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SubjectRe: [PATCH] fbmem: fix race condition between register_framebuffer() and fb_open()
On Tue, 24 May 2011 Fabio Erculiani <lxnay@sabayon.org> wrote:
> register_framebuffer() acquires registration_lock, then calls
> do_register_framebuffer() which calls fb_notifier_call_chain()
> which calls fb_open() which calls get_fb_info() which tries
> (at last) to acquire registration_lock again.
> This is a workaround.

What calls fb_open() instead of just calling fb_info->fbops->fb_open
when it is set?
In other words what kernel handler of fb_notifier chain does file-based
open of the framebuffer or is the event pushed out synchronously to
userspace?

> Signed-off-by: Fabio Erculiani <lxnay@sabayon.org>
> ---
> drivers/video/fbmem.c | 13 ++++++++++++-
> 1 files changed, 12 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
> index 5aac00e..b9831a0 100644
> --- a/drivers/video/fbmem.c
> +++ b/drivers/video/fbmem.c
> @@ -1628,6 +1628,11 @@ static int do_register_framebuffer(struct fb_info *fb_info)
> event.info = fb_info;
> if (!lock_fb_info(fb_info))
> return -ENODEV;
> +
> + /* FIXME: unlock registration mutex before registration
> + * notification is sent, in order to avoid deadlock.
> + */
> + mutex_unlock(&registration_lock);
> fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
> unlock_fb_info(fb_info);
> return 0;
> @@ -1692,7 +1697,13 @@ register_framebuffer(struct fb_info *fb_info)
>
> mutex_lock(&registration_lock);
> ret = do_register_framebuffer(fb_info);
> - mutex_unlock(&registration_lock);
> + if (ret != 0)
> + /*
> + * FIXME: mutex is unlocked only if ret == 0.
> + * This is the second part of the workaround
> + * that prevents deadlocking.
> + */
> + mutex_unlock(&registration_lock);
>
> return ret;
> }


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