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Diffstat (limited to 'Documentation/RCU/rcuref.txt')
| -rw-r--r-- | Documentation/RCU/rcuref.txt | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt new file mode 100644 index 000000000000..a23fee66064d --- /dev/null +++ b/Documentation/RCU/rcuref.txt | |||
| @@ -0,0 +1,74 @@ | |||
| 1 | Refcounter framework for elements of lists/arrays protected by | ||
| 2 | RCU. | ||
| 3 | |||
| 4 | Refcounting on elements of lists which are protected by traditional | ||
| 5 | reader/writer spinlocks or semaphores are straight forward as in: | ||
| 6 | |||
| 7 | 1. 2. | ||
| 8 | add() search_and_reference() | ||
| 9 | { { | ||
| 10 | alloc_object read_lock(&list_lock); | ||
| 11 | ... search_for_element | ||
| 12 | atomic_set(&el->rc, 1); atomic_inc(&el->rc); | ||
| 13 | write_lock(&list_lock); ... | ||
| 14 | add_element read_unlock(&list_lock); | ||
| 15 | ... ... | ||
| 16 | write_unlock(&list_lock); } | ||
| 17 | } | ||
| 18 | |||
| 19 | 3. 4. | ||
| 20 | release_referenced() delete() | ||
| 21 | { { | ||
| 22 | ... write_lock(&list_lock); | ||
| 23 | atomic_dec(&el->rc, relfunc) ... | ||
| 24 | ... delete_element | ||
| 25 | } write_unlock(&list_lock); | ||
| 26 | ... | ||
| 27 | if (atomic_dec_and_test(&el->rc)) | ||
| 28 | kfree(el); | ||
| 29 | ... | ||
| 30 | } | ||
| 31 | |||
| 32 | If this list/array is made lock free using rcu as in changing the | ||
| 33 | write_lock in add() and delete() to spin_lock and changing read_lock | ||
| 34 | in search_and_reference to rcu_read_lock(), the rcuref_get in | ||
| 35 | search_and_reference could potentially hold reference to an element which | ||
| 36 | has already been deleted from the list/array. rcuref_lf_get_rcu takes | ||
| 37 | care of this scenario. search_and_reference should look as; | ||
| 38 | |||
| 39 | 1. 2. | ||
| 40 | add() search_and_reference() | ||
| 41 | { { | ||
| 42 | alloc_object rcu_read_lock(); | ||
| 43 | ... search_for_element | ||
| 44 | atomic_set(&el->rc, 1); if (rcuref_inc_lf(&el->rc)) { | ||
| 45 | write_lock(&list_lock); rcu_read_unlock(); | ||
| 46 | return FAIL; | ||
| 47 | add_element } | ||
| 48 | ... ... | ||
| 49 | write_unlock(&list_lock); rcu_read_unlock(); | ||
| 50 | } } | ||
| 51 | 3. 4. | ||
| 52 | release_referenced() delete() | ||
| 53 | { { | ||
| 54 | ... write_lock(&list_lock); | ||
| 55 | rcuref_dec(&el->rc, relfunc) ... | ||
| 56 | ... delete_element | ||
| 57 | } write_unlock(&list_lock); | ||
| 58 | ... | ||
| 59 | if (rcuref_dec_and_test(&el->rc)) | ||
| 60 | call_rcu(&el->head, el_free); | ||
| 61 | ... | ||
| 62 | } | ||
| 63 | |||
| 64 | Sometimes, reference to the element need to be obtained in the | ||
| 65 | update (write) stream. In such cases, rcuref_inc_lf might be an overkill | ||
| 66 | since the spinlock serialising list updates are held. rcuref_inc | ||
| 67 | is to be used in such cases. | ||
| 68 | For arches which do not have cmpxchg rcuref_inc_lf | ||
| 69 | api uses a hashed spinlock implementation and the same hashed spinlock | ||
| 70 | is acquired in all rcuref_xxx primitives to preserve atomicity. | ||
| 71 | Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the | ||
| 72 | refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api | ||
| 73 | might lead to races. rcuref_inc_lf() must be used in lockfree | ||
| 74 | RCU critical sections only. | ||