diff options
| -rw-r--r-- | Documentation/RCU/RTFP.txt | 25 | ||||
| -rw-r--r-- | Documentation/RCU/checklist.txt | 6 | ||||
| -rw-r--r-- | Documentation/RCU/listRCU.txt | 21 | ||||
| -rw-r--r-- | Documentation/RCU/rcu.txt | 5 | ||||
| -rw-r--r-- | Documentation/RCU/rcuref.txt | 31 | ||||
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 29 |
6 files changed, 69 insertions, 48 deletions
diff --git a/Documentation/RCU/RTFP.txt b/Documentation/RCU/RTFP.txt index fcbcbc3..6221464 100644 --- a/Documentation/RCU/RTFP.txt +++ b/Documentation/RCU/RTFP.txt @@ -90,16 +90,20 @@ at OLS. The resulting abundance of RCU patches was presented the following year [McKenney02a], and use of RCU in dcache was first described that same year [Linder02a]. -Also in 2002, Michael [Michael02b,Michael02a] presented techniques -that defer the destruction of data structures to simplify non-blocking -synchronization (wait-free synchronization, lock-free synchronization, -and obstruction-free synchronization are all examples of non-blocking -synchronization). In particular, this technique eliminates locking, -reduces contention, reduces memory latency for readers, and parallelizes -pipeline stalls and memory latency for writers. However, these -techniques still impose significant read-side overhead in the form of -memory barriers. Researchers at Sun worked along similar lines in the -same timeframe [HerlihyLM02,HerlihyLMS03]. +Also in 2002, Michael [Michael02b,Michael02a] presented "hazard-pointer" +techniques that defer the destruction of data structures to simplify +non-blocking synchronization (wait-free synchronization, lock-free +synchronization, and obstruction-free synchronization are all examples of +non-blocking synchronization). In particular, this technique eliminates +locking, reduces contention, reduces memory latency for readers, and +parallelizes pipeline stalls and memory latency for writers. However, +these techniques still impose significant read-side overhead in the +form of memory barriers. Researchers at Sun worked along similar lines +in the same timeframe [HerlihyLM02,HerlihyLMS03]. These techniques +can be thought of as inside-out reference counts, where the count is +represented by the number of hazard pointers referencing a given data +structure (rather than the more conventional counter field within the +data structure itself). In 2003, the K42 group described how RCU could be used to create hot-pluggable implementations of operating-system functions. Later that @@ -113,7 +117,6 @@ number of operating-system kernels [PaulEdwardMcKenneyPhD], a paper describing how to make RCU safe for soft-realtime applications [Sarma04c], and a paper describing SELinux performance with RCU [JamesMorris04b]. - 2005 has seen further adaptation of RCU to realtime use, permitting preemption of RCU realtime critical sections [PaulMcKenney05a, PaulMcKenney05b]. diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index e118a7c..49e27cc 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -177,3 +177,9 @@ over a rather long period of time, but improvements are always welcome! If you want to wait for some of these other things, you might instead need to use synchronize_irq() or synchronize_sched(). + +12. Any lock acquired by an RCU callback must be acquired elsewhere + with irq disabled, e.g., via spin_lock_irqsave(). Failing to + disable irq on a given acquisition of that lock will result in + deadlock as soon as the RCU callback happens to interrupt that + acquisition's critical section. diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.txt index f8a54fa..1fd1753 100644 --- a/Documentation/RCU/listRCU.txt +++ b/Documentation/RCU/listRCU.txt @@ -232,7 +232,7 @@ entry does not exist. For this to be helpful, the search function must return holding the per-entry spinlock, as ipc_lock() does in fact do. Quick Quiz: Why does the search function need to return holding the -per-entry lock for this deleted-flag technique to be helpful? + per-entry lock for this deleted-flag technique to be helpful? If the system-call audit module were to ever need to reject stale data, one way to accomplish this would be to add a "deleted" flag and a "lock" @@ -275,8 +275,8 @@ flag under the spinlock as follows: { struct audit_entry *e; - /* Do not use the _rcu iterator here, since this is the only - * deletion routine. */ + /* Do not need to use the _rcu iterator here, since this + * is the only deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { spin_lock(&e->lock); @@ -304,9 +304,12 @@ function to reject newly deleted data. Answer to Quick Quiz - -If the search function drops the per-entry lock before returning, then -the caller will be processing stale data in any case. If it is really -OK to be processing stale data, then you don't need a "deleted" flag. -If processing stale data really is a problem, then you need to hold the -per-entry lock across all of the code that uses the value looked up. + Why does the search function need to return holding the per-entry + lock for this deleted-flag technique to be helpful? + + If the search function drops the per-entry lock before returning, + then the caller will be processing stale data in any case. If it + is really OK to be processing stale data, then you don't need a + "deleted" flag. If processing stale data really is a problem, + then you need to hold the