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Subject[PATCH tip/core/rcu 9/9] rcu: documentation update for CONFIG_PROVE_RCU
Date
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Adds a lockdep.txt file and updates checklist.txt and whatisRCU.txt
to reflect the new lockdep-enabled capabilities of RCU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/00-INDEX | 2 +
Documentation/RCU/checklist.txt | 34 +++++++++++--------
Documentation/RCU/lockdep.txt | 67 +++++++++++++++++++++++++++++++++++++++
Documentation/RCU/whatisRCU.txt | 13 +++++---
4 files changed, 97 insertions(+), 19 deletions(-)
create mode 100644 Documentation/RCU/lockdep.txt
diff --git a/Documentation/RCU/00-INDEX b/Documentation/RCU/00-INDEX
index 0a27ea9..71b6f50 100644
--- a/Documentation/RCU/00-INDEX
+++ b/Documentation/RCU/00-INDEX
@@ -6,6 +6,8 @@ checklist.txt
- Review Checklist for RCU Patches
listRCU.txt
- Using RCU to Protect Read-Mostly Linked Lists
+lockdep.txt
+ - RCU and lockdep checking
NMI-RCU.txt
- Using RCU to Protect Dynamic NMI Handlers
rcubarrier.txt
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 767cf06..cbc180f 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -127,10 +127,14 @@ over a rather long period of time, but improvements are always welcome!
perfectly legal (if redundant) for update-side code to
use rcu_dereference() and the "_rcu()" list-traversal
primitives. This is particularly useful in code that
- is common to readers and updaters. However, neither
- rcu_dereference() nor the "_rcu()" list-traversal
- primitives can substitute for a good concurrency design
- coordinating among multiple updaters.
+ is common to readers and updaters. However, lockdep
+ will complain if you access rcu_dereference() outside
+ of an RCU read-side critical section. See lockdep.txt
+ to learn what to do about this.
+
+ Of course, neither rcu_dereference() nor the "_rcu()"
+ list-traversal primitives can substitute for a good
+ concurrency design coordinating among multiple updaters.

b. If the list macros are being used, the list_add_tail_rcu()
and list_add_rcu() primitives must be used in order
@@ -249,7 +253,9 @@ over a rather long period of time, but improvements are always welcome!
must be protected by appropriate update-side locks. RCU
read-side critical sections are delimited by rcu_read_lock()
and rcu_read_unlock(), or by similar primitives such as
- rcu_read_lock_bh() and rcu_read_unlock_bh().
+ rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
+ the matching rcu_dereference() primitive must be used in order
+ to keep lockdep happy, in this case, rcu_dereference_bh().

The reason that it is permissible to use RCU list-traversal
primitives when the update-side lock is held is that doing so
@@ -302,15 +308,15 @@ over a rather long period of time, but improvements are always welcome!
not the case, a self-spawning RCU callback would prevent the
victim CPU from ever going offline.)

-14. SRCU (srcu_read_lock(), srcu_read_unlock(), synchronize_srcu(),
- and synchronize_srcu_expedited()) may only be invoked from
- process context. Unlike other forms of RCU, it -is- permissible
- to block in an SRCU read-side critical section (demarked by
- srcu_read_lock() and srcu_read_unlock()), hence the "SRCU":
- "sleepable RCU". Please note that if you don't need to sleep
- in read-side critical sections, you should be using RCU rather
- than SRCU, because RCU is almost always faster and easier to
- use than is SRCU.
+14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
+ synchronize_srcu(), and synchronize_srcu_expedited()) may only
+ be invoked from process context. Unlike other forms of RCU, it
+ -is- permissible to block in an SRCU read-side critical section
+ (demarked by srcu_read_lock() and srcu_read_unlock()), hence the
+ "SRCU": "sleepable RCU". Please note that if you don't need
+ to sleep in read-side critical sections, you should be using
+ RCU rather than SRCU, because RCU is almost always faster and
+ easier to use than is SRCU.

