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

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Message-ID: <20110415174320.GA2283@linux.vnet.ibm.com>
Date:	Fri, 15 Apr 2011 10:43:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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>
Subject: Re: [PATCH -v3 3/4] lib, Make gen_pool memory allocator lockless

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?

							Thanx, Paul
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