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Date:   Wed, 22 Jan 2020 23:38:51 +0100
From:   Marco Elver <elver@google.com>
To:     Qian Cai <cai@lca.pw>
Cc:     Will Deacon <will@kernel.org>, mingo@redhat.com,
        peterz@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
Message-ID: <20200122223851.GA45602@google.com>
References: <20200122165938.GA16974@willie-the-truck>
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On Wed, 22 Jan 2020, Qian Cai wrote:

>=20
>=20
> > On Jan 22, 2020, at 11:59 AM, Will Deacon <will@kernel.org> wrote:
> >=20
> > I don't understand this; 'next' is a local variable.
> >=20
> > Not keen on the onslaught of random "add a READ_ONCE() to shut the
> > sanitiser up" patches we're going to get from kcsan :(
>=20
> My fault. I suspect it is node->next. I=E2=80=99ll do a bit more testing =
to confirm.

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:
=C2=A0osq_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 =3D this_cpu_ptr(&osq_node);
		struct optimistic_spin_node *prev, *next;
		int curr =3D encode_cpu(smp_processor_id());
		int old;

		node->locked =3D 0;
		node->next =3D NULL;
-->		node->cpu =3D curr;


Thanks,
-- Marco