[PATCH 2/2] drm/v3d: Appease lockdep while updating GPU stats

[Maíra Canal]

Hi Tvrtko,

On 8/13/24 07:25, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
> 
> Lockdep thinks our seqcount_t usage is unsafe because the update path can
> be both from irq and worker context:
> 
>   [ ] ================================
>   [ ] WARNING: inconsistent lock state
>   [ ] 6.10.3-v8-16k-numa #159 Tainted: G        WC
>   [ ] --------------------------------
>   [ ] inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
>   [ ] swapper/0/0 [HC1[1]:SC0[0]:HE0:SE1] takes:
>   [ ] ffff80003d7c08d0 (&v3d_priv->stats[i].lock){?.+.}-{0:0}, at: v3d_irq+0xc8/0x660 [v3d]
>   [ ] {HARDIRQ-ON-W} state was registered at:
>   [ ]   lock_acquire+0x1f8/0x328
>   [ ]   v3d_job_start_stats.isra.0+0xd8/0x218 [v3d]
>   [ ]   v3d_bin_job_run+0x23c/0x388 [v3d]
>   [ ]   drm_sched_run_job_work+0x520/0x6d0 [gpu_sched]
>   [ ]   process_one_work+0x62c/0xb48
>   [ ]   worker_thread+0x468/0x5b0
>   [ ]   kthread+0x1c4/0x1e0
>   [ ]   ret_from_fork+0x10/0x20
>   [ ] irq event stamp: 337094
>   [ ] hardirqs last  enabled at (337093): [<ffffc0008144ce7c>] default_idle_call+0x11c/0x140
>   [ ] hardirqs last disabled at (337094): [<ffffc0008144a354>] el1_interrupt+0x24/0x58
>   [ ] softirqs last  enabled at (337082): [<ffffc00080061d90>] handle_softirqs+0x4e0/0x538
>   [ ] softirqs last disabled at (337073): [<ffffc00080010364>] __do_softirq+0x1c/0x28
>   [ ]
>                  other info that might help us debug this:
>   [ ]  Possible unsafe locking scenario:
> 
>   [ ]        CPU0
>   [ ]        ----
>   [ ]   lock(&v3d_priv->stats[i].lock);
>   [ ]   <Interrupt>
>   [ ]     lock(&v3d_priv->stats[i].lock);
>   [ ]
>                  *** DEADLOCK ***
> 
>   [ ] no locks held by swapper/0/0.
>   [ ]
>                 stack backtrace:
>   [ ] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        WC         6.10.3-v8-16k-numa #159
>   [ ] Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
>   [ ] Call trace:
>   [ ]  dump_backtrace+0x170/0x1b8
>   [ ]  show_stack+0x20/0x38
>   [ ]  dump_stack_lvl+0xb4/0xd0
>   [ ]  dump_stack+0x18/0x28
>   [ ]  print_usage_bug+0x3cc/0x3f0
>   [ ]  mark_lock+0x4d0/0x968
>   [ ]  __lock_acquire+0x784/0x18c8
>   [ ]  lock_acquire+0x1f8/0x328
>   [ ]  v3d_job_update_stats+0xec/0x2e0 [v3d]
>   [ ]  v3d_irq+0xc8/0x660 [v3d]
>   [ ]  __handle_irq_event_percpu+0x1f8/0x488
>   [ ]  handle_irq_event+0x88/0x128
>   [ ]  handle_fasteoi_irq+0x298/0x408
>   [ ]  generic_handle_domain_irq+0x50/0x78
> 
> But it is a false positive because all the queue-stats pairs have their
> own lock and jobs are also one at a time.
> 
> Nevertheless we can appease lockdep by disabling local interrupts to make
> it see lock usage is consistent.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
> Fixes: 6abe93b621ab ("drm/v3d: Fix race-condition between sysfs/fdinfo and interrupt handler")
> Cc: Maíra Canal <mcanal at igalia.com>

Applied to misc/kernel.git (drm-misc-next).

Best Regards,
- Maíra

> ---
>   drivers/gpu/drm/v3d/v3d_sched.c | 44 ++++++++++++++++++++++++++++++---
>   1 file changed, 40 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/v3d/v3d_sched.c b/drivers/gpu/drm/v3d/v3d_sched.c
> index cc2e5a89467b..e4fc3d401edd 100644
> --- a/drivers/gpu/drm/v3d/v3d_sched.c
> +++ b/drivers/gpu/drm/v3d/v3d_sched.c
> @@ -135,8 +135,31 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
>   	struct v3d_stats *global_stats = &v3d->queue[queue].stats;
>   	struct v3d_stats *local_stats = &file->stats[queue];
>   	u64 now = local_clock();
> +	unsigned long flags;
>   
> -	preempt_disable();
> +	/*
> +	 * We only need to disable local interrupts to appease lockdep who
> +	 * otherwise would think v3d_job_start_stats vs v3d_stats_update has an
> +	 * unsafe in-irq vs no-irq-off usage problem. This is a false positive
> +	 * becuase all the locks are per queue and stats type, and all jobs are
> +	 * completely one at a time serialised. More specifically:
> +	 *
> +	 * 1. Locks for GPU queues are updated from interrupt handlers under a
> +	 *    spin lock and started here with preemption disabled.
> +	 *
> +	 * 2. Locks for CPU queues are updated from the worker with preemption
> +	 *    disabled and equally started here with preemption disabled.
> +	 *
> +	 * Therefore both are consistent.
> +	 *
> +	 * 3. Because next job can only be queued after the previous one has
> +	 *    been signaled, and locks are per queue, there is also no scope for
> +	 *    the start part to race with the update part.
> +	 */
> +	if (IS_ENABLED(CONFIG_LOCKDEP))
> +		local_irq_save(flags);
> +	else
> +		preempt_disable();
>   
>   	write_seqcount_begin(&local_stats->lock);
>   	local_stats->start_ns = now;
> @@ -146,7 +169,10 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
>   	global_stats->start_ns = now;
>   	write_seqcount_end(&global_stats->lock);
>   
> -	preempt_enable();
> +	if (IS_ENABLED(CONFIG_LOCKDEP))
> +		local_irq_restore(flags);
> +	else
> +		preempt_enable();
>   }
>   
>   static void
> @@ -167,11 +193,21 @@ v3d_job_update_stats(struct v3d_job *job, enum v3d_queue queue)
>   	struct v3d_stats *global_stats = &v3d->queue[queue].stats;
>   	struct v3d_stats *local_stats = &file->stats[queue];
>   	u64 now = local_clock();
> +	unsigned long flags;
> +
> +	/* See comment in v3d_job_start_stats() */
> +	if (IS_ENABLED(CONFIG_LOCKDEP))
> +		local_irq_save(flags);
> +	else
> +		preempt_disable();
>   
> -	preempt_disable();
>   	v3d_stats_update(local_stats, now);
>   	v3d_stats_update(global_stats, now);
> -	preempt_enable();
> +
> +	if (IS_ENABLED(CONFIG_LOCKDEP))
> +		local_irq_restore(flags);
> +	else
> +		preempt_enable();
>   }
>   
>   static struct dma_fence *v3d_bin_job_run(struct drm_sched_job *sched_job)