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Message-ID: <CAJKOXPc=607yqBRThcRm+342-pBq2PozEqiE9zyHcKztDXDeBA@mail.gmail.com>
Date: Sat, 21 Nov 2015 22:07:53 +0900
From: Krzysztof Kozlowski <k.kozlowski@...sung.com>
To: Anand Moon <linux.amoon@...il.com>
Cc: Guenter Roeck <linux@...ck-us.net>,
Thierry Reding <thierry.reding@...il.com>,
Javier Martinez Canillas <javier@....samsung.com>,
linux-pwm@...r.kernel.org,
Linux Kernel <linux-kernel@...r.kernel.org>,
"linux-samsung-soc@...r.kernel.org"
<linux-samsung-soc@...r.kernel.org>,
Jonathan Richardson <jonathar@...adcom.com>
Subject: Re: [PATCH] pwm: Avoid double mutex lock on pwm_enable
2015-11-21 21:11 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
> hi Krzysztof,
>
> On 21 November 2015 at 15:22, Krzysztof Kozlowski
> <k.kozlowski@...sung.com> wrote:
>> 2015-11-21 18:40 GMT+09:00 Anand Moon <linux.amoon@...il.com>:
>>> hi Krzysztof,
>>>
>>> On 21 November 2015 at 09:56, Krzysztof Kozlowski <k.kozlowski@...sung.com>
>>> 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@...il.com>
>>>> > ---
>>>> > 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
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