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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc:     kernel test robot <lkp@intel.com>, kbuild-all@lists.01.org,
        linux-kernel@vger.kernel.org,
        Daniel Borkmann <daniel@iogearbox.net>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: kernel/bpf/devmap.c:1030:40: sparse: sparse: cast removes
 address space '__rcu' of expression
In-Reply-To: <20220601003834.ilvx2pik672yxuxt@mail>
References: <202205222029.xpW3PM1y-lkp@intel.com> <87y1yspmmh.fsf@toke.dk>
 <20220601003834.ilvx2pik672yxuxt@mail>
Date:   Wed, 01 Jun 2022 12:26:27 +0200
Message-ID: <87h7547k8c.fsf@toke.dk>
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Luc Van Oostenryck <luc.vanoostenryck@gmail.com> writes:

> On Mon, May 23, 2022 at 12:30:14PM +0200, Toke H=C3=B8iland-J=C3=B8rgense=
n wrote:
>> kernel test robot <lkp@intel.com> writes:
>>=20
>> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux=
.git master
>> > head:   eaea45fc0e7b6ae439526b4a41d91230c8517336
>> > commit: 782347b6bcad07ddb574422e01e22c92e05928c8 xdp: Add proper __rcu=
 annotations to redirect map entries
>> > date:   11 months ago
>> > config: ia64-randconfig-s031-20220522 (https://download.01.org/0day-ci=
/archive/20220522/202205222029.xpW3PM1y-lkp@intel.com/config)
>> > compiler: ia64-linux-gcc (GCC) 11.3.0
>>=20
>> Hmm, so this is ia64-only? Some kind of macro breakage? Paul, any ideas?
>
> Hi,
>
> It's surely IA64's cmpxchg() which contains lines like:
> 	_r_ =3D ia64_cmpxchg8_##sem((__u64 *) ptr, new, _o_);=20

Oh, right. Hmm, well, if the cmpxchg does an internal cast that
complicates things a bit. My immediate thought was to move the
unrcu_pointer() inside the calls to cmpxchg(), like:

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 980f8928e977..3b6dc6d34177 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -1098,7 +1098,7 @@ static int dev_map_notification(struct notifier_block=
 *notifier,
                                dev =3D rcu_dereference(dtab->netdev_map[i]=
);
                                if (!dev || netdev !=3D dev->dev)
                                        continue;
-                               odev =3D unrcu_pointer(cmpxchg(&dtab->netde=
v_map[i], RCU_INITIALIZER(dev), NULL));
+                               odev =3D cmpxchg(unrcu_pointer(&dtab->netde=
v_map[i]), dev, NULL);
                                if (dev =3D=3D odev)
                                        call_rcu(&dev->rcu,
                                                 __dev_map_entry_free);


But that seems to confuse sparse because these are ptr-to-ptr
constructs:

kernel/bpf/devmap.c:1101:40: error: incompatible types in comparison expres=
sion (different address spaces):
kernel/bpf/devmap.c:1101:40:    struct bpf_dtab_netdev [noderef] __rcu *[no=
deref] __rcu *
kernel/bpf/devmap.c:1101:40:    struct bpf_dtab_netdev [noderef] __rcu **
kernel/bpf/devmap.c:1101:40: error: incompatible types in comparison expres=
sion (different address spaces):
kernel/bpf/devmap.c:1101:40:    struct bpf_dtab_netdev [noderef] __rcu *[no=
deref] __rcu *
kernel/bpf/devmap.c:1101:40:    struct bpf_dtab_netdev [noderef] __rcu **

which I'm not sure how to fix. And not really sure if it's the
semantically right thing to do either in this case...

-Toke