linux-kernel - Re: [PATCH 5/5] platform/x86: think-lmi: mutex protection around multiple WMI calls

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Message-Id: <f6b0b842-3ef0-4c48-afca-6d9953993c9e@app.fastmail.com>
Date:   Thu, 25 May 2023 15:50:20 -0400
From:   "Mark Pearson" <mpearson-lenovo@...ebb.ca>
To:     "Hans de Goede" <hdegoede@...hat.com>
Cc:     "markgross@...nel.org" <markgross@...nel.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] platform/x86: think-lmi: mutex protection around multiple WMI
 calls



On Thu, May 25, 2023, at 3:41 PM, Hans de Goede wrote:
> Hi Mark,
>
> On 5/25/23 21:31, Mark Pearson wrote:
>> Add mutex protection around cases where an operation needs multiple
>> WMI calls - e.g. setting password.
>> 
>> Signed-off-by: Mark Pearson <mpearson-lenovo@...ebb.ca>
>> ---
>> Changes in V2: New commit added after review of other patches in series.
>> 
>>  drivers/platform/x86/think-lmi.c | 46 ++++++++++++++++++++++++--------
>>  1 file changed, 35 insertions(+), 11 deletions(-)
>> 
>> diff --git a/drivers/platform/x86/think-lmi.c b/drivers/platform/x86/think-lmi.c
>> index 64cd453d6e7d..f3e1e4dacba2 100644
>> --- a/drivers/platform/x86/think-lmi.c
>> +++ b/drivers/platform/x86/think-lmi.c
>> @@ -14,6 +14,7 @@
>>  #include <linux/acpi.h>
>>  #include <linux/errno.h>
>>  #include <linux/fs.h>
>> +#include <linux/mutex.h>
>>  #include <linux/string.h>
>>  #include <linux/types.h>
>>  #include <linux/dmi.h>
>> @@ -195,6 +196,7 @@ static const char * const level_options[] = {
>>  };
>>  static struct think_lmi tlmi_priv;
>>  static struct class *fw_attr_class;
>> +static DEFINE_MUTEX(tlmi_mutex);
>>  
>>  /* ------ Utility functions ------------*/
>>  /* Strip out CR if one is present */
>> @@ -463,23 +465,32 @@ static ssize_t new_password_store(struct kobject *kobj,
>>  			sprintf(pwd_type, "%s", setting->pwd_type);
>>  		}
>>  
>> +		mutex_lock(&tlmi_mutex);
>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordType", pwd_type);
>> -		if (ret)
>> +		if (ret) {
>> +			mutex_unlock(&tlmi_mutex);
>>  			goto out;
>> -
>> +		}
>>  		if (tlmi_priv.pwd_admin->valid) {
>>  			ret = tlmi_opcode_setting("WmiOpcodePasswordAdmin",
>>  					tlmi_priv.pwd_admin->password);
>> -			if (ret)
>> +			if (ret) {
>> +				mutex_unlock(&tlmi_mutex);
>>  				goto out;
>> +			}
>>  		}
>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordCurrent01", setting->password);
>> -		if (ret)
>> +		if (ret) {
>> +			mutex_unlock(&tlmi_mutex);
>>  			goto out;
>> +		}
>>  		ret = tlmi_opcode_setting("WmiOpcodePasswordNew01", new_pwd);
>> -		if (ret)
>> +		if (ret) {
>> +			mutex_unlock(&tlmi_mutex);
>>  			goto out;
>> +		}
>>  		ret = tlmi_simple_call(LENOVO_OPCODE_IF_GUID, "WmiOpcodePasswordSetUpdate;");
>> +		mutex_unlock(&tlmi_mutex);
>>  	} else {
>>  		/* Format: 'PasswordType,CurrentPw,NewPw,Encoding,KbdLang;' */
>>  		auth_str = kasprintf(GFP_KERNEL, "%s,%s,%s,%s,%s;",
>
>
> I haven't take a really close / good look yet. But at a first glance
> I think it would be cleaner to just take the mutex at the top
> and unlock it after the out label to which all the existing goto-s
> already go ?
>
I did consider that - and it was in my first implementation; but then I got concerned
about if the mutex_unlock could potentially get called without mutex_lock having been 
called beforehand. I couldn't find any good reference as to whether that was safe or not.

I ended up deciding that a few extra brackets and unlock calls wasn't that ugly and was 'safer'...and 
so went that route.

Happy to change it - but do you happen to know if it's safe to call unlock without a lock? If it is then
that implementation is cleaner.

Mark