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Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200123093604.GT14914@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Jan 2020 22:12:58 -0500
Cc:     Marco Elver <elver@google.com>, Will Deacon <will@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "paul E. McKenney" <paulmck@kernel.org>
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To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jan 23, 2020, at 4:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Wed, Jan 22, 2020 at 11:38:51PM +0100, Marco Elver wrote:
> 
>> If possible, decode and get the line numbers. I have observed a data
>> race in osq_lock before, however, this is the only one I have recently
>> seen in osq_lock:
>> 
>> read to 0xffff88812c12d3d4 of 4 bytes by task 23304 on cpu 0:
>>  osq_lock+0x170/0x2f0 kernel/locking/osq_lock.c:143
>> 
>> 	while (!READ_ONCE(node->locked)) {
>> 		/*
>> 		 * If we need to reschedule bail... so we can block.
>> 		 * Use vcpu_is_preempted() to avoid waiting for a preempted
>> 		 * lock holder:
>> 		 */
>> -->		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
>> 			goto unqueue;
>> 
>> 		cpu_relax();
>> 	}
>> 
>> where
>> 
>> 	static inline int node_cpu(struct optimistic_spin_node *node)
>> 	{
>> -->		return node->cpu - 1;
>> 	}
>> 
>> 
>> write to 0xffff88812c12d3d4 of 4 bytes by task 23334 on cpu 1:
>> osq_lock+0x89/0x2f0 kernel/locking/osq_lock.c:99
>> 
>> 	bool osq_lock(struct optimistic_spin_queue *lock)
>> 	{
>> 		struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
>> 		struct optimistic_spin_node *prev, *next;
>> 		int curr = encode_cpu(smp_processor_id());
>> 		int old;
>> 
>> 		node->locked = 0;
>> 		node->next = NULL;
>> -->		node->cpu = curr;
>> 
> 
> Yeah, that's impossible. This store happens before the node is
> published, so no matter how the load in node_cpu() is shattered, it must
> observe the right value.

Marco, any thought on how to do something about this? The worry is that
too many false positives like this will render the tool usefulness as a
general debug option.