Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable

From: Anand Moon
Date: Thu Dec 10 2015 - 23:08:15 EST

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Hi Krzysztof,

On 22 November 2015 at 05:43, Krzysztof Kozlowski
<k.kozlowski@xxxxxxxxxxx> wrote:
> 2015-11-22 3:14 GMT+09:00 Anand Moon <linux.amoon@xxxxxxxxx>:
>> Hi Krzysztof,
>>
>> On 21 November 2015 at 18:37, Krzysztof Kozlowski
>> <k.kozlowski@xxxxxxxxxxx> wrote:
>>> 2015-11-21 21:11 GMT+09:00 Anand Moon <linux.amoon@xxxxxxxxx>:
>>>> hi Krzysztof,
>>>>
>>>> On 21 November 2015 at 15:22, Krzysztof Kozlowski
>>>> <k.kozlowski@xxxxxxxxxxx> wrote:
>>>>> 2015-11-21 18:40 GMT+09:00 Anand Moon <linux.amoon@xxxxxxxxx>:
>>>>>> hi Krzysztof,
>>>>>>
>>>>>> On 21 November 2015 at 09:56, Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
>>>>>> wrote:
>>>>>>>
>>>>>>> W dniu 21.11.2015 o 01:59, Anand Moon pisze:
>>>>>>> > Commit d1cd21427747f15920cd726f5f67a07880e7dee4
>>>>>>> > (pwm: Set enable state properly on failed call to enable)
>>>>>>> > introduce double lock of mutex on pwm_enable
>>>>>>> >
>>>>>>> > Changes fix the following debug lock warning.
>>>>>>> >
>>>>>>> > [ 2.701720] WARNING: CPU: 3 PID: 0 at kernel/locking/mutex.c:526
>>>>>>> > __mutex_lock_slowpath+0x3bc/0x404()
>>>>>>> > [ 2.701731] DEBUG_LOCKS_WARN_ON(in_interrupt())
>>>>>>>
>>>>>>> Hi Anand!
>>>>>>>
>>>>>>> Yeah, I am hitting this as well. Annoying. However your description is
>>>>>>> inaccurate. Double lock of mutex does not cause warnings of sleeping or
>>>>>>> locking in atomic context (if you build with debug sleep atomic you will
>>>>>>> see different warning). More over you are partially reverting the commit
>>>>>>> d1cd21427747 ("pwm: Set enable state properly on failed call to enable")
>>>>>>> without proper explanation:
>>>>>>> 1. why this locking is inappropriate;
>>>>>>> 2. why it is safe to remove the mutex_lock().
>>>>>>>
>>>>>>> Please find the real problem, fix the real problem and throughly explain
>>>>>>> the real issue.
>>>>>>>
>>>>>>> I was suspecting some weird changes in timers. For !irqsafe timer I
>>>>>>> think it is safe to use mutexes... but apparently not. Maybe a work
>>>>>>> should be scheduled from function called by timer?
>>>>>>>
>>>>>>> Warning with debug atomic sleep:
>>>>>>> [ 16.047517] BUG: sleeping function called from invalid context at
>>>>>>> ../kernel/locking/mutex.c:617
>>>>>>> [ 16.054866] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/0
>>>>>>> [ 16.061402] INFO: lockdep is turned off.
>>>>>>> [ 16.065281] Preemption disabled at:[< (null)>] (null)
>>>>>>> [ 16.070524]
>>>>>>> [ 16.072002] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
>>>>>>> 4.4.0-rc1-00040-g28c429565d4f #290
>>>>>>> [ 16.080150] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
>>>>>>> [ 16.086215] [<c0016780>] (unwind_backtrace) from [<c00132f0>]
>>>>>>> (show_stack+0x10/0x14)
>>>>>>> [ 16.093938] [<c00132f0>] (show_stack) from [<c0223ba4>]
