[Intel-xe] [PATCH] drm/xe: Fix lockdep warning in xe_force_wake calls

[Aravind Iddamsetty]

On 11/24/23 14:07, Ville Syrjälä wrote:
> On Fri, Nov 24, 2023 at 02:01:27PM +0530, Aravind Iddamsetty wrote:
>> On 11/24/23 12:49, Ville Syrjälä wrote:
>>> On Fri, Nov 24, 2023 at 12:14:08PM +0530, Aravind Iddamsetty wrote:
>>>> Introduce atomic version for xe_force_wake calls which uses spin_lock
>>>> while the non atomic version uses spin_lock_irq
>>>>
>>>> Fix for below:
>>>> [13994.811263] ========================================================
>>>> [13994.811295] WARNING: possible irq lock inversion dependency detected
>>>> [13994.811326] 6.6.0-rc3-xe #2 Tainted: G     U
>>>> [13994.811358] --------------------------------------------------------
>>>> [13994.811388] swapper/0/0 just changed the state of lock:
>>>> [13994.811416] ffff895c7e044db8 (&cpuctx_lock){-...}-{2:2}, at:
>>>> __perf_event_read+0xb7/0x3a0
>>>> [13994.811494] but this lock took another, HARDIRQ-unsafe lock in the
>>>> past:
>>>> [13994.811528]  (&fw->lock){+.+.}-{2:2}
>>>> [13994.811544]
>>>>
>>>>                and interrupts could create inverse lock ordering between
>>>> them.
>>>>
>>>> [13994.811606]
>>>>                other info that might help us debug this:
>>>> [13994.811636]  Possible interrupt unsafe locking scenario:
>>>>
>>>> [13994.811667]        CPU0                    CPU1
>>>> [13994.811691]        ----                    ----
>>>> [13994.811715]   lock(&fw->lock);
>>>> [13994.811744]                                local_irq_disable();
>>>> [13994.811773]                                lock(&cpuctx_lock);
>>>> [13994.811810]                                lock(&fw->lock);
>>>> [13994.811846]   <Interrupt>
>>>> [13994.811865]     lock(&cpuctx_lock);
>>>> [13994.811895]
>>>>                 *** DEADLOCK ***
>>>>
>>>> v2: Use spin_lock in atomic context and spin_lock_irq in a non atomic
>>>> context (Matthew Brost)
>>> No idea what this "atomic context" means, but looks like
>>> you just want to use spin_lock_irqsave() & co.
>> atomic context: where sleeping is not allowed.
> That has nothing to do with your lockdep spew. Also spinlocks don't
> sleep by definition (if we ignore the RT spinlock->mutex magic).
>
>> Well that is what I had in
>> v1 and Matt suggested we should explicitly know from where we are calling
>> force wake and depending on it use spin_lock or spin_lock_irq versions.
> Duplicating tons of code for that is silly. I seriously doubt someone
> benchmarked this and saw a meaningful improvement from skipping the
> save/restore.

@Matt any thoughts ?


>
>>>> Cc: Matthew Brost <matthew.brost at intel.com>
>>>> Cc: Anshuman Gupta <anshuman.gupta at intel.com>
>>>> Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
>>>> Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty at linux.intel.com>
>>>> ---
>>>>  drivers/gpu/drm/xe/xe_force_wake.c | 62 +++++++++++++++++++++++++++++-
>>>>  drivers/gpu/drm/xe/xe_force_wake.h |  4 ++
>>>>  drivers/gpu/drm/xe/xe_pmu.c        |  4 +-
>>>>  3 files changed, 66 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/xe/xe_force_wake.c b/drivers/gpu/drm/xe/xe_force_wake.c
>>>> index 32d6c4dd2807..1693097f72d3 100644
>>>> --- a/drivers/gpu/drm/xe/xe_force_wake.c
>>>> +++ b/drivers/gpu/drm/xe/xe_force_wake.c
>>>> @@ -147,7 +147,7 @@ int xe_force_wake_get(struct xe_force_wake *fw,
>>>>  	enum xe_force_wake_domains tmp, woken = 0;
>>>>  	int ret, ret2 = 0;
>>>>  
>>>> -	spin_lock(&fw->lock);
>>>> +	spin_lock_irq(&fw->lock);
>>>>  	for_each_fw_domain_masked(domain, domains, fw, tmp) {
>>>>  		if (!domain->ref++) {
>>>>  			woken |= BIT(domain->id);
>>>> @@ -162,7 +162,7 @@ int xe_force_wake_get(struct xe_force_wake *fw,
>>>>  				   domain->id, ret);
>>>>  	}
>>>>  	fw->awake_domains |= woken;
>>>> -	spin_unlock(&fw->lock);
>>>> +	spin_unlock_irq(&fw->lock);
>>>>  
>>>>  	return ret2;
>>>>  }
>>>> @@ -176,6 +176,64 @@ int xe_force_wake_put(struct xe_force_wake *fw,
>>>>  	enum xe_force_wake_domains tmp, sleep = 0;
>>>>  	int ret, ret2 = 0;
>>>>  
>>>> +	spin_lock_irq(&fw->lock);
>>>> +	for_each_fw_domain_masked(domain, domains, fw, tmp) {
>>>> +		if (!--domain->ref) {
>>>> +			sleep |= BIT(domain->id);
>>>> +			domain_sleep(gt, domain);
>>>> +		}
>>>> +	}
>>>> +	for_each_fw_domain_masked(domain, sleep, fw, tmp) {
>>>> +		ret = domain_sleep_wait(gt, domain);
>>> Why on earth are we waiting here?
>>>
>>> Why is this all this stuff called "sleep something"?
>> to my knowledge the HW can take sometime to ack the forcewake request
> We are *releasing* the forcewake here, not acquiring it.

ya I meant forcewake request to get and put.

Thanks,
Aravind.
>
>> that is why we have a wait, regarding the naming it was existing from before
>> may be Matt can answer that.
>>
>>
>> Thanks,
>> Aravind.