linux-kernel - array of pointers with rcu

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Date:	Sun, 28 Jun 2009 16:22:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com
Subject: array of pointers with rcu

Paul,
I'd like to implement a static array of pointers with rcu.
(Note that Documentation/RCU/arrayRCU.txt addresses only the case of
static arrays where the data (rather than a pointer to the data) is
located in each array element).
The array is implemented today in kvm as follows:

struct kvm_io_bus {
        int                   dev_count;
#define NR_IOBUS_DEVS 6
        struct kvm_io_device *devs[NR_IOBUS_DEVS];
};

It's easy to use rcu_assign_pointer and
rcu_dereference to access each value in the array,
so that's fine.

However, I also have the dev_count value to handle.
This value is usually used in loops like this

for (i = 0; i < bus->dev_count; ++i) {
	... uses bus->devs[i] ...
}

I can assign the dev_count value with rcu_assign_pointer and even though
it is not a pointer I think it should work fine.  However, to access
dev_count I think that rcu_dereference will not do what I want:

#define rcu_dereference(p)     ({ \
                                typeof(p) _________p1 = ACCESS_ONCE(p);
\
                                smp_read_barrier_depends(); \
                                (_________p1); \
                                })

The use of dev_count is not through a dependency so
smp_read_barrier_depends will not be enough on most architectures.

So it seems that what I really need is something like:
#define rcu_read_value(p)     ({ \
                                typeof(p) _________p1 = ACCESS_ONCE(p);
\
                                smp_rmb(); \
                                (_________p1); \
                                })

And maybe
#define rcu_assign_value rcu_assign_pointer
for symmetry.

Comments?

-- 
MST
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