Messages in this thread | | | Date | Wed, 17 Feb 2016 11:40:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within up() |
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On Wed, Feb 17, 2016 at 06:11:51PM +0900, Byungchul Park wrote: > change from v2 to v3 > - the way to solve it in v2 is racy, so I changed the approach entirely. > - just make semaphore's trylock use spinlock's trylock. > > change from v1 to v2 > - remove unnecessary overhead by the redundant spin(un)lock. > > Since I faced a infinite recursive printk() bug, I've tried to propose > patches the title of which is "lib/spinlock_debug.c: prevent a recursive > cycle in the debug code". But I noticed the root problem cannot be fixed > by that, through some discussion thanks to Sergey and Peter. So I focused > on preventing the deadlock. > > -----8<----- > From 43e029ca920890ac644e30d873be69cf5d01efdb Mon Sep 17 00:00:00 2001 > From: Byungchul Park <byungchul.park@lge.com> > Date: Wed, 17 Feb 2016 17:22:18 +0900 > Subject: [PATCH v3] lock/semaphore: Avoid an unnecessary deadlock within up() > > One of semaphore acquisition functions, down_trylock() is implemented > using raw_spin_lock_irqsave(&sem->lock) even though it's enough to use > raw_spin_trylock_irqsave(). Furthermore, using raw_spin_lock_irqsave() > can cause a unnecessary deadlock as described below. Just make it use > the spinlock trylock to implement the semaphore trylock so that we can > avoid the unnecessary deadlock happened. > > The scenario the bad thing can happen is, > > printk > console_trylock > console_unlock > up_console_sem > up > raw_spin_lock_irqsave(&sem->lock, flags) > __up > wake_up_process > try_to_wake_up > raw_spin_lock_irqsave(&p->pi_lock) > __spin_lock_debug > spin_dump > printk > console_trylock > raw_spin_lock_irqsave(&sem->lock, flags) > >>> DEADLOCK <<< > > Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Mucking with the semaphore implementation just because printk() is terminally broken shite really doesn't fly.
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