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+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
+ 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" along with
+ rcu_read_lock_held(). This is useful in code that is
+ invoked by both RCU readers and updaters.
+ rcu_dereference_bh_check(p, c):
+ Use explicit check expression "c" along with
+ rcu_read_lock_bh_held(). This is useful in code that
+ is invoked by both RCU-bh readers and updaters.
+ rcu_dereference_sched_check(p, c):
+ Use explicit check expression "c" along with
+ rcu_read_lock_sched_held(). This is useful in code that
+ is invoked by both RCU-sched readers and updaters.
+ srcu_dereference_check(p, c):
+ Use explicit check expression "c" along with
+ srcu_read_lock_held()(). This is useful in code that
+ is invoked by both SRCU readers and updaters.
+ rcu_dereference_index_check(p, c):
+ Use explicit check expression "c", but the caller
+ must supply one of the rcu_read_lock_held() functions.
+ This is useful in code that uses RCU-protected arrays
+ that is invoked by both RCU readers and updaters.
+ rcu_dereference_raw(p):
+ Don't check. (Use sparingly, if at all.)
+ rcu_dereference_protected(p, c):
+ Use explicit check expression "c", and omit all barriers
+ and compiler constraints. This is useful when the data
+ structure cannot change, for example, in code that is
+ invoked only by updaters.
+ rcu_access_pointer(p):
+ Return the value of the pointer and omit all barriers,
+ but retain the compiler constraints that prevent duplicating
+ or coalescsing. This is useful when when testing the
+ value of the pointer itself, for example, against NULL.
+The rcu_dereference_check() check expression can be any boolean
+expression, but would normally include 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],
+ 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 (implicit), 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. If the above statement was invoked only from updater
+code, it could instead be written as follows:
+ file = rcu_dereference_protected(fdt->fd[fd],
+ lockdep_is_held(&files->file_lock) ||
+ atomic_read(&files->count) == 1);
+This would verify cases #2 and #3 above, and furthermore lockdep would
+complain if this was used in an RCU read-side critical section unless one
+of these two cases held. Because rcu_dereference_protected() omits all
+barriers and compiler constraints, it generates better code than do the
+other flavors of rcu_dereference(). On the other hand, it is illegal
+to use rcu_dereference_protected() if either the RCU-protected pointer
+or the RCU-protected data that it points to can change concurrently.
+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.