>>>>>>> (dump_stack+0x70/0xbc)
>>>>>>> [ 16.101122] [<c0223ba4>] (dump_stack) from [<c05ed8e0>]
>>>>>>> (mutex_lock_nested+0x24/0x474)
>>>>>>> [ 16.109009] [<c05ed8e0>] (mutex_lock_nested) from [<c0259154>]
>>>>>>> (pwm_enable+0x20/0x7c)
>>>>>>> [ 16.116799] [<c0259154>] (pwm_enable) from [<c04400bc>]
>>>>>>> (led_heartbeat_function+0xdc/0x140)
>>>>>>> [ 16.125119] [<c04400bc>] (led_heartbeat_function) from [<c00864c8>]
>>>>>>> (call_timer_fn+0x6c/0xf4)
>>>>>>> [ 16.133606] [<c00864c8>] (call_timer_fn) from [<c00866f8>]
>>>>>>> (run_timer_softirq+0x1a8/0x230)
>>>>>>> [ 16.141846] [<c00866f8>] (run_timer_softirq) from [<c0028e68>]
>>>>>>> (__do_softirq+0x134/0x2c0)
>>>>>>> [ 16.149982] [<c0028e68>] (__do_softirq) from [<c0029334>]
>>>>>>> (irq_exit+0xd0/0x114)
>>>>>>> [ 16.157255] [<c0029334>] (irq_exit) from [<c0076610>]
>>>>>>> (__handle_domain_irq+0x70/0xe4)
>>>>>>> [ 16.165056] [<c0076610>] (__handle_domain_irq) from [<c00094e8>]
>>>>>>> (gic_handle_irq+0x4c/0x94)
>>>>>>> [ 16.173376] [<c00094e8>] (gic_handle_irq) from [<c0013db8>]
>>>>>>> (__irq_svc+0x58/0x98)
>>>>>>> [ 16.180817] Exception stack(0xc08cdf58 to 0xc08cdfa0)
>>>>>>> [ 16.185823] df40:
>>>>>>> c0010dcc 00000000
>>>>>>> [ 16.193997] df60: c08cdfa8 00000000 c05f57c4 00000000 c08ca520
>>>>>>> c08ce4bc c08c7364 c08ce4b4
>>>>>>> [ 16.202141] df80: c09121ca 00000000 00000001 c08cdfa8 c0010dcc
>>>>>>> c0010dd0 600f0013 ffffffff
>>>>>>> [ 16.210291] [<c0013db8>] (__irq_svc) from [<c0010dd0>]
>>>>>>> (arch_cpu_idle+0x20/0x3c)
>>>>>>> [ 16.217661] [<c0010dd0>] (arch_cpu_idle) from [<c0063174>]
>>>>>>> (cpu_startup_entry+0x17c/0x26c)
>>>>>>> [ 16.225899] [<c0063174>] (cpu_startup_entry) from [<c0860c40>]
>>>>>>> (start_kernel+0x368/0x3d0)
>>>>>>>
>>>>>>> Best regards,
>>>>>>> Krzysztof
>>>>>>>
>>>>>>>
>>>>>>> > [ 2.701737] Modules linked in:
>>>>>>> > [ 2.701748] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.4.0-rc1-xu4f
>>>>>>> > #24
>>>>>>> > [ 2.701753] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
>>>>>>> > [ 2.701787] [<c0015f48>] (unwind_backtrace) from [<c0012d04>]
>>>>>>> > (show_stack+0x10/0x14)
>>>>>>> > [ 2.701808] [<c0012d04>] (show_stack) from [<c01f83fc>]
>>>>>>> > (dump_stack+0x84/0xc4)
>>>>>>> > [ 2.701824] [<c01f83fc>] (dump_stack) from [<c0023494>]
>>>>>>> > (warn_slowpath_common+0x80/0xb0)
>>>>>>> > [ 2.701836] [<c0023494>] (warn_slowpath_common) from [<c00234f4>]
>>>>>>> > (warn_slowpath_fmt+0x30/0x40)
>>>>>>> > [ 2.701849] [<c00234f4>] (warn_slowpath_fmt) from [<c056e6b8>]
>>>>>>> > (__mutex_lock_slowpath+0x3bc/0x404)
>>>>>>> > [ 2.701864] [<c056e6b8>] (__mutex_lock_slowpath) from [<c056e70c>]
>>>>>>> > (mutex_lock+0xc/0x24)
>>>>>>> > [ 2.701884] [<c056e70c>] (mutex_lock) from [<c0228984>]
>>>>>>> > (pwm_enable+0x20/0x7c)
>>>>>>> > [ 2.701903] [<c0228984>] (pwm_enable) from [<c03f0000>]
>>>>>>> > (led_heartbeat_function+0x74/0x144)
