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>Protecting A Collection of Objects: Reference Counts</H1
><P
> Locking a collection of objects is fairly easy: you get a
single spinlock, and you make sure you grab it before
searching, adding or deleting an object.
</P
><P
> The purpose of this lock is not to protect the individual
objects: you might have a separate lock inside each one for
that. It is to protect the <I
CLASS="EMPHASIS"
>data structure
containing the objects</I
> from race conditions. Often
the same lock is used to protect the contents of all the
objects as well, for simplicity, but they are inherently
orthogonal (and many other big words designed to confuse).
</P
><P
> Changing this to a read-write lock will often help markedly if
reads are far more common that writes. If not, there is
another approach you can use to reduce the time the lock is
held: reference counts.
</P
><P
> In this approach, an object has an owner, who sets the
reference count to one. Whenever you get a pointer to the
object, you increment the reference count (a `get' operation).
Whenever you relinquish a pointer, you decrement the reference
count (a `put' operation). When the owner wants to destroy
it, they mark it dead, and do a put.
</P
><P
> Whoever drops the reference count to zero (usually implemented
with <TT
CLASS="FUNCTION"
>atomic_dec_and_test()</TT
>) actually cleans
up and frees the object.
</P
><P
> This means that you are guaranteed that the object won't
vanish underneath you, even though you no longer have a lock
for the collection.
</P
><P
> Here's some skeleton code:
</P
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> void create_foo(struct foo *x)
{
atomic_set(&x->use, 1);
spin_lock_bh(&list_lock);
... insert in list ...
spin_unlock_bh(&list_lock);
}
struct foo *get_foo(int desc)
{
struct foo *ret;
spin_lock_bh(&list_lock);
... find in list ...
if (ret) atomic_inc(&ret->use);
spin_unlock_bh(&list_lock);
return ret;
}
void put_foo(struct foo *x)
{
if (atomic_dec_and_test(&x->use))
kfree(foo);
}
void destroy_foo(struct foo *x)
{
spin_lock_bh(&list_lock);
... remove from list ...
spin_unlock_bh(&list_lock);
put_foo(x);
}
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>Macros To Help You</H2
><P
> There are a set of debugging macros tucked inside
<TT
CLASS="FILENAME"
>include/linux/netfilter_ipv4/lockhelp.h</TT
>
and <TT
CLASS="FILENAME"
>listhelp.h</TT
>: these are very
useful for ensuring that locks are held in the right places to protect
infrastructure.
</P
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