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docs: RCU: Convert rcuref.txt to ReST

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- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to RCU/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Mauro Carvalho Chehab 2020-04-21 19:04:07 +02:00 committed by Paul E. McKenney
parent 43cb5451df
commit 90c73cb2c6
2 changed files with 101 additions and 93 deletions
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1
Documentation/RCU/index.rst
View File

@ -18,6 +18,7 @@ RCU concepts
whatisRCU
rcu
rculist_nulls
rcuref
torture
listRCU
NMI-RCU

193
Documentation/RCU/rcuref.txt → Documentation/RCU/rcuref.rst
View File

@ -1,4 +1,8 @@
Reference-count design for elements of lists/arrays protected by RCU.
.. SPDX-License-Identifier: GPL-2.0
====================================================================
Reference-count design for elements of lists/arrays protected by RCU
====================================================================
Please note that the percpu-ref feature is likely your first
@ -12,32 +16,33 @@ please read on.
Reference counting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straightforward:
CODE LISTING A:
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); }
}
CODE LISTING A::
3. 4.
release_referenced() delete()
{ {
... write_lock(&list_lock);
if(atomic_dec_and_test(&el->rc)) ...
kfree(el);
... remove_element
} write_unlock(&list_lock);
...
if (atomic_dec_and_test(&el->rc))
kfree(el);
...
}
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);
if(atomic_dec_and_test(&el->rc)) ...
kfree(el);
... remove_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()
@ -46,34 +51,35 @@ search_and_reference() could potentially hold reference to an element which
has already been deleted from the list/array. Use atomic_inc_not_zero()
in this scenario as follows:
CODE LISTING B:
1. 2.
add() search_and_reference()
{ {
alloc_object rcu_read_lock();
... search_for_element
atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
spin_lock(&list_lock); rcu_read_unlock();
return FAIL;
add_element }
... ...
spin_unlock(&list_lock); rcu_read_unlock();
} }
3. 4.
release_referenced() delete()
{ {
... spin_lock(&list_lock);
if (atomic_dec_and_test(&el->rc)) ...
call_rcu(&el->head, el_free); remove_element
... spin_unlock(&list_lock);
} ...
if (atomic_dec_and_test(&el->rc))
call_rcu(&el->head, el_free);
...
}
CODE LISTING B::
1. 2.
add() search_and_reference()
{ {
alloc_object rcu_read_lock();
... search_for_element
atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
spin_lock(&list_lock); rcu_read_unlock();
return FAIL;
add_element }
... ...
spin_unlock(&list_lock); rcu_read_unlock();
} }
3. 4.
release_referenced() delete()
{ {
... spin_lock(&list_lock);
if (atomic_dec_and_test(&el->rc)) ...
call_rcu(&el->head, el_free); remove_element
... spin_unlock(&list_lock);
} ...
if (atomic_dec_and_test(&el->rc))
call_rcu(&el->head, el_free);
...
}
Sometimes, a reference to the element needs to be obtained in the
update (write) stream. In such cases, atomic_inc_not_zero() might be
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.
@ -82,39 +88,40 @@ search_and_reference() code path. In such cases, the
atomic_dec_and_test() may be moved from delete() to el_free()
as follows:
CODE LISTING C:
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); ...
CODE LISTING C::
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();
}
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
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.
@ -130,21 +137,21 @@ the eventual invocation of kfree(), which is usually not a problem on
modern computer systems, even the small ones.
In cases where delete() can sleep, synchronize_rcu() can be called from
delete(), so that el_free() can be subsumed into delete as follows:
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);
...
}
4.
delete()
{
spin_lock(&list_lock);
...
remove_element
spin_unlock(&list_lock);
...
synchronize_rcu();
if (atomic_dec_and_test(&el->rc))
kfree(el);
...
}
As additional examples in the kernel, the pattern in listing C is used by
reference counting of struct pid, while the pattern in listing B is used by