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SubjectRe: [PATCH] platform/chromeos: cros_ec: Use per-device lockdep key
On Mon, Jan 9, 2023 at 1:46 PM Tzung-Bi Shih <tzungbi@kernel.org> wrote:
>
> On Sat, Jan 07, 2023 at 01:43:57PM +0800, Chen-Yu Tsai wrote:
> > On Fri, Jan 6, 2023 at 5:08 PM Tzung-Bi Shih <tzungbi@kernel.org> wrote:
> > >
> > > On Fri, Jan 06, 2023 at 12:55:37PM +0800, Chen-Yu Tsai wrote:
> > > > Lockdep reports a bogus possible deadlock on MT8192 Chromebooks due to
> > > > the following lock sequences:
> > > >
> > > > 1. lock(i2c_register_adapter) [1]; lock(&ec_dev->lock)
> > > > 2. lock(&ec_dev->lock); lock(prepare_lock);
> > > >
> > > > The actual dependency chains are much longer. The shortened version
> > > > looks somewhat like:
> > > >
> > > > 1. cros-ec-rpmsg on mtk-scp
> > > > ec_dev->lock -> prepare_lock
> > > > 2. In rt5682_i2c_probe() on native I2C bus:
> > > > prepare_lock -> regmap->lock -> (possibly) i2c_adapter->bus_lock
> > > > 3. In rt5682_i2c_probe() on native I2C bus:
> > > > regmap->lock -> i2c_adapter->bus_lock
> > > > 4. In sbs_probe() on cros-ec-i2c (passthrough) I2C bus on cros-ec
> > > > i2c_adapter->bus_lock -> ec_dev->lock
> > > >
> > > > While lockdep is correct that the shared lockdep classes have a circular
> > > > dependency, it is bogus because
> > > >
> > > > a) 2+3 happen on a native I2C bus
> > > > b) 4 happens on the actual EC on ChromeOS devices
> > > > c) 1 happens on the SCP coprocessor on MediaTek Chromebooks that just
> > > > happen to expose a cros-ec interface, but do not have a passthrough
> > > > I2C bus
> > > >
> > > > In short, the "dependencies" are actually on different devices.
> > >
> > > Path of 4 looks weird to me.
> > >
> > > Could you point out where sbs_probe() gets to acquire ec_dev->lock?
> >
> > sbs_probe() calls sbs_get_battery_presence_and_health(), which
> >
> > -> does an I2C transfer. This SBS instance is connected on the I2C bus
> > on the EC, so the I2C transfer
> >
> > -> acquires i2c_adapter->bus_lock, and
>
> I see.
>
> Another question: the i2c_adapter here should be different from the native
> I2C bus in 2 and 3. Did they really form the circular dependencies?

That's why it's a false positive. lockdep normally doesn't track individual
instances, only classes of locks. The class is declared as part of the
mutex_init() macro.

ChenYu

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