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    SubjectRe: [Linux-fbdev-devel] [2.6.29-rc2] fb_mmap: circular locking dependency on hibernation
    On Sat, 31 Jan 2009, Andrea Righi wrote:
    > On 2009-01-30 05:15, Andrey Borzenkov wrote:
    > > On 29 of January 2009 12:10:11 Geert Uytterhoeven wrote:
    > >> On Tue, 27 Jan 2009, Andrey Borzenkov wrote:
    > fbcon: avoid circular locking dependency between fb_info->lock and mm->mmap_sem
    >
    > In fbcon notifier the handler for FB_EVENT_SET_CONSOLE_MAP doesn't need
    > to hold fb_info->lock.
    >
    > Simply unlock it before calling set_con2fb_map(), that could try to
    > acquire mm->mmap_sem to avoid a circular locking dependency with
    > fb_mmap() (that acquires mm->mmap_sem -> fb_info-lock).

    However, set_con2fb_map() accesses the array of fb_info pointers
    registered_fb[], which seems to be a bit unsafe now the BKL is no longer held.

    > Reported-by: Andrey Borzenkov <arvidjaar@mail.ru>
    > Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
    > ---
    > drivers/video/console/fbcon.c | 6 ++++++
    > 1 files changed, 6 insertions(+), 0 deletions(-)
    >
    > diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
    > index 1657b96..81fc2f7 100644
    > --- a/drivers/video/console/fbcon.c
    > +++ b/drivers/video/console/fbcon.c
    > @@ -3169,9 +3169,15 @@ static int fbcon_event_notify(struct notifier_block *self,
    > ret = fbcon_fb_unregistered(info);
    > break;
    > case FB_EVENT_SET_CONSOLE_MAP:
    > + /*
    > + * XXX: info is not used here, we can unlock it to avoid
    > + * circular locking with &current->mm->mmap_sem in fb_mmap().
    > + */
    > + unlock_fb_info(info);
    > con2fb = event->data;
    > ret = set_con2fb_map(con2fb->console - 1,
    > con2fb->framebuffer, 1);
    > + lock_fb_info(info);
    > break;
    > case FB_EVENT_GET_CONSOLE_MAP:
    > con2fb = event->data;
    >

    Gr{oetje,eeting}s,

    Geert

    --
    Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

    In personal conversations with technical people, I call myself a hacker. But
    when I'm talking to journalists I just say "programmer" or something like that.
    -- Linus Torvalds


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