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" 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_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.