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

From: Anand Moon
Date: Sat Nov 21 2015 - 13:14:38 EST

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In reply to: Anand Moon: "Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable"
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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)

I don't understand why their is requirement for mutex lock in
pwm_enable function.
mutexes are sleeping locks which behave similarly to binary semaphores.

Please correct me if I am wrong.

-Anand Moon
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Next message: Lars-Peter Clausen: "Re: [PATCH] iio: Move IIO Dummy Driver out of staging"
Previous message: Jonathan Cameron: "Re: [RFC 6/9] iio: ina2xx: add direct IO support for TI INA2xx Power Monitors"
In reply to: Anand Moon: "Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable"
Next in thread: Krzysztof Kozlowski: "Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable"
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