Re: [PATCH v2 5/5] PM: domains: Do not call device_pm_check_callbacks() when holding genpd_lock()

[Krzysztof Kozlowski]

On 17/01/2023 16:11, Ulf Hansson wrote:
> On Mon, 19 Dec 2022 at 16:15, Krzysztof Kozlowski
> <krzysztof.kozlowski@xxxxxxxxxx> wrote:
>>
>> If PM domain on PREEMPT_RT is marked as GENPD_FLAG_RT_SAFE(), the
>> genpd_lock() will be a raw spin lock, thus device_pm_check_callbacks()
>> must be called outside of the domain lock.
>>
>> This solves on PREEMPT_RT:
>>
>>   [ BUG: Invalid wait context ]
>>   6.1.0-rt5-00325-g8a5f56bcfcca #8 Tainted: G        W
>>   -----------------------------
>>   swapper/0/1 is trying to lock:
>>   ffff76e045dec9a0 (&dev->power.lock){+.+.}-{3:3}, at: device_pm_check_callbacks+0x20/0xf0
>>   other info that might help us debug this:
>>   context-{5:5}
>>   3 locks held by swapper/0/1:
>>    #0: ffff76e045deb8e8 (&dev->mutex){....}-{4:4}, at: __device_attach+0x38/0x1c0
>>    #1: ffffa92b81f825e0 (gpd_list_lock){+.+.}-{4:4}, at: __genpd_dev_pm_attach+0x7c/0x250
>>    #2: ffff76e04105c7a0 (&genpd->rslock){....}-{2:2}, at: genpd_lock_rawspin+0x1c/0x30
>>   stack backtrace:
>>   CPU: 5 PID: 1 Comm: swapper/0 Tainted: G        W          6.1.0-rt5-00325-g8a5f56bcfcca #8
>>   Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
>>   Call trace:
>>    dump_backtrace.part.0+0xe0/0xf0
>>    show_stack+0x18/0x40
>>    dump_stack_lvl+0x8c/0xb8
>>    dump_stack+0x18/0x34
>>    __lock_acquire+0x938/0x2100
>>    lock_acquire.part.0+0x104/0x28c
>>    lock_acquire+0x68/0x84
>>    rt_spin_lock+0x40/0x100
>>    device_pm_check_callbacks+0x20/0xf0
>>    dev_pm_domain_set+0x54/0x64
>>    genpd_add_device+0x258/0x340
>>    __genpd_dev_pm_attach+0xa8/0x250
>>    genpd_dev_pm_attach_by_id+0xc4/0x190
>>    genpd_dev_pm_attach_by_name+0x3c/0x60
>>    dev_pm_domain_attach_by_name+0x20/0x30
>>    dt_idle_attach_cpu+0x24/0x90
>>    psci_cpuidle_probe+0x300/0x4b0
>>    platform_probe+0x68/0xe0
>>    really_probe+0xbc/0x2dc
>>    __driver_probe_device+0x78/0xe0
>>    driver_probe_device+0x3c/0x160
>>    __device_attach_driver+0xb8/0x140
>>    bus_for_each_drv+0x78/0xd0
>>    __device_attach+0xa8/0x1c0
>>    device_initial_probe+0x14/0x20
>>    bus_probe_device+0x9c/0xa4
>>    device_add+0x3b4/0x8dc
>>    platform_device_add+0x114/0x234
>>    platform_device_register_full+0x108/0x1a4
>>    psci_idle_init+0x6c/0xb0
>>    do_one_initcall+0x74/0x450
>>    kernel_init_freeable+0x2e0/0x350
>>    kernel_init+0x24/0x130
>>    ret_from_fork+0x10/0x20
>>
>> Cc: Adrien Thierry <athierry@xxxxxxxxxx>
>> Cc: Brian Masney <bmasney@xxxxxxxxxx>
>> Cc: linux-rt-users@xxxxxxxxxxxxxxx
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
>> ---
>>  drivers/base/power/domain.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
>> index 4dfce1d476f4..db499ba40497 100644
>> --- a/drivers/base/power/domain.c
>> +++ b/drivers/base/power/domain.c
>> @@ -1666,10 +1666,14 @@ static int genpd_add_device(struct generic_pm_domain *genpd, struct device *dev,
>>         if (ret)
>>                 goto out;
>>
>> +
>> +       /* PREEMPT_RT: Must be outside of genpd_lock */
>> +       device_pm_check_callbacks(dev);
>> +
>>         genpd_lock(genpd);
>>
>>         genpd_set_cpumask(genpd, gpd_data->cpu);
>> -       dev_pm_domain_set(dev, &genpd->domain);
>> +       dev_pm_domain_set_no_cb(dev, &genpd->domain);
>>
>>         genpd->device_count++;
>>         if (gd)
> 
> Rather than splitting up the assignment in two steps, I think it
> should be perfectly fine to move the call to dev_pm_domain_set()
> outside the genpd lock.
> 
> Note that, genpd_add_device() is always being called with
> gpd_list_lock mutex being held. This prevents the genpd from being
> removed, while we use it here.

Hm, indeed, should be fine.

> 
> Moreover, we need a similar change for the call to dev_pm_domain_set()
> in genpd_remove_device().

Right.

Best regards,
Krzysztof