>>>>>>> > [ 2.701919] [<c03f0000>] (led_heartbeat_function) from [<c0074368>]
>>>>>>> > (call_timer_fn+0x24/0x98)
>>>>>>> > [ 2.701932] [<c0074368>] (call_timer_fn) from [<c007453c>]
>>>>>>> > (run_timer_softirq+0x160/0x21c)
>>>>>>> > [ 2.701946] [<c007453c>] (run_timer_softirq) from [<c0026e10>]
>>>>>>> > (__do_softirq+0x110/0x228)
>>>>>>> > [ 2.701959] [<c0026e10>] (__do_softirq) from [<c00271c8>]
>>>>>>> > (irq_exit+0xc0/0xfc)
>>>>>>> > [ 2.701976] [<c00271c8>] (irq_exit) from [<c0065180>]
>>>>>>> > (__handle_domain_irq+0x80/0xec)
>>>>>>> > [ 2.701990] [<c0065180>] (__handle_domain_irq) from [<c0009494>]
>>>>>>> > (gic_handle_irq+0x54/0x94)
>>>>>>> > [ 2.702001] [<c0009494>] (gic_handle_irq) from [<c0013794>]
>>>>>>> > (__irq_svc+0x54/0x90)
>>>>>>> > [ 2.702008] Exception stack(0xee8bdf88 to 0xee8bdfd0)
>>>>>>> > [ 2.702019] df80: 00000001 00000000 00000000
>>>>>>> > c001b720 ee8bc000 c0573354
>>>>>>> > [ 2.702031] dfa0: 00000000 00000000 ee8bdfe0 c07f92e4 c08004b4
>>>>>>> > c08004bc f0806640 ee8bdfd8
>>>>>>> > [ 2.702039] dfc0: c0010180 c0010184 60000013 ffffffff
>>>>>>> > [ 2.702053] [<c0013794>] (__irq_svc) from [<c0010184>]
>>>>>>> > (arch_cpu_idle+0x38/0x3c)
>>>>>>> > [ 2.702073] [<c0010184>] (arch_cpu_idle) from [<c0058ed4>]
>>>>>>> > (cpu_startup_entry+0x1ec/0x270)
>>>>>>> > [ 2.702086] [<c0058ed4>] (cpu_startup_entry) from [<4000956c>]
>>>>>>> > (0x4000956c)
>>>>>>> > [ 2.702093] ---[ end trace 539af70491f4f1a9 ]---
>>>>>>> >
>>>>>>> > Signed-off-by: Anand Moon <linux.amoon@xxxxxxxxx>
>>>>>>> > ---
>>>>>>> > drivers/pwm/core.c | 4 ----
>>>>>>> > 1 file changed, 4 deletions(-)
>>>>>>> >
>>>>>>> > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
>>>>>>> > index d24ca5f..b8f035a 100644
>>>>>>> > --- a/drivers/pwm/core.c
>>>>>>> > +++ b/drivers/pwm/core.c
>>>>>>> > @@ -506,16 +506,12 @@ int pwm_enable(struct pwm_device *pwm)
>>>>>>> > if (!pwm)
>>>>>>> > return -EINVAL;
>>>>>>> >
>>>>>>> > - mutex_lock(&pwm->lock);
>>>>>>> > -
>>>>>>> > if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {
>>>>>>> > err = pwm->chip->ops->enable(pwm->chip, pwm);
>>>>>>> > if (err)
>>>>>>> > clear_bit(PWMF_ENABLED, &pwm->flags);
>>>>>>> > }
>>>>>>> >
>>>>>>> > - mutex_unlock(&pwm->lock);
>>>>>>> > -
>>>>>>> > return err;
>>>>>>> > }
>>>>>>> > EXPORT_SYMBOL_GPL(pwm_enable);
>>>>>>> >
>>>>>>
>>>>>>
>>>>>> Adding Jonathan Richardson.
>>>>>>
>>>>>> Yes I am
>>>>>>
>>>>>> aware I am reverting some part of the d1cd21427747 ("pwm: Set enable state
>>>>>> properly on failed call to enable")
>>>>>>
>>>>>> Please take a look at this below commit.
>>>>>>
>>>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/pwm/core.c?id=d1cd21427747f15920cd726f5f67a07880e7dee4
>>>>>>
>>>>>> Actually reverting this change it work fine.
>>>>>>
>>>>>> This changes introduce the new mutex lock pwm->lock to protect enabled bit
>>>>>> by drivers while setting polarity.
>>>>>>
>>>>>> Well pwm_set_polarity already acquires the pwm->lock and calls
