linux-kernel - Re: [PATCH -v3 3/4] lib, Make gen

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Date:	Mon, 18 Apr 2011 19:42:16 +0800
From:	huang ying <huang.ying.caritas@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Huang Ying <ying.huang@...el.com>, Len Brown <lenb@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH -v3 3/4] lib, Make gen_pool memory allocator lockless

Hi, Paul,

On Sat, Apr 16, 2011 at 1:44 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
>> On Fri, Apr 15, 2011 at 12:46:29PM -0400, Mathieu Desnoyers wrote:
>> > * Huang Ying (ying.huang@...el.com) wrote:
>> > > On 04/14/2011 05:07 AM, Mathieu Desnoyers wrote:
>> > > > * Huang Ying (ying.huang@...el.com) wrote:
>> > > > [...]
>> > > >> + * rcu_read_lock and rcu_read_unlock is not used int gen_pool_alloc,
>> > > >> + * gen_pool_free, gen_pool_avail and gen_pool_size etc, because chunks
>> > > >> + * are only added into pool, not deleted from pool unless the pool
>> > > >> + * itself is destroyed.  If chunk will be deleted from pool,
>> > > >> + * rcu_read_lock and rcu_read_unlock should be uses in these
>> > > >> + * functions.
>> > > >
>> > > > So how do you protect between pool destruction and adding chunks into
>> > > > the pool ?
>> > >
>> > > Because the pool itself will be freed when destruction, we need some
>> > > mechanism outside of pool.  For example, if gen_pool_add() is called via
>> > > device file IOCTL, we must un-register the device file first, and
>> > > destroy the pool after the last reference to device has gone.
>> >
>> > I am concerned about the list_for_each_entry_rcu() (and thus
>> > rcu_dereference_raw()) used outside of rcu_read_lock/unlock pairs.
>> > Validation infrastructure as recently been added to RCU: it triggers
>> > warnings when these situations are encountered in some RCU debugging
>> > configurations. The case of RCU list iteration is not covered by the
>> > checks, but it would make sense to be aware of it.
>> >
>> > So although it seems like your code does not require rcu read lock
>> > critical sections, I'd prefer to let Paul McKenney have a look.
>>
>> As long as you add elements and never remove them, then you can get
>> away with using list_for_each_entry_rcu() outside of an RCU read-side
>> critical section.  But please comment this -- it is all too easy
>> for someone to decide later to start deleting elements without also
>> inserting the needed rcu_read_lock() and rcu_read_unlock() pairs.
>>
>> But I have lost the thread -- what code am I supposed to look at?
>
> You can have a look at https://lkml.org/lkml/2011/4/13/56

What do you think about this patch and its usage of RCU?

Best Regards,
Huang Ying
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