Re: [PATCH v3 3/9] PM / QoS: Fix constraints alloc vs reclaim locking

[Rob Clark]

On Fri, Aug 4, 2023 at 10:07 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Fri, Aug 4, 2023 at 12:02 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
> >
> > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> >
> > In the process of adding lockdep annotation for drm GPU scheduler's
> > job_run() to detect potential deadlock against shrinker/reclaim, I hit
> > this lockdep splat:
> >
> >    ======================================================
> >    WARNING: possible circular locking dependency detected
> >    6.2.0-rc8-debug+ #558 Tainted: G        W
> >    ------------------------------------------------------
> >    ring0/125 is trying to acquire lock:
> >    ffffffd6d6ce0f28 (dev_pm_qos_mtx){+.+.}-{3:3}, at: dev_pm_qos_update_request+0x38/0x68
> >
> >    but task is already holding lock:
> >    ffffff8087239208 (&gpu->active_lock){+.+.}-{3:3}, at: msm_gpu_submit+0xec/0x178
> >
> >    which lock already depends on the new lock.
> >
> >    the existing dependency chain (in reverse order) is:
> >
> >    -> #4 (&gpu->active_lock){+.+.}-{3:3}:
> >           __mutex_lock+0xcc/0x3c8
> >           mutex_lock_nested+0x30/0x44
> >           msm_gpu_submit+0xec/0x178
> >           msm_job_run+0x78/0x150
> >           drm_sched_main+0x290/0x370
> >           kthread+0xf0/0x100
> >           ret_from_fork+0x10/0x20
> >
> >    -> #3 (dma_fence_map){++++}-{0:0}:
> >           __dma_fence_might_wait+0x74/0xc0
> >           dma_resv_lockdep+0x1f4/0x2f4
> >           do_one_initcall+0x104/0x2bc
> >           kernel_init_freeable+0x344/0x34c
> >           kernel_init+0x30/0x134
> >           ret_from_fork+0x10/0x20
> >
> >    -> #2 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}:
> >           fs_reclaim_acquire+0x80/0xa8
> >           slab_pre_alloc_hook.constprop.0+0x40/0x25c
> >           __kmem_cache_alloc_node+0x60/0x1cc
> >           __kmalloc+0xd8/0x100
> >           topology_parse_cpu_capacity+0x8c/0x178
> >           get_cpu_for_node+0x88/0xc4
> >           parse_cluster+0x1b0/0x28c
> >           parse_cluster+0x8c/0x28c
> >           init_cpu_topology+0x168/0x188
> >           smp_prepare_cpus+0x24/0xf8
> >           kernel_init_freeable+0x18c/0x34c
> >           kernel_init+0x30/0x134
> >           ret_from_fork+0x10/0x20
> >
> >    -> #1 (fs_reclaim){+.+.}-{0:0}:
> >           __fs_reclaim_acquire+0x3c/0x48
> >           fs_reclaim_acquire+0x54/0xa8
> >           slab_pre_alloc_hook.constprop.0+0x40/0x25c
> >           __kmem_cache_alloc_node+0x60/0x1cc
> >           kmalloc_trace+0x50/0xa8
> >           dev_pm_qos_constraints_allocate+0x38/0x100
> >           __dev_pm_qos_add_request+0xb0/0x1e8
> >           dev_pm_qos_add_request+0x58/0x80
> >           dev_pm_qos_expose_latency_limit+0x60/0x13c
> >           register_cpu+0x12c/0x130
> >           topology_init+0xac/0xbc
> >           do_one_initcall+0x104/0x2bc
> >           kernel_init_freeable+0x344/0x34c
> >           kernel_init+0x30/0x134
> >           ret_from_fork+0x10/0x20
> >
> >    -> #0 (dev_pm_qos_mtx){+.+.}-{3:3}:
> >           __lock_acquire+0xe00/0x1060
> >           lock_acquire+0x1e0/0x2f8
> >           __mutex_lock+0xcc/0x3c8
> >           mutex_lock_nested+0x30/0x44
> >           dev_pm_qos_update_request+0x38/0x68
> >           msm_devfreq_boost+0x40/0x70
> >           msm_devfreq_active+0xc0/0xf0
> >           msm_gpu_submit+0x10c/0x178
> >           msm_job_run+0x78/0x150
> >           drm_sched_main+0x290/0x370
> >           kthread+0xf0/0x100
> >           ret_from_fork+0x10/0x20
> >
> >    other info that might help us debug this:
> >
> >    Chain exists of:
> >      dev_pm_qos_mtx --> dma_fence_map --> &gpu->active_lock
> >
> >     Possible unsafe locking scenario:
> >
> >           CPU0                    CPU1
> >           ----                    ----
> >      lock(&gpu->active_lock);
> >                                   lock(dma_fence_map);
> >                                   lock(&gpu->active_lock);
> >      lock(dev_pm_qos_mtx);
> >
> >     *** DEADLOCK ***
> >
> >    3 locks held by ring0/123:
> >     #0: ffffff8087251170 (&gpu->lock){+.+.}-{3:3}, at: msm_job_run+0x64/0x150
> >     #1: ffffffd00b0e57e8 (dma_fence_map){++++}-{0:0}, at: msm_job_run+0x68/0x150
> >     #2: ffffff8087251208 (&gpu->active_lock){+.+.}-{3:3}, at: msm_gpu_submit+0xec/0x178
> >
> >    stack backtrace:
> >    CPU: 6 PID: 123 Comm: ring0 Not tainted 6.2.0-rc8-debug+ #559
> >    Hardware name: Google Lazor (rev1 - 2) with LTE (DT)
> >    Call trace:
> >     dump_backtrace.part.0+0xb4/0xf8
