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    SubjectRe: [PATCH] fb: avoid possible deadlock caused by fb_set_suspend
    Guennadi, could you have a look at (completely untested) patch which avoids
    possible deadlock due to inverted lock taking order on hotplug as well
    as "readding" lock_fb_info() for fb_set_suspend() call after Herton's
    patch to fb_set_suspend().

    Thanks,
    Bruno


    On Sat, 18 June 2011 Bruno Prémont <bonbons@linux-vserver.org> wrote:
    > On Fri, 17 June 2011 Florian Tobias Schandinat <FlorianSchandinat@gmx.de> wrote:
    > > From: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
    > >
    > > A lock ordering issue can cause deadlocks: in framebuffer/console code,
    > > all needed struct fb_info locks are taken before acquire_console_sem(),
    > > in places which need to take console semaphore.
    > >
    > > But fb_set_suspend is always called with console semaphore held, and
    > > inside it we call lock_fb_info which gets the fb_info lock, inverse
    > > locking order of what the rest of the code does. This causes a real
    > > deadlock issue, when we write to state fb sysfs attribute (which calls
    > > fb_set_suspend) while a framebuffer is being unregistered by
    > > remove_conflicting_framebuffers, as can be shown by following show
    > > blocked state trace on a test program which loads i915 and runs another
    > > forked processes writing to state attribute:
    > >
    > > Test process with semaphore held and trying to get fb_info lock:
    >
    > ...
    >
    > > fb-test2 which reproduces above is available on kernel.org bug #26232.
    > > To solve this issue, avoid calling lock_fb_info inside fb_set_suspend,
    > > and move it out to where needed (callers of fb_set_suspend must call
    > > lock_fb_info before if needed). So far, the only place which needs to
    > > call lock_fb_info is store_fbstate, all other places which calls
    > > fb_set_suspend are suspend/resume hooks that should not need the lock as
    > > they should be run only when processes are already frozen in
    > > suspend/resume.
    >
    > From a quick look through FB drivers in 2.6.39 I've found one that would need
    > more work:
    > - drivers/video/sh_mobile_hdmi.c: sh_hdmi_edid_work_fn()
    > Should get changed to
    > a) right locking order in case (hdmi->hp_state == HDMI_HOTPLUG_CONNECTED)
    > b) lock fb_info in the other case
    > For this one fb_set_suspend() does get call in a hotplug worker,
    > thus independently on suspend/resume process.
    >
    > The rest does match the suspend/resume hook pattern mentioned.
    >
    > Bruno
    >
    >
    > > References: https://bugzilla.kernel.org/show_bug.cgi?id=26232
    > > Signed-off-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
    > > Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
    > > Cc: stable@kernel.org
    > > ---
    > > drivers/video/fbmem.c | 3 ---
    > > drivers/video/fbsysfs.c | 3 +++
    > > 2 files changed, 3 insertions(+), 3 deletions(-)
    > >
    > > diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
    > > index 5aac00e..ad93629 100644
    > > --- a/drivers/video/fbmem.c
    > > +++ b/drivers/video/fbmem.c
    > > @@ -1738,8 +1738,6 @@ void fb_set_suspend(struct fb_info *info, int state)
    > > {
    > > struct fb_event event;
    > >
    > > - if (!lock_fb_info(info))
    > > - return;
    > > event.info = info;
    > > if (state) {
    > > fb_notifier_call_chain(FB_EVENT_SUSPEND, &event);
    > > @@ -1748,7 +1746,6 @@ void fb_set_suspend(struct fb_info *info, int state)
    > > info->state = FBINFO_STATE_RUNNING;
    > > fb_notifier_call_chain(FB_EVENT_RESUME, &event);
    > > }
    > > - unlock_fb_info(info);
    > > }
    > >
    > > /**
    > > diff --git a/drivers/video/fbsysfs.c b/drivers/video/fbsysfs.c
    > > index 04251ce..67afa9c 100644
    > > --- a/drivers/video/fbsysfs.c
    > > +++ b/drivers/video/fbsysfs.c
    > > @@ -399,9 +399,12 @@ static ssize_t store_fbstate(struct device *device,
    > >
    > > state = simple_strtoul(buf, &last, 0);
    > >
    > > + if (!lock_fb_info(fb_info))
    > > + return -ENODEV;
    > > console_lock();
    > > fb_set_suspend(fb_info, (int)state);
    > > console_unlock();
    > > + unlock_fb_info(fb_info);
    > >
    > > return count;
    > > }


    diff --git a/drivers/video/sh_mobile_hdmi.c b/drivers/video/sh_mobile_hdmi.c
    index 2b9e56a..b1a13ab 100644
    --- a/drivers/video/sh_mobile_hdmi.c
    +++ b/drivers/video/sh_mobile_hdmi.c
    @@ -1151,27 +1151,27 @@ static void sh_hdmi_edid_work_fn(struct work_struct *work)

    ch = info->par;

    - console_lock();
    + if (lock_fb_info(info)) {
    + console_lock();

    - /* HDMI plug in */
    - if (!sh_hdmi_must_reconfigure(hdmi) &&
    - info->state == FBINFO_STATE_RUNNING) {
    - /*
    - * First activation with the default monitor - just turn
    - * on, if we run a resume here, the logo disappears
    - */
    - if (lock_fb_info(info)) {
    + /* HDMI plug in */
    + if (!sh_hdmi_must_reconfigure(hdmi) &&
    + info->state == FBINFO_STATE_RUNNING) {
    + /*
    + * First activation with the default monitor - just turn
    + * on, if we run a resume here, the logo disappears
    + */
    info->var.width = hdmi->var.width;
    info->var.height = hdmi->var.height;
    sh_hdmi_display_on(hdmi, info);
    - unlock_fb_info(info);
    + } else {
    + /* New monitor or have to wake up */
    + fb_set_suspend(info, 0);
    }
    - } else {
    - /* New monitor or have to wake up */
    - fb_set_suspend(info, 0);
    - }

    - console_unlock();
    + console_unlock();
    + unlock_fb_info(info);
    + }
    } else {
    ret = 0;
    if (!hdmi->info)
    @@ -1181,12 +1181,15 @@ static void sh_hdmi_edid_work_fn(struct work_struct *work)
    fb_destroy_modedb(hdmi->monspec.modedb);
    hdmi->monspec.modedb = NULL;

    - console_lock();
    + if (lock_fb_info(info)) {
    + console_lock();

    - /* HDMI disconnect */
    - fb_set_suspend(hdmi->info, 1);
    + /* HDMI disconnect */
    + fb_set_suspend(hdmi->info, 1);

    - console_unlock();
    + console_unlock();
    + unlock_fb_info(info);
    + }
    pm_runtime_put(hdmi->dev);
    }

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