Also unlike other forms of RCU, explicit initialization
and cleanup is required via init_srcu_struct() and
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
new file mode 100644
index 0000000..fe24b58
--- /dev/null
+++ b/Documentation/RCU/lockdep.txt
@@ -0,0 +1,67 @@
+RCU and lockdep checking
+
+All flavors of RCU have lockdep checking available, so that lockdep is
+aware of when each task enters and leaves any flavor of RCU read-side
+critical section. Each flavor of RCU is tracked separately (but note
+that this is not the case in 2.6.32 and earlier). This allows lockdep's
+tracking to include RCU state, which can sometimes help when debugging
+deadlocks and the like.
+
+In addition, RCU provides the following primitives that check lockdep's
+state:
+
+ rcu_read_lock_held() for normal RCU.
+ rcu_read_lock_bh_held() for RCU-bh.
+ rcu_read_lock_sched_held() for RCU-sched.
+ srcu_read_lock_held() for SRCU.
+
+These functions are conservative, and will therefore return 1 if they
+aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
+This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
+positives when lockdep is disabled.
+
+In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
+checking of rcu_dereference() primitives:
+
+ rcu_dereference(p):
+ Check for RCU read-side critical section.
+ rcu_dereference_bh(p):
+ Check for RCU-bh read-side critical section.
+ rcu_dereference_sched(p):
+ Check for RCU-sched read-side critical section.
+ srcu_dereference(p, sp):
+ Check for SRCU read-side critical section.
+ rcu_dereference_check(p, c):
+ Use explicit check expression "c".
+ rcu_dereference_raw(p)
+ Don't check. (Use sparingly, if at all.)
+
+The rcu_dereference_check() check expression can be any boolean
+expression, but would normally include one of the rcu_read_lock_held()
+family of functions and a lockdep expression. However, any boolean
+expression can be used. For a moderately ornate example, consider
+the following:
+
+ file = rcu_dereference_check(fdt->fd[fd],
+ rcu_read_lock_held() ||
+ lockdep_is_held(&files->file_lock) ||
+ atomic_read(&files->count) == 1);
+
+This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
+and, if CONFIG_PROVE_RCU is configured, verifies that this expression
+is used in:
+
+1. An RCU read-side critical section, or
+2. with files->file_lock held, or
+3. on an unshared files_struct.
+
+In case (1), the pointer is picked up in an RCU-safe manner for vanilla
+RCU read-side critical sections, in case (2) the ->file_lock prevents
+any change from taking place, and finally, in case (3) the current task
+is the only task accessing the file_struct, again preventing any change
+from taking place.
+
+There are currently only "universal" versions of the rcu_assign_pointer()
+and RCU list-/tree-traversal primitives, which do not (yet) check for
+being in an RCU read-side critical section. In the future, separate
+versions of these primitives might be created.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 469a58b..1dc00ee 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -323,15 +323,17 @@ used as follows:
Defer Protect

a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
- call_rcu()
+ call_rcu() rcu_dereference()

b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
+ rcu_dereference_bh()

c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
preempt_disable() / preempt_enable()
local_irq_save() / local_irq_restore()
hardirq enter / hardirq exit
NMI enter / NMI exit
+ rcu_dereference_sched()

These three mechanisms are used as follows:

@@ -781,9 +783,8 @@ Linux-kernel source code, but it helps to have a full list of the
APIs, since there does not appear to be a way to categorize them
in docbook. Here is the list, by category.

-RCU pointer/list traversal:
+RCU list traversal:

- rcu_dereference
list_for_each_entry_rcu
hlist_for_each_entry_rcu
hlist_nulls_for_each_entry_rcu
@@ -809,7 +810,7 @@ RCU: Critical sections Grace period Barrier

rcu_read_lock synchronize_net rcu_barrier
rcu_read_unlock synchronize_rcu
- synchronize_rcu_expedited
+ rcu_dereference synchronize_rcu_expedited
call_rcu


@@ -817,7 +818,7 @@ bh: Critical sections Grace period Barrier

rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
rcu_read_unlock_bh synchronize_rcu_bh
- synchronize_rcu_bh_expedited
+ rcu_dereference_bh synchronize_rcu_bh_expedited


sched: Critical sections Grace period Barrier
@@ -826,12 +827,14 @@ sched: Critical sections Grace period Barrier
rcu_read_unlock_sched call_rcu_sched
[preempt_disable] synchronize_sched_expedited
[and friends]
+ rcu_dereference_sched


SRCU: Critical sections Grace period Barrier

srcu_read_lock synchronize_srcu N/A
srcu_read_unlock synchronize_srcu_expedited
+ srcu_dereference

SRCU: Initialization/cleanup
init_srcu_struct
--
1.5.2.5


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