> >     show_stack+0x20/0x38
> >     dump_stack_lvl+0x9c/0xd0
> >     dump_stack+0x18/0x34
> >     print_circular_bug+0x1b4/0x1f0
> >     check_noncircular+0x78/0xac
> >     __lock_acquire+0xe00/0x1060
> >     lock_acquire+0x1e0/0x2f8
> >     __mutex_lock+0xcc/0x3c8
> >     mutex_lock_nested+0x30/0x44
> >     dev_pm_qos_update_request+0x38/0x68
> >     msm_devfreq_boost+0x40/0x70
> >     msm_devfreq_active+0xc0/0xf0
> >     msm_gpu_submit+0x10c/0x178
> >     msm_job_run+0x78/0x150
> >     drm_sched_main+0x290/0x370
> >     kthread+0xf0/0x100
> >     ret_from_fork+0x10/0x20
> >
> > The issue is that dev_pm_qos_mtx is held in the runpm suspend/resume (or
> > freq change) path, but it is also held across allocations that could
> > recurse into shrinker.
> >
> > Solve this by changing dev_pm_qos_constraints_allocate() into a function
> > that can be called unconditionally before the device qos object is
> > needed and before aquiring dev_pm_qos_mtx.  This way the allocations can
> > be done without holding the mutex.  In the case that we raced with
> > another thread to allocate the qos object, detect this *after* acquiring
> > the dev_pm_qos_mtx and simply free the redundant allocations.
> >
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > ---
> >  drivers/base/power/qos.c | 60 +++++++++++++++++++++++++++-------------
> >  1 file changed, 41 insertions(+), 19 deletions(-)
> >
> > diff --git a/drivers/base/power/qos.c b/drivers/base/power/qos.c
> > index 8e93167f1783..f3e0c6b65635 100644
> > --- a/drivers/base/power/qos.c
> > +++ b/drivers/base/power/qos.c
> > @@ -185,18 +185,24 @@ static int apply_constraint(struct dev_pm_qos_request *req,
> >  }
> >
> >  /*
> > - * dev_pm_qos_constraints_allocate
> > + * dev_pm_qos_constraints_ensure_allocated
> >   * @dev: device to allocate data for
> >   *
> > - * Called at the first call to add_request, for constraint data allocation
> > - * Must be called with the dev_pm_qos_mtx mutex held
> > + * Called to ensure that devices qos is allocated, before acquiring
> > + * dev_pm_qos_mtx.
> >   */
> > -static int dev_pm_qos_constraints_allocate(struct device *dev)
> > +static int dev_pm_qos_constraints_ensure_allocated(struct device *dev)
> >  {
> >         struct dev_pm_qos *qos;
> >         struct pm_qos_constraints *c;
> >         struct blocking_notifier_head *n;
> >
> > +       if (!dev)
> > +               return -ENODEV;
> > +
> > +       if (!IS_ERR_OR_NULL(dev->power.qos))
> > +               return 0;
> > +
> >         qos = kzalloc(sizeof(*qos), GFP_KERNEL);
> >         if (!qos)
> >                 return -ENOMEM;
> > @@ -227,10 +233,26 @@ static int dev_pm_qos_constraints_allocate(struct device *dev)
> >
> >         INIT_LIST_HEAD(&qos->flags.list);
> >
> > +       mutex_lock(&dev_pm_qos_mtx);
> > +
> > +       if (!IS_ERR_OR_NULL(dev->power.qos)) {
> > +               /*
> > +                * We have raced with another task to create the qos.
> > +                * No biggie, just free the resources we've allocated
> > +                * outside of dev_pm_qos_mtx and move on with life.
> > +                */
> > +               kfree(n);
> > +               kfree(qos);
> > +               goto unlock;
> > +       }
> > +
> >         spin_lock_irq(&dev->power.lock);
> >         dev->power.qos = qos;
> >         spin_unlock_irq(&dev->power.lock);
> >
> > +unlock:
> > +       mutex_unlock(&dev_pm_qos_mtx);
> > +
> >         return 0;
> >  }
> >
> > @@ -331,17 +353,15 @@ static int __dev_pm_qos_add_request(struct device *dev,
> >  {
> >         int ret = 0;
> >
> > -       if (!dev || !req || dev_pm_qos_invalid_req_type(dev, type))
> > +       if (!req || dev_pm_qos_invalid_req_type(dev, type))
> >                 return -EINVAL;
> >
> >         if (WARN(dev_pm_qos_request_active(req),
> >                  "%s() called for already added request\n", __func__))
> >                 return -EINVAL;
> >
> > -       if (IS_ERR(dev->power.qos))
> > +       if (IS_ERR_OR_NULL(dev->power.qos))
> >                 ret = -ENODEV;
> > -       else if (!dev->power.qos)
> > -               ret = dev_pm_qos_constraints_allocate(dev);
> >
> >         trace_dev_pm_qos_add_request(dev_name(dev), type, value);
> >         if (ret)
> > @@ -390,6 +410,10 @@ int dev_pm_qos_add_request(struct device *dev, struct dev_pm_qos_request *req,
> >  {
> >         int ret;
> >
> > +       ret = dev_pm_qos_constraints_ensure_allocated(dev);
> > +       if (ret)
> > +               return ret;
> > +
>
> It is a bit unfortunate that the mutex is dropped and then immediately
> re-acquired again.  I don't think that this is strictly necessary.

