Re: [PATCH] net/bluetooth: fix erroneous use of bitmap_from_u64()

From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yury Norov <yury.norov@gmail.com>,
	Marcel Holtmann <marcel@holtmann.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	"David S . Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Guo Ren <guoren@kernel.org>,
	linux-bluetooth <linux-bluetooth@vger.kernel.org>,
	Netdev <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-csky@vger.kernel.org,
	Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	David Hildenbrand <david@redhat.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Sven Schnelle <svens@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [PATCH] net/bluetooth: fix erroneous use of bitmap_from_u64()
Date: Mon, 6 Jun 2022 23:00:13 -0700	[thread overview]
Message-ID: <CABBYNZJfqAU-o7f9HhLCgTmL46WfwNQbM5NsCACsVVDLACMLYw@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wicWxvuaL7GCj+1uEvpvpntdcB=AHot_h3j4wpenwyZ2Q@mail.gmail.com>

Hi Linus,

On Sun, Jun 5, 2022 at 5:02 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sun, Jun 5, 2022 at 11:51 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > *Most* of the accesses to those connection flags seem to be with
> > hci_dev_lock() held, and the ones that aren't can't possibly depend on
> > atomicity since those things are currently copied around with random
> > other "copy bitmaps" functions.
>
> I've committed that patch as commit e1cff7002b71 ("bluetooth: don't
> use bitmaps for random flag accesses").
>
> That basically ends up reverting
>
>   a9a347655d22 ("Bluetooth: MGMT: Add conditions for setting
> HCI_CONN_FLAG_REMOTE_WAKEUP")
>   6126ffabba6b ("Bluetooth: Introduce HCI_CONN_FLAG_DEVICE_PRIVACY device flag")
>
> which did horrible things, and would end up overwriting the end of the
> bitmap allocation on 32-bit architectures.
>
> Luiz, if the reason for the change to use a bitmap type was because of
> some atomicity concerns, then you can do that by
>
>  (a) change 'hci_conn_flags_t' to be an 'atomic_t' instead of a 'u8'
>
>  (b) change the regular accesses to it to use 'atomic_read/write()'
>
>  (c) change the "bitfield" operations to use 'atomic_or/andnot()'
>
> but honestly, when it used to mix atomic ops
> (set_bit/clear_bit/test_bit) with random non-atomic users
> (bitmap_from_u64(), bitmap_to_arr32() etc) it was never atomic to
> begin with.
>
> Regardless, trying to use bitmaps for this was absolutely not the
> right thing to ever do. It looks like gcc randomly started complaining
> when 'bitmap_from_u64()' was changed, but it was buggy before that
> too.

Right, thanks for fixing it. About some of the changes perhaps we
should use BIT when declaring values in enum hci_conn_flags?

-- 
Luiz Augusto von Dentz