linux-kernel - Re: per cpun+ spin locks coexistence?

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Date:	Thu, 20 Mar 2008 00:25:04 +0800
From:	Peter Teoh <htmldeveloper@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Peter Teoh <htmldeveloper@...il.com>,
	Johannes Weiner <hannes@...urebad.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Tejun Heo <htejun@...il.com>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: per cpun+ spin locks coexistence?

Eric Dumazet wrote:
> Peter Teoh a écrit :
>> On 3/18/08, Eric Dumazet <dada1@...mosbay.com> wrote:
>>
>>  
>>> You are right Peter, that fs/file.c contains some leftover from 
>>> previous
>>>  implementation of defer queue,
>>>  that was using a timer.
>>>
>>>  So we can probably provide a patch that :
>>>
>>>  - Use spin_lock() & spin_unlock() instead of spin_lock_bh() &
>>>  spin_unlock_bh() in free_fdtable_work()
>>>  since we dont anymore use a softirq (timer) to reschedule the 
>>> workqueue.
>>>
>>>  ( this timer was deleted by the following patch :
>>>  http://readlist.com/lists/vger.kernel.org/linux-kernel/50/251040.html
>>>
>>>
>>>  But, you cannot avoid use of spin_lock()/spin_unlock() because
>>>  schedule_work() makes no garantee that the work will be done by 
>>> this cpu.
>>>     
>>
>> Ah.....u have hit the nail....and combine with Johannes Weiner's
>> explanation, I have pieced together the full scenario:
>>
>> First, the following is possible:
>>
>>                 fddef = &get_cpu_var(fdtable_defer_list);
>>                 spin_lock(&fddef->lock);
>>                 fdt->next = fddef->next;
>>                 fddef->next = fdt;==============>executing at CPU A
>>                 /* vmallocs are handled from the workqueue context */
>>                 schedule_work(&fddef->wq);
>>                 spin_unlock(&fddef->lock);==============>executing at 
>> CPU B
>>                 put_cpu_var(fdtable_defer_list);
>>
>> where the execution can switch CPU after the schedule_work() API, then
>> LOGICALLY u definitely need the spin_lock(), and the per_cpu data is
>> really not necessary.
>>
>> But without the per_cpu structure, then the following "dedicated
>> chunk" can only execute on one processor, with the possibility of
>> switching to another processor after schedule_work():
>>   
> Hum, you misunderstood the point.
>
> schedule_work(); wont switch your current CPU, since you are inside a 
> spin_lock
> ()/spin_unlock() pair, so preemption is not possible.
>
>
>
>> So then we introduce the per_cpu structure - so that the "dedicated
>> chunk" can be executing on multiple processor ALL AT THE SAME TIME,
>> without interferring each other, as fddef are per-cpu (rightfully
>> owned only before schedule_work() is called, but after schedule_work()
>> is called, an arbitrary CPU will be executing this fddef).
>>
>> spin_lock() is necessary because of the possibility of CPU switch
>> (schedule_work()).
>>
>> and per_cpu is so that the same chunk of code can be executing at
>> multiple CPUs all at the same time.
>>
>> Now the key issue rises up - as I have just asked before u answered 
>> my question:
>>
>> http://mail.nl.linux.org/kernelnewbies/2008-03/msg00236.html
>>
>> can schedule_work() sleep?   (just like schedule(), whcih can sleep 
>> right?)
>> schedule_work() is guaranteed to execute the work queue at least once,
>> and so this thread may or may not sleep. correct?   Or wrong?
>>
>>   
> schedule_work() cannot sleep. It only queues a work to be done later 
> by a special thread.
>
> We need this because vfree() should not be called from softirq handler 
> (rcu in this case), so we queue a (small) job.
>> Problem is when u sleep and never wake up, then the spin_lock become
>> permanently locked, and when later the same CPU (have to be the same
>> fddef CPU) is being reschedule to execute the get_cpu_var() again, it
>> will spin_lock() infinitely, resulting in 100% CPU utilization error.
>>
>> To prevent these types of error, spin_lock are always not to be used
>> with to wrap around functions that can sleep, and can only containing
>> short routines between lock and unlock.
>>
>> Is my analysis correct?
>>
>>   
> Not exactly :) , but please continue to learn :)
>

Thank you everyone here for a very informative education.   I will go 
back and analyse in more detail. :-).


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