linux-kernel - Re: [PATCH v2] lock/semaphore: Avoid an unnecessary deadlock within up()

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Date:	Wed, 3 Feb 2016 16:42:23 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Byungchul Park <byungchul.park@....com>, willy@...ux.intel.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	akinobu.mita@...il.com, jack@...e.cz,
	sergey.senozhatsky.work@...il.com, peter@...leysoftware.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] lock/semaphore: Avoid an unnecessary deadlock within
 up()

On (02/03/16 08:28), Ingo Molnar wrote:
[..]
> So why not move printk away from semaphores? Semaphores are classical constructs 
> that have legacies and are somewhat non-obvious to use, compared to modern, 
> simpler locking primitives. I'd not touch their implementation, unless we are 
> absolutely sure this is a safe optimization.

semaphore's spin_lock is not the only spin lock that printk acquires. it also takes the
logbuf_lock (and different locks in console drivers (up to console driver)).

Jan Kara posted a patch that offloads printing job (console_trylock()-console_unlock())
from printk() call (when printk can offload it). so semaphore and console driver's locks
will go away (mostly) with Jan's patch. logbug spin_lock, however, will stay.

	-ss