linux-kernel - Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

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Date:	Thu, 05 Mar 2015 13:18:25 +0200
From:	Alex Dowad <alexinbeijing@...il.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kees Cook <keescook@...omium.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual
 use


On 05/03/15 01:07, David Rientjes wrote:
> On Wed, 4 Mar 2015, Alex Dowad wrote:
>
>> The 'stack_size' argument is never used to pass a stack size. It's only used when
>> forking a kernel thread, in which case it is an argument which should be passed
>> to the 'main' function which the kernel thread executes. Hence, rename it to
>> 'kthread_arg'.
>>
>> Signed-off-by: Alex Dowad <alexinbeijing@...il.com>
>> ---
>>
>> Hi,
>>
>> Please have a look at this patch. If this is accepted, I have a series of patches
>> ready for a similar cleanup to all the arch-specific implementations of copy_thread()
>> (as suggested by Andrew Morton in a private e-mail).
>>
>> Thank you,
>> Alex Dowad
>>
>>   kernel/fork.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index cf65139..b38a2ae 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
>>    * It copies the registers, and all the appropriate
>>    * parts of the process environment (as per the clone
>>    * flags). The actual kick-off is left to the caller.
>> + *
>> + * When copying a kernel thread, 'stack_start' is the function to run.
>>    */
>>   static struct task_struct *copy_process(unsigned long clone_flags,
>>   					unsigned long stack_start,
>> -					unsigned long stack_size,
>> +					unsigned long kthread_arg,
>>   					int __user *child_tidptr,
>>   					struct pid *pid,
>>   					int trace)
>> @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>   	retval = copy_io(clone_flags, p);
>>   	if (retval)
>>   		goto bad_fork_cleanup_namespaces;
>> -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
>> +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
>>   	if (retval)
>>   		goto bad_fork_cleanup_io;
>>   
>> @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
>>    * it and waits for it to finish using the VM if required.
>>    */
>>   long do_fork(unsigned long clone_flags,
>> -	      unsigned long stack_start,
>> -	      unsigned long stack_size,
>> +	      unsigned long stack_start, /* or function for kthread to run */
>> +	      unsigned long kthread_arg,
>>   	      int __user *parent_tidptr,
>>   	      int __user *child_tidptr)
>>   {
> Looks fine, but I'm not sure about commenting functional formals.  Since
> copy_process() and do_fork() can have formals with different meanings,
> then why not just rename them "arg1" and "arg2" respectively and then
> define in the comment above the function what the possible combinations
> are?

The second argument is *only* ever used for one thing: an argument 
passed to a kernel thread. That's why I would like to rename it to 
"kthread_arg". The previous argument (currently named "stack_start") is 
indeed used for 2 different things: a new stack pointer for a user 
thread, or a function to be executed by a kernel thread. Rather than 
"arg1", what would you think of something like "sp_or_fn", or "usp_or_fn"?

Thanks for your feedback!

>
>> @@ -1656,7 +1658,7 @@ long do_fork(unsigned long clone_flags,
>>   			trace = 0;
>>   	}
>>   
>> -	p = copy_process(clone_flags, stack_start, stack_size,
>> +	p = copy_process(clone_flags, stack_start, kthread_arg,
>>   			 child_tidptr, NULL, trace);
>>   	/*
>>   	 * Do this prior waking up the new thread - the thread pointer
>> @@ -1740,7 +1742,7 @@ SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
>>   		 int, tls_val)
>>   #elif defined(CONFIG_CLONE_BACKWARDS3)
>>   SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
>> -		int, stack_size,
>> +		int, ignored,
>>   		int __user *, parent_tidptr,
>>   		int __user *, child_tidptr,
>>   		int, tls_val)

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