linux-kernel - Re: [PATCH v1] soc/tegra: pmc: Drop locking from tegra_powergate_is

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Message-ID: <11fc94af-efe2-e571-3163-efdaa91f34fc@gmail.com>
Date:   Wed, 24 Oct 2018 13:44:32 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Jon Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] soc/tegra: pmc: Drop locking from
 tegra_powergate_is_powered()

On 10/24/18 1:20 PM, Jon Hunter wrote:
> 
> On 21/10/2018 19:36, Dmitry Osipenko wrote:
>> This fixes splats like the one below if CONFIG_DEBUG_ATOMIC_SLEEP=y
>> and machine (Tegra30) booted with SMP=n or all secondary CPU's are put
>> offline. Locking isn't needed because it protects atomic operation.
>>
>> # echo 0 | tee /sys/devices/system/cpu/cpu[1-3]/online
>>
>> BUG: sleeping function called from invalid context at kernel/locking/mutex.c:254
>> in_atomic(): 1, irqs_disabled(): 128, pid: 0, name: swapper/0
>> CPU: 0 PID: 0 Comm: swapper/0 Tainted: G         C        4.18.0-next-20180821-00180-gc3ebb6544e44-dirty #823
>> Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
>> [<c01134f4>] (unwind_backtrace) from [<c010db2c>] (show_stack+0x20/0x24)
>> [<c010db2c>] (show_stack) from [<c0bd0f3c>] (dump_stack+0x94/0xa8)
>> [<c0bd0f3c>] (dump_stack) from [<c0151df8>] (___might_sleep+0x13c/0x174)
>> [<c0151df8>] (___might_sleep) from [<c0151ea0>] (__might_sleep+0x70/0xa8)
>> [<c0151ea0>] (__might_sleep) from [<c0bec2b8>] (mutex_lock+0x2c/0x70)
>> [<c0bec2b8>] (mutex_lock) from [<c0589844>] (tegra_powergate_is_powered+0x44/0xa8)
>> [<c0589844>] (tegra_powergate_is_powered) from [<c0581a60>] (tegra30_cpu_rail_off_ready+0x30/0x74)
>> [<c0581a60>] (tegra30_cpu_rail_off_ready) from [<c0122244>] (tegra30_idle_lp2+0xa0/0x108)
>> [<c0122244>] (tegra30_idle_lp2) from [<c0853438>] (cpuidle_enter_state+0x140/0x540)
>> [<c0853438>] (cpuidle_enter_state) from [<c08538a4>] (cpuidle_enter+0x40/0x4c)
>> [<c08538a4>] (cpuidle_enter) from [<c01595e0>] (call_cpuidle+0x30/0x48)
>> [<c01595e0>] (call_cpuidle) from [<c01599f8>] (do_idle+0x238/0x28c)
>> [<c01599f8>] (do_idle) from [<c0159d28>] (cpu_startup_entry+0x28/0x2c)
>> [<c0159d28>] (cpu_startup_entry) from [<c0be76c8>] (rest_init+0xd8/0xdc)
>> [<c0be76c8>] (rest_init) from [<c1200f50>] (start_kernel+0x41c/0x430)
>>
>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  drivers/soc/tegra/pmc.c | 2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/soc/tegra/pmc.c b/drivers/soc/tegra/pmc.c
>> index 1fa840e3d930..1714cd818d69 100644
>> --- a/drivers/soc/tegra/pmc.c
>> +++ b/drivers/soc/tegra/pmc.c
>> @@ -543,9 +543,7 @@ int tegra_powergate_is_powered(unsigned int id)
>>  	if (!tegra_powergate_is_valid(id))
>>  		return -EINVAL;
>>  
>> -	mutex_lock(&pmc->powergates_lock);
>>  	status = tegra_powergate_state(id);
>> -	mutex_unlock(&pmc->powergates_lock);
>>  
>>  	return status;
> 
> Can we just ...
> 
> 	return tegra_powergate_state(id);

We can, but it is not necessary. Feel free to adjust the code as you wish while applying ;) I deliberately left the 'status' variable because I think that makes code a bit more expressive.