per-entry lock across all of the code + that uses the value that was returned. diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt index 6fa0922..02e27bf 100644 --- a/Documentation/RCU/rcu.txt +++ b/Documentation/RCU/rcu.txt @@ -111,6 +111,11 @@ o What are all these files in this directory? You are reading it! + rcuref.txt + + Describes how to combine use of reference counts + with RCU. + whatisRCU.txt Overview of how the RCU implementation works. Along diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt index 3f60db41..451de2a 100644 --- a/Documentation/RCU/rcuref.txt +++ b/Documentation/RCU/rcuref.txt @@ -1,7 +1,7 @@ -Refcounter design for elements of lists/arrays protected by RCU. +Reference-count 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: +Reference counting on elements of lists which are protected by traditional +reader/writer spinlocks or semaphores are straightforward: 1. 2. add() search_and_reference() @@ -28,12 +28,12 @@ release_referenced() delete() ... } -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 +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 atomic_get in search_and_reference could potentially hold reference to an element which -has already been deleted from the list/array. atomic_inc_not_zero takes -care of this scenario. search_and_reference should look as; +has already been deleted from the list/array. Use atomic_inc_not_zero() +in this scenario as follows: 1. 2. add() search_and_reference() @@ -51,17 +51,16 @@ add() search_and_reference() 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)) ... + call_rcu(&el->head, el_free); 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, 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. - +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. diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 15da168..5ed85af 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -200,10 +200,11 @@ rcu_assign_pointer() the new value, and also executes any memory-barrier instructions required for a given CPU architecture. - Perhaps more important, it serves to document which pointers - are protected by RCU. That said, rcu_assign_pointer() is most - frequently used indirectly, via the _rcu list-manipulation - primitives such as list_add_rcu(). + Perhaps just as important, it serves to document (1) which + pointers are protected by RCU and (2) the point at which a + given structure becomes accessible to other CPUs. That said, + rcu_assign_pointer() is most frequently used indirectly, via + the _rcu list-manipulation primitives such as list_add_rcu(). rcu_dereference() @@ -258,9 +259,11 @@ rcu_dereference() locking. As with rcu_assign_pointer(), an important function of - rcu_dereference() is to document which pointers are protected - by RCU. And, again like rcu_assign_pointer(), rcu_dereference() - is typically used indirectly, via the _rcu list-manipulation + rcu_dereference() is to document which pointers are protected by + RCU, in particular, flagging a pointer that is subject to changing + at any time, including immediately after the rcu_dereference(). + And, again like rcu_assign_pointer(), rcu_dereference() is + typically used indirectly, via the _rcu list-manipulation primitives, such as list_for_each_entry_rcu(). The following diagram shows how each API communicates among the @@ -327,7 +330,7 @@ for specialized uses, but are relatively uncommon. 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? This section shows a simple use of the core RCU API to protect a -global pointer to a dynamically allocated structure. More typical +global pointer to a dynamically allocated structure. More-typical uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. struct foo { @@ -410,6 +413,8 @@ o Use synchronize_rcu() -after- removing a data element from an data item. See checklist.txt for additional rules to follow when using RCU. +And again, more-typical uses of RCU may be found in listRCU.txt, +arrayRCU.txt, and NMI-RCU.txt. 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? @@ -513,7 +518,7 @@ production-quality implementation, and see: for papers describing the Linux kernel RCU implementation. The OLS'01 and OLS'02 papers are a good introduction, and the dissertation provides -more details on the current implementation. +more details on the current implementation as of early 2004. 5A. "TOY" IMPLEMENTATION #1: LOCKING @@ -768,7 +773,6 @@ RCU pointer/list traversal: rcu_dereference list_for_each_rcu (to be deprecated in favor of list_for_each_entry_rcu) - list_for_each_safe_rcu (deprecated, not used) list_for_each_entry_rcu list_for_each_continue_rcu (to be deprecated in favor of new list_for_each_entry_continue_rcu) @@ -807,7 +811,8 @@ Quick Quiz #1: Why is this argument naive? How could a deadlock Answer: Consider the following sequence of events: 1. CPU 0 acquires some unrelated lock, call it - "problematic_lock". + "problematic_lock", disabling irq via + spin_lock_irqsave(). 2. CPU 1 enters synchronize_rcu(), write-acquiring rcu_gp_mutex. @@ -894,7 +899,7 @@ Answer: Just as PREEMPT_RT permits preemption of spinlock ACKNOWLEDGEMENTS My thanks to the people who helped make this human-readable, including -Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood. +Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern. For more information, see http://www.rdrop.com/users/paulmck/RCU. |