netdev - Re: [PATCH] net/bluetooth: fix erroneous use of bitmap_from

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Date:   Mon, 6 Jun 2022 23:00:13 -0700
From:   Luiz Augusto von Dentz <luiz.dentz@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Yury Norov <yury.norov@...il.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Guo Ren <guoren@...nel.org>,
        linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-csky@...r.kernel.org,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH] net/bluetooth: fix erroneous use of bitmap_from_u64()

Hi Linus,

On Sun, Jun 5, 2022 at 5:02 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Jun 5, 2022 at 11:51 AM Linus Torvalds
> <torvalds@...ux-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