Re: [PATCH] kernel/locking: Add context to ww_mutex_trylock.

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 07, 2021 at 03:20:44PM +0200, Maarten Lankhorst wrote:
> i915 will soon gain an eviction path that trylock a whole lot of locks
> for eviction, getting dmesg failures like below:
> 
> BUG: MAX_LOCK_DEPTH too low!
> turning off the locking correctness validator.
> depth: 48  max: 48!
> 48 locks held by i915_selftest/5776:
>  #0: ffff888101a79240 (&dev->mutex){....}-{3:3}, at: __driver_attach+0x88/0x160
>  #1: ffffc900009778c0 (reservation_ww_class_acquire){+.+.}-{0:0}, at: i915_vma_pin.constprop.63+0x39/0x1b0 [i915]
>  #2: ffff88800cf74de8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_vma_pin.constprop.63+0x5f/0x1b0 [i915]
>  #3: ffff88810c7f9e38 (&vm->mutex/1){+.+.}-{3:3}, at: i915_vma_pin_ww+0x1c4/0x9d0 [i915]
>  #4: ffff88810bad5768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
>  #5: ffff88810bad60e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
> ...
>  #46: ffff88811964d768 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
>  #47: ffff88811964e0e8 (reservation_ww_class_mutex){+.+.}-{3:3}, at: i915_gem_evict_something+0x110/0x860 [i915]
> INFO: lockdep is turned off.

> As an intermediate solution, add an acquire context to ww_mutex_trylock,
> which allows us to do proper nesting annotations on the trylocks, making
> the above lockdep splat disappear.

Fair enough I suppose.

> +/**
> + * ww_mutex_trylock - tries to acquire the w/w mutex with optional acquire context
> + * @lock: mutex to lock
> + * @ctx: optional w/w acquire context
> + *
> + * Trylocks a mutex with the optional acquire context; no deadlock detection is
> + * possible. Returns 1 if the mutex has been acquired successfully, 0 otherwise.
> + *
> + * Unlike ww_mutex_lock, no deadlock handling is performed. However, if a @ctx is
> + * specified, -EALREADY and -EDEADLK handling may happen in calls to ww_mutex_lock.
> + *
> + * A mutex acquired with this function must be released with ww_mutex_unlock.
> + */
> +int __sched
> +ww_mutex_trylock(struct ww_mutex *ww, struct ww_acquire_ctx *ctx)
> +{
> +	bool locked;
> +
> +	if (!ctx)
> +		return mutex_trylock(&ww->base);
> +
> +#ifdef CONFIG_DEBUG_MUTEXES
> +	DEBUG_LOCKS_WARN_ON(ww->base.magic != &ww->base);
> +#endif
> +
> +	preempt_disable();
> +	locked = __mutex_trylock(&ww->base);
> +
> +	if (locked) {
> +		ww_mutex_set_context_fastpath(ww, ctx);
> +		mutex_acquire_nest(&ww->base.dep_map, 0, 1, &ctx->dep_map, _RET_IP_);
> +	}
> +	preempt_enable();
> +
> +	return locked;
> +}
> +EXPORT_SYMBOL(ww_mutex_trylock);

You'll need a similar hunk in ww_rt_mutex.c