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From: Elizabeth Figura <zfigura@codeweavers.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 Jonathan Corbet <corbet@lwn.net>, Shuah Khan <shuah@kernel.org>,
 linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
 wine-devel@winehq.org,
 =?ISO-8859-1?Q?Andr=E9?= Almeida <andrealmeid@igalia.com>,
 Wolfram Sang <wsa@kernel.org>, Arkadiusz Hiler <ahiler@codeweavers.com>,
 Andy Lutomirski <luto@kernel.org>, linux-doc@vger.kernel.org,
 linux-kselftest@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>,
 Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
 Waiman Long <longman@redhat.com>, Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH v4 02/27] ntsync: Introduce NTSYNC_IOC_WAIT_ALL.
Date: Wed, 17 Apr 2024 15:03:05 -0500
Message-ID: <3479054.QJadu78ljV@camazotz>
In-Reply-To: <20240417113703.GL30852@noisy.programming.kicks-ass.net>
References:
 <20240416010837.333694-1-zfigura@codeweavers.com>
 <20240416010837.333694-3-zfigura@codeweavers.com>
 <20240417113703.GL30852@noisy.programming.kicks-ass.net>
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On Wednesday, 17 April 2024 06:37:03 CDT Peter Zijlstra wrote:
> On Mon, Apr 15, 2024 at 08:08:12PM -0500, Elizabeth Figura wrote:
> > +	if (atomic_read(&sem->all_hint) > 0) {
> > +		spin_lock(&dev->wait_all_lock);
> > +		spin_lock_nest_lock(&sem->lock, &dev->wait_all_lock);
> > =20
> > +		prev_count =3D sem->u.sem.count;
> > +		ret =3D post_sem_state(sem, args);
> > +		if (!ret) {
> > +			try_wake_all_obj(dev, sem);
> > +			try_wake_any_sem(sem);
> > +		}
> > =20
> > +		spin_unlock(&sem->lock);
> > +		spin_unlock(&dev->wait_all_lock);
> > +	} else {
> > +		spin_lock(&sem->lock);
> > +
> > +		prev_count =3D sem->u.sem.count;
> > +		ret =3D post_sem_state(sem, args);
> > +		if (!ret)
> > +			try_wake_any_sem(sem);
> > +
> > +		spin_unlock(&sem->lock);
> > +	}
> > =20
> >  	if (!ret && put_user(prev_count, user_args))
> >  		ret =3D -EFAULT;
>=20
> vs.
>=20
> > +	/* queue ourselves */
> > +
> > +	spin_lock(&dev->wait_all_lock);
> > +
> > +	for (i =3D 0; i < args.count; i++) {
> > +		struct ntsync_q_entry *entry =3D &q->entries[i];
> > +		struct ntsync_obj *obj =3D entry->obj;
> > +
> > +		atomic_inc(&obj->all_hint);
> > +
> > +		/*
> > +		 * obj->all_waiters is protected by dev->wait_all_lock rather
> > +		 * than obj->lock, so there is no need to acquire obj->lock
> > +		 * here.
> > +		 */
> > +		list_add_tail(&entry->node, &obj->all_waiters);
> > +	}
>=20
> This looks racy, consider:
>=20
> 	atomic_read(all_hints) /* 0 */
>=20
> 				spin_lock(wait_all_lock)
> 				atomic_inc(all_hint)	/* 1 */
> 				list_add_tail()
>=20
> 	spin_lock(sem->lock)
> 	/* try_wake_all_obj() missing */
>=20
>=20
>=20
>=20
> I've not yet thought about if this is harmful or not, but if not, it
> definitely needs a comment.
>=20
> Anyway, I need a break, maybe more this evening.

Ach. I wrote this with the idea that the race isn't meaningful, but
looking at it again you're right=E2=80=94there is a harmful race here.

I think it should be fixable by moving the atomic_read inside the lock,
though.