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docs: RCU: Convert rcuref.txt to ReST 

- 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 whatisRCU
rcu rcu
rculist_nulls rculist_nulls
rcuref
torture torture
listRCU listRCU
NMI-RCU 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 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 Reference counting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straightforward: reader/writer spinlocks or semaphores are straightforward:
CODE LISTING A: 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); }
}
3. 4. 1. 2.
release_referenced() delete() add() search_and_reference()
{ { { {
... write_lock(&list_lock); alloc_object read_lock(&list_lock);
if(atomic_dec_and_test(&el->rc)) ... ... search_for_element
kfree(el); atomic_set(&el->rc, 1); atomic_inc(&el->rc);
... remove_element write_lock(&list_lock); ...
} write_unlock(&list_lock); add_element read_unlock(&list_lock);
... ... ...
if (atomic_dec_and_test(&el->rc)) write_unlock(&list_lock); }
kfree(el); }
...
} 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 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() 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() has already been deleted from the list/array. Use atomic_inc_not_zero()
in this scenario as follows: in this scenario as follows:
CODE LISTING B: CODE LISTING B::
1. 2.
add() search_and_reference() 1. 2.
{ { add() search_and_reference()
alloc_object rcu_read_lock(); { {
... search_for_element alloc_object rcu_read_lock();
atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) { ... search_for_element
spin_lock(&list_lock); rcu_read_unlock(); atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) {
return FAIL; spin_lock(&list_lock); rcu_read_unlock();
add_element } return FAIL;
... ... add_element }
spin_unlock(&list_lock); rcu_read_unlock(); ... ...
} } spin_unlock(&list_lock); rcu_read_unlock();
3. 4. } }
release_referenced() delete() 3. 4.
{ { release_referenced() delete()
... spin_lock(&list_lock); { {
if (atomic_dec_and_test(&el->rc)) ... ... spin_lock(&list_lock);
call_rcu(&el->head, el_free); remove_element if (atomic_dec_and_test(&el->rc)) ...
... spin_unlock(&list_lock); 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); 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 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 overkill, since we hold the update-side spinlock. One might instead
use atomic_inc() in such cases. 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() atomic_dec_and_test() may be moved from delete() to el_free()
as follows: as follows:
CODE LISTING C: 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); ...
add_element rcu_read_unlock(); 1. 2.
... } add() search_and_reference()
spin_unlock(&list_lock); 4. { {
} delete() alloc_object rcu_read_lock();
3. { ... search_for_element
release_referenced() spin_lock(&list_lock); atomic_set(&el->rc, 1); atomic_inc(&el->rc);
{ ... spin_lock(&list_lock); ...
... remove_element
if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock); add_element rcu_read_unlock();
kfree(el); ... ... }
... call_rcu(&el->head, el_free); spin_unlock(&list_lock); 4.
} ... } delete()
5. } 3. {
void el_free(struct rcu_head *rhp) release_referenced() spin_lock(&list_lock);
{ { ...
release_referenced(); ... 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 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 until after a grace period has elapsed following removal. This means that
search_and_reference() cannot find this element, which means that the value 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 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 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 element can therefore safely be freed. This in turn guarantees that if
any reader finds the element, that reader may safely acquire a reference any reader finds the element, that reader may safely acquire a reference
without checking the value of the reference counter. 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. modern computer systems, even the small ones.
In cases where delete() can sleep, synchronize_rcu() can be called from 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. 4.
delete() delete()
{ {
spin_lock(&list_lock); spin_lock(&list_lock);
... ...
remove_element remove_element
spin_unlock(&list_lock); spin_unlock(&list_lock);
... ...
synchronize_rcu(); synchronize_rcu();
if (atomic_dec_and_test(&el->rc)) if (atomic_dec_and_test(&el->rc))
kfree(el); kfree(el);
... ...
} }
As additional examples in the kernel, the pattern in listing C is used by 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 reference counting of struct pid, while the pattern in listing B is used by