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Diffstat (limited to 'Documentation/RCU/lockdep.txt')
| -rw-r--r-- | Documentation/RCU/lockdep.txt | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt new file mode 100644 index 00000000000..fe24b58627b --- /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. |