linux-kernel - Re: [PATCH v2 2/3] futex: Create set_robust

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Message-ID: <8373eb11-d61c-40c4-9289-1047ec35c4d6@igalia.com>
Date: Mon, 4 Nov 2024 18:55:45 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>,
 Arnd Bergmann <arnd@...db.de>, sonicadvance1@...il.com,
 linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
 linux-api@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2 2/3] futex: Create set_robust_list2

Hi Peter,

Em 04/11/2024 08:22, Peter Zijlstra escreveu:
> On Fri, Nov 01, 2024 at 01:21:46PM -0300, André Almeida wrote:
>> @@ -1046,24 +1095,44 @@ static inline void exit_pi_state_list(struct task_struct *curr) { }
>>   
>>   static void futex_cleanup(struct task_struct *tsk)
>>   {
>> +	struct robust_list2_entry *curr, *n;
>> +	struct list_head *list2 = &tsk->robust_list2;
>> +
>>   #ifdef CONFIG_64BIT
>>   	if (unlikely(tsk->robust_list)) {
>> -		exit_robust_list64(tsk);
>> +		exit_robust_list64(tsk, tsk->robust_list);
>>   		tsk->robust_list = NULL;
>>   	}
>>   #else
>>   	if (unlikely(tsk->robust_list)) {
>> -		exit_robust_list32(tsk);
>> +		exit_robust_list32(tsk, (struct robust_list_head32 *) tsk->robust_list);
>>   		tsk->robust_list = NULL;
>>   	}
>>   #endif
>>   
>>   #ifdef CONFIG_COMPAT
>>   	if (unlikely(tsk->compat_robust_list)) {
>> -		exit_robust_list32(tsk);
>> +		exit_robust_list32(tsk, tsk->compat_robust_list);
>>   		tsk->compat_robust_list = NULL;
>>   	}
>>   #endif
>> +	/*
>> +	 * Walk through the linked list, parsing robust lists and freeing the
>> +	 * allocated lists
>> +	 */
>> +	if (unlikely(!list_empty(list2))) {
>> +		list_for_each_entry_safe(curr, n, list2, list) {
>> +			if (curr->head != NULL) {
>> +				if (curr->list_type == ROBUST_LIST_64BIT)
>> +					exit_robust_list64(tsk, curr->head);
>> +				else if (curr->list_type == ROBUST_LIST_32BIT)
>> +					exit_robust_list32(tsk, curr->head);
>> +				curr->head = NULL;
>> +			}
>> +			list_del_init(&curr->list);
>> +			kfree(curr);
>> +		}
>> +	}
>>   
>>   	if (unlikely(!list_empty(&tsk->pi_state_list)))
>>   		exit_pi_state_list(tsk);
> 
> I'm still digesting this, but the above seems particularly silly.
> 
> Should not the legacy lists also be on the list of lists? I mean, it
> makes no sense to have two completely separate means of tracking lists.
> 

You are asking if, whenever someone calls set_robust_list() or 
compat_set_robust_list() to be inserted into &current->robust_list2 
instead of using tsk->robust_list and tsk->compat_robust_list?

I was thinking of doing that, but my current implementation has a 
kmalloc() call for every insertion, and I wasn't sure if I could add 
this new latency to the old set_robust_list() syscall. Assuming it is 
usually called just once during the thread initialization perhaps it 
shouldn't cause much harm I guess.