@...e.com>,
	Calvin Owens <calvinowens@...com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	kernel test robot <ying.huang@...ux.intel.com>,
	kernel-team@...com,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] printk: clear console_may_schedule on panic flushing

On (01/11/16 13:59), Andrew Morton wrote:
> > Commit "printk: do cond_resched() between lines while outputting to
> > consoles" made console flushing perform cond_resched() after each line
> > if the context allows as determined by whether the console lock was
> > acquired with console_lock().  The condition is carried in
> > console_may_schedule.
> > 
> > During panic, console lock status is ignored and the messages are
> > forcifully flushed which is implemented by performing
> > console_trylock(); console_unlock(); sequence ignoring whether trylock
> > succeeds or fails.  This means that the emergency flushing, after
> > trylock failure, may enter flushing path with console_may_schedule set
> > from the actual holder.
> > 
> > As a system may panic from any context, this can lead to
> > cond_resched() being invoked from a non-sleepable context triggering
> > an extra warning dump while panicking which is noisy and can be
> > confusing.  Besides, even when panicking from a sleepable context, we
> > don't want to be yielding during emergency message dumping.
> > 
> > Currently, the emergency dumping is opencoded in panic().  This patch
> > replaces the open coded implementation with a new function,
> > console_flush_on_panic() and makes it explicitly clear
> > console_may_schedule before starting the emergency flushing.
> > 
> 
> Got that, thanks.  Because the patch has significant information
> content I'd normally make it a standalone thing, but as it's destined
> for -stable I think I'll scrunch it into
> printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch
> and merge the changelogs.
> 
> You didn't comment on Sergey's observations
> (https://lkml.org/lkml/2015/12/2/1192), but I'm believing this is
> a separate issue and that this patch is still good.

Thanks for Cc-ing.

Yes, this problem is different.


FWIW,
what I ended up implementing in my private builds is a bit different
thing -- I added a new console_panic_mode() function which basically
calls zap_locks(). The reason I did it this way was that console_unlock()
can potentially have other locks in it some day, not just `console_sem'.
For example, like in one of Jan's deferred printk implementations. Sort
of a defensive move to avoid possible problems.

	-ss