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

[Maarten Lankhorst]

Op 08-09-2021 om 12:14 schreef Peter Zijlstra:
> 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

What tree has that file?