path: root/Documentation/RCU/lockdep.txt
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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.