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rcu: Document alternative RCU/reference-count algorithms 

The approach for mixing RCU and reference counting listed in the RCU
documentation only describes one possible approach.  This approach can
result in failure on the read side, which is nice if you want fresh data,
but not so good if you want simple code.  This commit therefore adds
two additional approaches that feature unconditional reference-count
acquisition by RCU readers.  These approaches are very similar to that
used in the security code.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2012-10-27 16:34:51 -07:00
parent 57d34a6cee
commit a4d611fdca
1 changed files with 59 additions and 2 deletions
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61
Documentation/RCU/rcuref.txt
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@ -20,7 +20,7 @@ release_referenced() delete()
{ {
... write_lock(&list_lock);
atomic_dec(&el->rc, relfunc) ...
... delete_element
... remove_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
@ -52,7 +52,7 @@ release_referenced() delete()
{ {
... spin_lock(&list_lock);
if (atomic_dec_and_test(&el->rc)) ...
call_rcu(&el->head, el_free); delete_element
call_rcu(&el->head, el_free); remove_element
... spin_unlock(&list_lock);
} ...
if (atomic_dec_and_test(&el->rc))
@ -64,3 +64,60 @@ Sometimes, a reference to the element needs to be obtained in the
update (write) stream. In such cases, atomic_inc_not_zero() might be
overkill, since we hold the update-side spinlock. One might instead
use atomic_inc() in such cases.
It is not always convenient to deal with "FAIL" in the
search_and_reference() code path. In such cases, the
atomic_dec_and_test() may be moved from delete() to el_free()
as follows:
1. 2.
add() search_and_reference()
{ {
alloc_object rcu_read_lock();
... search_for_element
atomic_set(&el->rc, 1); atomic_inc(&el->rc);
spin_lock(&list_lock); ...
add_element rcu_read_unlock();
... }
spin_unlock(&list_lock); 4.
} delete()
3. {
release_referenced() spin_lock(&list_lock);
{ ...
... remove_element
if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock);
kfree(el); ...
... call_rcu(&el->head, el_free);
} ...
5. }
void el_free(struct rcu_head *rhp)
{
release_referenced();
}
The key point is that the initial reference added by add() is not removed
until after a grace period has elapsed following removal. This means that
search_and_reference() cannot find this element, which means that the value
of el->rc cannot increase. Thus, once it reaches zero, there are no
readers that can or ever will be able to reference the element. The
element can therefore safely be freed. This in turn guarantees that if
any reader finds the element, that reader may safely acquire a reference
without checking the value of the reference counter.
In cases where delete() can sleep, synchronize_rcu() can be called from
delete(), so that el_free() can be subsumed into delete as follows:
4.
delete()
{
spin_lock(&list_lock);
...
remove_element
spin_unlock(&list_lock);
...
synchronize_rcu();
if (atomic_dec_and_test(&el->rc))
kfree(el);
...
}