linux-kernel - Re: [PATCH] lglock: Use spinlock_t instead of arch_spinlock

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Date:	Tue, 31 Mar 2015 11:17:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Daniel Wagner <daniel.wagner@...-carit.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Ingo Molnar <mingo@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <andi@...stfloor.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lglock: Use spinlock_t instead of arch_spinlock_t


* Daniel Wagner <daniel.wagner@...-carit.de> wrote:

> On 03/26/2015 05:03 PM, Peter Zijlstra wrote:
> > On Thu, Mar 26, 2015 at 04:02:08PM +0100, Daniel Wagner wrote:
> >> @@ -67,9 +67,9 @@ void lg_global_lock(struct lglock *lg)
> >>  	preempt_disable();
> >>  	lock_acquire_exclusive(&lg->lock_dep_map, 0, 0, NULL, _RET_IP_);
> >>  	for_each_possible_cpu(i) {
> >> -		arch_spinlock_t *lock;
> >> +		spinlock_t *lock;
> >>  		lock = per_cpu_ptr(lg->lock, i);
> >> -		arch_spin_lock(lock);
> >> +		spin_lock(lock);
> >>  	}
> >>  }
> > 
> > Nope, that'll blow up in two separate places.
> > 
> > One: lockdep, it can only track a limited number of held locks, and it
> > will further report a recursion warning on the 2nd cpu.
> 
> I was wondering why I haven't seen it explode. As it turns out I haven't
> looked closely enough at dmesg:
> 
> [  +0.001231] BUG: MAX_LOCK_DEPTH too low!
> [  +0.000092] turning off the locking correctness validator.

Yeah, we try really hard to not crash the kernel from debugging code, 
whenever we can avoid it! That sometimes creates a false sense of good 
kernel health.

Thanks,

	Ingo
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