We could have dev_pm_qos_constraints_ensure_allocated() return with
the lock held in the success case if we had to.. but that seems a bit
funny looking.  And the dev_pm_qos_update_user_latency_tolerance()
path would need to shuffle slightly to move the kzalloc out of the
lock.

BR,
-R


> >         mutex_lock(&dev_pm_qos_mtx);
> >         ret = __dev_pm_qos_add_request(dev, req, type, value);
> >         mutex_unlock(&dev_pm_qos_mtx);
> > @@ -537,15 +561,11 @@ int dev_pm_qos_add_notifier(struct device *dev, struct notifier_block *notifier,
> >  {
> >         int ret = 0;
> >
> > -       mutex_lock(&dev_pm_qos_mtx);
> > -
> > -       if (IS_ERR(dev->power.qos))
> > -               ret = -ENODEV;
> > -       else if (!dev->power.qos)
> > -               ret = dev_pm_qos_constraints_allocate(dev);
> > -
> > +       ret = dev_pm_qos_constraints_ensure_allocated(dev);
> >         if (ret)
> > -               goto unlock;
> > +               return ret;
> > +
> > +       mutex_lock(&dev_pm_qos_mtx);
> >
> >         switch (type) {
> >         case DEV_PM_QOS_RESUME_LATENCY:
> > @@ -565,7 +585,6 @@ int dev_pm_qos_add_notifier(struct device *dev, struct notifier_block *notifier,
> >                 ret = -EINVAL;
> >         }
> >
> > -unlock:
> >         mutex_unlock(&dev_pm_qos_mtx);
> >         return ret;
> >  }
> > @@ -905,10 +924,13 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
> >  {
> >         int ret;
> >
> > +       ret = dev_pm_qos_constraints_ensure_allocated(dev);
> > +       if (ret)
> > +               return ret;
> > +
> >         mutex_lock(&dev_pm_qos_mtx);
> >
> > -       if (IS_ERR_OR_NULL(dev->power.qos)
> > -           || !dev->power.qos->latency_tolerance_req) {
> > +       if (!dev->power.qos->latency_tolerance_req) {
> >                 struct dev_pm_qos_request *req;
> >
> >                 if (val < 0) {
> > --
> > 2.41.0
> >