Based on kernel version 4.2. Page generated on 2015-09-09 12:14 EST.
1 RCU and lockdep checking 2 3 All flavors of RCU have lockdep checking available, so that lockdep is 4 aware of when each task enters and leaves any flavor of RCU read-side 5 critical section. Each flavor of RCU is tracked separately (but note 6 that this is not the case in 2.6.32 and earlier). This allows lockdep's 7 tracking to include RCU state, which can sometimes help when debugging 8 deadlocks and the like. 9 10 In addition, RCU provides the following primitives that check lockdep's 11 state: 12 13 rcu_read_lock_held() for normal RCU. 14 rcu_read_lock_bh_held() for RCU-bh. 15 rcu_read_lock_sched_held() for RCU-sched. 16 srcu_read_lock_held() for SRCU. 17 18 These functions are conservative, and will therefore return 1 if they 19 aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). 20 This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false 21 positives when lockdep is disabled. 22 23 In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables 24 checking of rcu_dereference() primitives: 25 26 rcu_dereference(p): 27 Check for RCU read-side critical section. 28 rcu_dereference_bh(p): 29 Check for RCU-bh read-side critical section. 30 rcu_dereference_sched(p): 31 Check for RCU-sched read-side critical section. 32 srcu_dereference(p, sp): 33 Check for SRCU read-side critical section. 34 rcu_dereference_check(p, c): 35 Use explicit check expression "c" along with 36 rcu_read_lock_held(). This is useful in code that is 37 invoked by both RCU readers and updaters. 38 rcu_dereference_bh_check(p, c): 39 Use explicit check expression "c" along with 40 rcu_read_lock_bh_held(). This is useful in code that 41 is invoked by both RCU-bh readers and updaters. 42 rcu_dereference_sched_check(p, c): 43 Use explicit check expression "c" along with 44 rcu_read_lock_sched_held(). This is useful in code that 45 is invoked by both RCU-sched readers and updaters. 46 srcu_dereference_check(p, c): 47 Use explicit check expression "c" along with 48 srcu_read_lock_held()(). This is useful in code that 49 is invoked by both SRCU readers and updaters. 50 rcu_dereference_raw(p): 51 Don't check. (Use sparingly, if at all.) 52 rcu_dereference_protected(p, c): 53 Use explicit check expression "c", and omit all barriers 54 and compiler constraints. This is useful when the data 55 structure cannot change, for example, in code that is 56 invoked only by updaters. 57 rcu_access_pointer(p): 58 Return the value of the pointer and omit all barriers, 59 but retain the compiler constraints that prevent duplicating 60 or coalescsing. This is useful when when testing the 61 value of the pointer itself, for example, against NULL. 62 63 The rcu_dereference_check() check expression can be any boolean 64 expression, but would normally include a lockdep expression. However, 65 any boolean expression can be used. For a moderately ornate example, 66 consider the following: 67 68 file = rcu_dereference_check(fdt->fd[fd], 69 lockdep_is_held(&files->file_lock) || 70 atomic_read(&files->count) == 1); 71 72 This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner, 73 and, if CONFIG_PROVE_RCU is configured, verifies that this expression 74 is used in: 75 76 1. An RCU read-side critical section (implicit), or 77 2. with files->file_lock held, or 78 3. on an unshared files_struct. 79 80 In case (1), the pointer is picked up in an RCU-safe manner for vanilla 81 RCU read-side critical sections, in case (2) the ->file_lock prevents 82 any change from taking place, and finally, in case (3) the current task 83 is the only task accessing the file_struct, again preventing any change 84 from taking place. If the above statement was invoked only from updater 85 code, it could instead be written as follows: 86 87 file = rcu_dereference_protected(fdt->fd[fd], 88 lockdep_is_held(&files->file_lock) || 89 atomic_read(&files->count) == 1); 90 91 This would verify cases #2 and #3 above, and furthermore lockdep would 92 complain if this was used in an RCU read-side critical section unless one 93 of these two cases held. Because rcu_dereference_protected() omits all 94 barriers and compiler constraints, it generates better code than do the 95 other flavors of rcu_dereference(). On the other hand, it is illegal 96 to use rcu_dereference_protected() if either the RCU-protected pointer 97 or the RCU-protected data that it points to can change concurrently. 98 99 There are currently only "universal" versions of the rcu_assign_pointer() 100 and RCU list-/tree-traversal primitives, which do not (yet) check for 101 being in an RCU read-side critical section. In the future, separate 102 versions of these primitives might be created.