Re: [PATCH v2 3/3] gpiolib: pin GPIO devices in place during descriptor lookup
From: Bartosz Golaszewski
Date: Mon Jan 08 2024 - 10:40:50 EST
On Mon, Jan 8, 2024 at 2:03 PM Marek Szyprowski
<m.szyprowski@xxxxxxxxxxx> wrote:
>
> On 02.01.2024 16:59, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> >
> > There's time between when we locate the relevant descriptor during
> > lookup and when we actually take the reference to its parent GPIO
> > device where - if the GPIO device in question is removed - we'll end up
> > with a dangling pointer to freed memory. Make sure devices cannot be
> > removed until we hold a new reference to the device.
> >
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
>
> This patch landed in linux-next as commit db660b9a9f86 ("gpiolib: pin
> GPIO devices in place during descriptor lookup"). Unfortunately it
> introduces a following lock-dep warning:
>
> ============================================
> WARNING: possible recursive locking detected
> 6.7.0-rc7-00062-gdb660b9a9f86 #7819 Not tainted
> --------------------------------------------
> kworker/u4:2/27 is trying to acquire lock:
> c13f4e1c (gpio_devices_sem){++++}-{3:3}, at: gpio_device_find+0x30/0x94
>
> but task is already holding lock:
> c13f4e1c (gpio_devices_sem){++++}-{3:3}, at:
> gpiod_find_and_request+0x44/0x594
>
> other info that might help us debug this:
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock(gpio_devices_sem);
> lock(gpio_devices_sem);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
> 4 locks held by kworker/u4:2/27:
> #0: c1c06ca8 ((wq_completion)events_unbound){+.+.}-{0:0}, at:
> process_one_work+0x148/0x608
> #1: e093df20 ((work_completion)(&entry->work)){+.+.}-{0:0}, at:
> process_one_work+0x148/0x608
> #2: c1f3048c (&dev->mutex){....}-{3:3}, at:
> __driver_attach_async_helper+0x38/0xec
> #3: c13f4e1c (gpio_devices_sem){++++}-{3:3}, at:
> gpiod_find_and_request+0x44/0x594
>
> stack backtrace:
> CPU: 0 PID: 27 Comm: kworker/u4:2 Not tainted
> 6.7.0-rc7-00062-gdb660b9a9f86 #7819
> Hardware name: Samsung Exynos (Flattened Device Tree)
> Workqueue: events_unbound async_run_entry_fn
> unwind_backtrace from show_stack+0x10/0x14
> show_stack from dump_stack_lvl+0x58/0x70
> dump_stack_lvl from __lock_acquire+0x1300/0x2984
> __lock_acquire from lock_acquire+0x130/0x37c
> lock_acquire from down_read+0x44/0x224
> down_read from gpio_device_find+0x30/0x94
> gpio_device_find from of_get_named_gpiod_flags+0xa4/0x3a8
> of_get_named_gpiod_flags from of_find_gpio+0x80/0x168
> of_find_gpio from gpiod_find_and_request+0x120/0x594
> gpiod_find_and_request from gpiod_get_optional+0x54/0x90
> gpiod_get_optional from reg_fixed_voltage_probe+0x200/0x400
> reg_fixed_voltage_probe from platform_probe+0x5c/0xb8
> platform_probe from really_probe+0xe0/0x400
> really_probe from __driver_probe_device+0x9c/0x1f0
> __driver_probe_device from driver_probe_device+0x30/0xc0
> driver_probe_device from __driver_attach_async_helper+0x54/0xec
> __driver_attach_async_helper from async_run_entry_fn+0x40/0x154
> async_run_entry_fn from process_one_work+0x204/0x608
> process_one_work from worker_thread+0x1e0/0x498
> worker_thread from kthread+0x104/0x138
> kthread from ret_from_fork+0x14/0x28
> Exception stack(0xe093dfb0 to 0xe093dff8)
> ...
>
>
> Taking gpio_devices_sem more than once for reading is safe, but it looks
> that it needs some lock-dep annotations to to make it happy and avoid
> the above warning.
>
>
> > ---
> > drivers/gpio/gpiolib.c | 40 +++++++++++++++++++++++-----------------
> > 1 file changed, 23 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > index 4c93cf73a826..be57f8d6aeae 100644
> > --- a/drivers/gpio/gpiolib.c
> > +++ b/drivers/gpio/gpiolib.c
> > @@ -4134,27 +4134,33 @@ static struct gpio_desc *gpiod_find_and_request(struct device *consumer,
> > struct gpio_desc *desc;
> > int ret;
> >
> > - desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx, &flags, &lookupflags);
> > - if (gpiod_not_found(desc) && platform_lookup_allowed) {
> > + scoped_guard(rwsem_read, &gpio_devices_sem) {
> > + desc = gpiod_find_by_fwnode(fwnode, consumer, con_id, idx,
> > + &flags, &lookupflags);
> > + if (gpiod_not_found(desc) && platform_lookup_allowed) {
> > + /*
> > + * Either we are not using DT or ACPI, or their lookup
> > + * did not return a result. In that case, use platform
> > + * lookup as a fallback.
> > + */
> > + dev_dbg(consumer,
> > + "using lookup tables for GPIO lookup\n");
> > + desc = gpiod_find(consumer, con_id, idx, &lookupflags);
> > + }
> > +
> > + if (IS_ERR(desc)) {
> > + dev_dbg(consumer, "No GPIO consumer %s found\n",
> > + con_id);
> > + return desc;
> > + }
> > +
> > /*
> > - * Either we are not using DT or ACPI, or their lookup did not
> > - * return a result. In that case, use platform lookup as a
> > - * fallback.
> > + * If a connection label was passed use that, else attempt to
> > + * use the device name as label
> > */
> > - dev_dbg(consumer, "using lookup tables for GPIO lookup\n");
> > - desc = gpiod_find(consumer, con_id, idx, &lookupflags);
> > + ret = gpiod_request(desc, label);
> > }
> >
> > - if (IS_ERR(desc)) {
> > - dev_dbg(consumer, "No GPIO consumer %s found\n", con_id);
> > - return desc;
> > - }
> > -
> > - /*
> > - * If a connection label was passed use that, else attempt to use
> > - * the device name as label
> > - */
> > - ret = gpiod_request(desc, label);
> > if (ret) {
> > if (!(ret == -EBUSY && flags & GPIOD_FLAGS_BIT_NONEXCLUSIVE))
> > return ERR_PTR(ret);
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>
Thanks for the report. I think it may have come too late in the
release cycle as it has the potential to break a lot of things. I will
back it out of my for-next branch. I'll resend it for v6.9.
Bart