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Date: Thu, 26 Mar 2015 17:03:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Jeff Layton <jlayton@poochiereds.net>, Ingo Molnar <mingo@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Thomas Gleixner <tglx@linutronix.de>, Andi Kleen <andi@firstfloor.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] lglock: Use spinlock_t instead of arch_spinlock_t
Message-ID: <20150326160325.GA21418@twins.programming.kicks-ass.net>
References: <1427382128-12541-1-git-send-email-daniel.wagner@bmw-carit.de>
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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.

Second: preempt_count_add(), spin_lock() does preempt_disable(), with
enough CPUs you'll overflow the preempt counter (255).