linux-kernel - Re: [PATCH] mm: memory-failure: use rcu lock instead of tasklist_lock when collect

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Message-ID: <132f7a1a-3219-bdac-5ca4-ad5a54b09616@huawei.com>
Date:   Mon, 21 Aug 2023 14:35:30 +0800
From:   Tong Tiangen <tongtiangen@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        <wangkefeng.wang@...wei.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: memory-failure: use rcu lock instead of tasklist_lock
 when collect_procs()



在 2023/8/21 12:34, Matthew Wilcox 写道:
> On Mon, Aug 21, 2023 at 10:25:34AM +0800, Tong Tiangen wrote:
>> +++ b/mm/memory-failure.c
>> @@ -546,24 +546,26 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>>    * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
>>    * on behalf of the thread group. Return task_struct of the (first found)
>>    * dedicated thread if found, and return NULL otherwise.
>> - *
>> - * We already hold read_lock(&tasklist_lock) in the caller, so we don't
>> - * have to call rcu_read_lock/unlock() in this function.
>>    */
>>   static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
>>   {
>>   	struct task_struct *t;
>>   
>> +	rcu_read_lock();
>>   	for_each_thread(tsk, t) {
>>   		if (t->flags & PF_MCE_PROCESS) {
>>   			if (t->flags & PF_MCE_EARLY)
>> -				return t;
>> +				goto found;
>>   		} else {
>>   			if (sysctl_memory_failure_early_kill)
>> -				return t;
>> +				goto found;
>>   		}
>>   	}
>> -	return NULL;
>> +
>> +	t = NULL;
>> +found:
>> +	rcu_read_unlock();
>> +	return t;
>>   }
> 
> I don't understand why you need to modify find_early_kill_thread() at
> all.  It's still true that the caller holds _a_ lock protecting it; the
> comment needs to be updated to reflect that it might be the RCU lock
> or the tasklist_lock (or did you change all callers?), but there's no
> need for this function to take the RCU lock itself, afaics?
> 
> .

I've checked that all the paths that call find_early_kill_thread() 
already hold the rcu lock, and there's really no need to hold the rcu 
lock here.
In the next patch version, here only the comments are modified.

Thanks,
Tong.