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

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Date:	Wed, 3 Feb 2016 09:04:47 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
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, peter@...leysoftware.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] lock/semaphore: Avoid an unnecessary deadlock within
 up()


* Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> 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.

Well, but this patch of yours only affects the semaphore code, so it does not 
change the logbuf_lock situation.

Furthermore, logbuf_lock already has recursion protection:

        /*
         * Ouch, printk recursed into itself!
         */
        if (unlikely(logbuf_cpu == this_cpu)) {

so it should not be possible to re-enter the printk() logbuf_lock critical section 
from the spinlock code. (There are other ways to get the logbuf_lock - if those 
are still triggerable then they should be fixed.)

In any case, recursion protection is generally done in the debugging facilities 
trying to behave lockless.

Thanks,

	Ingo