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[PATCH] rcu file: use atomic primitives

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Use atomic_inc_not_zero for rcu files instead of special case rcuref.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Nick Piggin
2006-01-08 01:02:19 -08:00
committed by Linus Torvalds
parent a57004e1af
commit 095975da26
9 changed files with 48 additions and 291 deletions
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87
Documentation/RCU/rcuref.txt
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@@ -1,74 +1,67 @@
Refcounter framework for elements of lists/arrays protected by
RCU.
Refcounter design for elements of lists/arrays protected by RCU.
Refcounting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straight forward as in:
1. 2.
add() search_and_reference()
{ {
alloc_object read_lock(&list_lock);
... search_for_element
atomic_set(&el->rc, 1); atomic_inc(&el->rc);
write_lock(&list_lock); ...
add_element read_unlock(&list_lock);
... ...
write_unlock(&list_lock); }
1. 2.
add() search_and_reference()
{ {
alloc_object read_lock(&list_lock);
... search_for_element
atomic_set(&el->rc, 1); atomic_inc(&el->rc);
write_lock(&list_lock); ...
add_element read_unlock(&list_lock);
... ...
write_unlock(&list_lock); }
}
3. 4.
release_referenced() delete()
{ {
... write_lock(&list_lock);
atomic_dec(&el->rc, relfunc) ...
... delete_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
kfree(el);
...
... write_lock(&list_lock);
atomic_dec(&el->rc, relfunc) ...
... delete_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
kfree(el);
...
}
If this list/array is made lock free using rcu as in changing the
write_lock in add() and delete() to spin_lock and changing read_lock
in search_and_reference to rcu_read_lock(), the rcuref_get in
in search_and_reference to rcu_read_lock(), the atomic_get in
search_and_reference could potentially hold reference to an element which
has already been deleted from the list/array. rcuref_lf_get_rcu takes
has already been deleted from the list/array. atomic_inc_not_zero takes
care of this scenario. search_and_reference should look as;
1. 2.
add() search_and_reference()
{ {
alloc_object rcu_read_lock();
... search_for_element
atomic_set(&el->rc, 1); if (rcuref_inc_lf(&el->rc)) {
write_lock(&list_lock); rcu_read_unlock();
return FAIL;
add_element }
... ...
write_unlock(&list_lock); rcu_read_unlock();
alloc_object rcu_read_lock();
... search_for_element
atomic_set(&el->rc, 1); if (atomic_inc_not_zero(&el->rc)) {
write_lock(&list_lock); rcu_read_unlock();
return FAIL;
add_element }
... ...
write_unlock(&list_lock); rcu_read_unlock();
} }
3. 4.
release_referenced() delete()
{ {
... write_lock(&list_lock);
rcuref_dec(&el->rc, relfunc) ...
... delete_element
} write_unlock(&list_lock);
...
if (rcuref_dec_and_test(&el->rc))
call_rcu(&el->head, el_free);
...
... write_lock(&list_lock);
atomic_dec(&el->rc, relfunc) ...
... delete_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
call_rcu(&el->head, el_free);
...
}
Sometimes, reference to the element need to be obtained in the
update (write) stream. In such cases, rcuref_inc_lf might be an overkill
since the spinlock serialising list updates are held. rcuref_inc
update (write) stream. In such cases, atomic_inc_not_zero might be an
overkill since the spinlock serialising list updates are held. atomic_inc
is to be used in such cases.
For arches which do not have cmpxchg rcuref_inc_lf
api uses a hashed spinlock implementation and the same hashed spinlock
is acquired in all rcuref_xxx primitives to preserve atomicity.
Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the
refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api
might lead to races. rcuref_inc_lf() must be used in lockfree
RCU critical sections only.