linux-kernel - Re: Question about usage of RCU in the input layer

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Date:	Sat, 21 Mar 2009 10:13:38 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	Arjan van de Ven <arjan@...radead.org>, paulmck@...ux.vnet.ibm.com,
	dipankar@...ibm.com, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

Dmitry Torokhov a écrit :
> On Thursday 19 March 2009 20:20:32 Arjan van de Ven wrote:
>>> I don't claim to understand the code in question, so it is entirely
>>> possible that the following is irrelevant.  But one other reason for
>>> synchronize_rcu() is:
>>>
>>> 1.    Make change.
>>>
>>> 2.    synchronize_rcu()
>>>
>>> 3.    Now you are guaranteed that all CPUs/tasks/whatever
>>> currently running either are not messing with you on the one hand, or
>>>       have seen the change on the other.
>> ok so this is for the case where someone is already iterating the list.
>>
>> I don't see anything in the code that assumes this..
> 
> This is something that input core guarantees to its users: by the time
> input core calls hander->start() or, in its absence, by the time
> input_register_handle() returns, events from input drivers will be
> passed into the handle being registered, i.e. the presence of the
> new item in the list is noticed by all CPUs.
> 
> Now, it is possible to stop using RCU primitives in the input core
> but I think that you'd want to figure out why synchronize_rcu()
> takes so long first, otheriwse you may find another "abuser"
> down the road.
> 

If a cpu does a loop, it nearly impossible that synchronize_rcu() can
be fast.

We had same problem in ksoftirqd(), where I had to add a call
to rcu_qsctr_inc() to unblock other threads blocked in synchronize_rcu()

http://git2.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=64ca5ab913f1594ef316556e65f5eae63ff50cee

If a driver does a loop with no call to scheduler, it might have same problem


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