[PATCH 3/3] drm/i915/hwmon: Block waiting for GuC reset to complete

[Dixit, Ashutosh]

On Mon, 17 Apr 2023 22:35:58 -0700, Rodrigo Vivi wrote:
>

Hi Rodrigo,

> On Mon, Apr 10, 2023 at 03:35:09PM -0700, Ashutosh Dixit wrote:
> > Instead of erroring out when GuC reset is in progress, block waiting for
> > GuC reset to complete which is a more reasonable uapi behavior.
> >
> > v2: Avoid race between wake_up_all and waiting for wakeup (Rodrigo)
> >
> > Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_hwmon.c | 38 +++++++++++++++++++++++++++----
> >  1 file changed, 33 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> > index 9ab8971679fe3..8471a667dfc71 100644
> > --- a/drivers/gpu/drm/i915/i915_hwmon.c
> > +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> > @@ -51,6 +51,7 @@ struct hwm_drvdata {
> >	char name[12];
> >	int gt_n;
> >	bool reset_in_progress;
> > +	wait_queue_head_t waitq;
> >  };
> >
> >  struct i915_hwmon {
> > @@ -395,16 +396,41 @@ hwm_power_max_read(struct hwm_drvdata *ddat, long *val)
> >  static int
> >  hwm_power_max_write(struct hwm_drvdata *ddat, long val)
> >  {
> > +#define GUC_RESET_TIMEOUT msecs_to_jiffies(2000)
> > +
> > +	int ret = 0, timeout = GUC_RESET_TIMEOUT;
> >	struct i915_hwmon *hwmon = ddat->hwmon;
> >	intel_wakeref_t wakeref;
> > -	int ret = 0;
> > +	DEFINE_WAIT(wait);
> >	u32 nval;
> >
> > -	mutex_lock(&hwmon->hwmon_lock);
> > -	if (hwmon->ddat.reset_in_progress) {
> > -		ret = -EAGAIN;
> > -		goto unlock;
> > +	/* Block waiting for GuC reset to complete when needed */
> > +	for (;;) {
> > +		mutex_lock(&hwmon->hwmon_lock);
>
> I'm really afraid of how this mutex is handled with the wait queue.
> some initial thought it looks like it is trying to reimplement ww_mutex?

Sorry, but I am missing the relation with ww_mutex. No such relation is
intended.

> all other examples of the wait_queue usages like this or didn't use
> locks or had it in a total different flow that I could not correlate.

Actually there are several examples of prepare_to_wait/finish_wait
sequences with both spinlock and mutex in the kernel. See
e.g. rpm_suspend(), wait_for_rtrs_disconnection(), softsynthx_read().

Also, as I mentioned, except for the lock, the sequence here is identical
to intel_guc_wait_for_pending_msg().

>
> > +
> > +		prepare_to_wait(&ddat->waitq, &wait, TASK_INTERRUPTIBLE);
> > +
> > +		if (!hwmon->ddat.reset_in_progress)
> > +			break;
>
> If this breaks we never unlock it?

Correct, this is the original case in Patch 2 where the mutex is acquired
in the beginning of the function and released just before the final exit
from the function (so the mutex is held for the entire duration of the
function).

>
> > +
> > +		if (signal_pending(current)) {
> > +			ret = -EINTR;
> > +			break;
> > +		}
> > +
> > +		if (!timeout) {
> > +			ret = -ETIME;
> > +			break;
> > +		}
> > +
> > +		mutex_unlock(&hwmon->hwmon_lock);
>
> do we need to lock the signal pending and timeout as well?
> or only wrapping it around the hwmon->ddat access would be
> enough?

Strictly, the mutex is only needed for the hwmon->ddat.reset_in_progress
flag. But because this is not a performance path, implementing it as done
in the patch simplifies the code flow (since there are several if/else,
goto's, mutex lock/unlock and prepare_to_wait/finish_wait to consider).

So if possible I *really* want to not try to over-optimize here (I did try
a few other things when writing the patch but it was getting ugly). The
only real requirement is to drop the lock before calling schedule_timeout()
below (and we are reacquiring the lock as soon as we are scheduled back in,
as you can see in the loop above).

>
> > +
> > +		timeout = schedule_timeout(timeout);
> >	}
> > +	finish_wait(&ddat->waitq, &wait);
> > +	if (ret)
> > +		goto unlock;
> > +
> >	wakeref = intel_runtime_pm_get(ddat->uncore->rpm);
> >
> >	/* Disable PL1 limit and verify, because the limit cannot be disabled on all platforms */
> > @@ -508,6 +534,7 @@ void i915_hwmon_power_max_restore(struct drm_i915_private *i915, bool old)
> >	intel_uncore_rmw(hwmon->ddat.uncore, hwmon->rg.pkg_rapl_limit,
> >			 PKG_PWR_LIM_1_EN, old ? PKG_PWR_LIM_1_EN : 0);
> >	hwmon->ddat.reset_in_progress = false;
> > +	wake_up_all(&hwmon->ddat.waitq);
> >
> >	mutex_unlock(&hwmon->hwmon_lock);
> >  }
> > @@ -784,6 +811,7 @@ void i915_hwmon_register(struct drm_i915_private *i915)
> >	ddat->uncore = &i915->uncore;
> >	snprintf(ddat->name, sizeof(ddat->name), "i915");
> >	ddat->gt_n = -1;
> > +	init_waitqueue_head(&ddat->waitq);
> >
> >	for_each_gt(gt, i915, i) {
> >		ddat_gt = hwmon->ddat_gt + i;
> > --
> > 2.38.0
> >

From what I understand is the locking above is fine and is not the
point. The real race is between schedule_timeout() (which suspends the
thread) and wake_up_all() (which schedules it back in). But this
prepare_to_wait/finish_wait pattern is so widespread that the kernel
guarantees that this works correctly as long as you do things in the
correct order (otherwise we'd see a lot more kernel hangs/deadlocks).

Thanks,
Ashutosh