>>>>>> pwm_is_enabled function.
>>>>>> Again within pwm_is_enabled we are trying to acquire the same mutex lock.
>>>>>
>>>>> You are describing a lockdown by trying to acquire the same mutex twice.
>>>>>
>>>>> However pwm_is_enabled() does not acquire mutex.
>>>>>
>>>>> Again, please look at generated warnings:
>>>>> 1. BUG: sleeping function called from invalid context
>>>>> 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
>>>>>
>>>>> They are not related anyhow to what you described (acquiring already
>>>>> locked mutex).
>>>>>
>>>>> Best regards,
>>>>> Krzysztof
>>>>
>>>> My last reply mail went in HTML format so resend this.
>>>>
>>>> First it was a typo on my part.
>>>> It not pwm_is_enabled function its pwm_enabled.
>>>
>>> There is no such function as "pwm_enabled".
>>>
>>> Sorry, I don't get your point.
>>>
>>> Instead of pasting some commit use a descriptive way to show the calls
>>> leading lockdown. But please use real function names.
>>>
>>> Best regards,
>>> Krzysztof
>>
>> Earlier my assumption of double mutex lock up totally rubbish.
>>
>> After reverting my changes and building image with CONFIG_DEBUG_ATOMIC_SLEEP.
>>
>> [ 390.415370] BUG: sleeping function called from invalid context at
>> kernel/locking/mutex.c:97
>> [ 390.422274] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
>> [ 390.428831] Preemption disabled at:[< (null)>] (null)
>> [ 391.970352] BUG: sleeping function called from invalid context at
>> kernel/locking/mutex.c:97
>> [ 391.977251] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
>> [ 391.983814] Preemption disabled at:[< (null)>] (null)
>> [ 393.520376] BUG: sleeping function called from invalid context at
>> kernel/locking/mutex.c:97
>> [ 393.527312] in_atomic(): 1, irqs_disabled(): 0, pid: 0, name: swapper/1
>> [ 393.533925] Preemption disabled at:[< (null)>] (null)
>
> Yes, now you pasted the same warning I did...
>
> This is still the same issue. I already wrote it:
>> 1. BUG: sleeping function called from invalid context
>> 2. DEBUG_LOCKS_WARN_ON(in_interrupt())
>
> We can repeat it many times but that won't change anything...
>
> Best regards,
> Krzysztof

Would you consider below changes to fix the above issue.
I have tested this change by enabling CONFIG_DEBUG_ATOMIC_SLEEP
And I don't observed issue.

1. BUG: sleeping function called from invalid context
2. DEBUG_LOCKS_WARN_ON(in_interrupt())

Please share your thought on this changes.

root@odroidxu4:/usr/src/odroidxu3-4.y-devel# git diff drivers/pwm/core.c
diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index d24ca5f..f3f6cf9 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -506,6 +506,9 @@ int pwm_enable(struct pwm_device *pwm)
if (!pwm)
return -EINVAL;

+ if (!mutex_is_locked(&pwm->lock))
+ return -EINVAL;
+
mutex_lock(&pwm->lock);

if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {

-Anand Moon
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