diff options
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 31 | ||||
| -rw-r--r-- | Documentation/memory-barriers.txt | 58 | ||||
| -rw-r--r-- | kernel/rcupdate.c | 10 | ||||
| -rw-r--r-- | kernel/rcutiny_plugin.h | 2 | ||||
| -rw-r--r-- | kernel/rcutorture.c | 1 |
5 files changed, 95 insertions, 7 deletions
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index cfaac34c4557..6ef692667e2f 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -849,6 +849,37 @@ All: lockdep-checked RCU-protected pointer access See the comment headers in the source code (or the docbook generated from them) for more information. +However, given that there are no fewer than four families of RCU APIs +in the Linux kernel, how do you choose which one to use? The following +list can be helpful: + +a. Will readers need to block? If so, you need SRCU. + +b. What about the -rt patchset? If readers would need to block + in an non-rt kernel, you need SRCU. If readers would block + in a -rt kernel, but not in a non-rt kernel, SRCU is not + necessary. + +c. Do you need to treat NMI handlers, hardirq handlers, + and code segments with preemption disabled (whether + via preempt_disable(), local_irq_save(), local_bh_disable(), + or some other mechanism) as if they were explicit RCU readers? + If so, you need RCU-sched. + +d. Do you need RCU grace periods to complete even in the face + of softirq monopolization of one or more of the CPUs? For + example, is your code subject to network-based denial-of-service + attacks? If so, you need RCU-bh. + +e. Is your workload too update-intensive for normal use of + RCU, but inappropriate for other synchronization mechanisms? + If so, consider SLAB_DESTROY_BY_RCU. But please be careful! + +f. Otherwise, use RCU. + +Of course, this all assumes that you have determined that RCU is in fact +the right tool for your job. + 8. ANSWERS TO QUICK QUIZZES diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 631ad2f1b229..f0d3a8026a56 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -21,6 +21,7 @@ Contents: - SMP barrier pairing. - Examples of memory barrier sequences. - Read memory barriers vs load speculation. + - Transitivity (*) Explicit kernel barriers. @@ -959,6 +960,63 @@ the speculation will be cancelled and the value reloaded: retrieved : : +-------+ +TRANSITIVITY +------------ + +Transitivity is a deeply intuitive notion about ordering that is not +always provided by real computer systems. The following example +demonstrates transitivity (also called "cumulativity"): + + CPU 1 CPU 2 CPU 3 + ======================= ======================= ======================= + { X = 0, Y = 0 } + STORE X=1 LOAD X STORE Y=1 + <general barrier> <general barrier> + LOAD Y LOAD X + +Suppose that CPU 2's load from X returns 1 and its load from Y returns 0. +This indicates that CPU 2's load from X in some sense follows CPU 1's +store to X and that CPU 2's load from Y in some sense preceded CPU 3's +store to Y. The question is then "Can CPU 3's load from X return 0?" + +Because CPU 2's load from X in some sense came after CPU 1's store, it +is natural to expect that CPU 3's load from X must therefore return 1. +This expectation is an example of transitivity: if a load executing on +CPU A follows a load from the same variable executing on CPU B, then +CPU A's load must either return the same value that CPU B's load did, +or must return some later value. + +In the Linux kernel, use of general memory barriers guarantees +transitivity. Therefore, in the above example, if CPU 2's load from X +returns 1 and its load from Y returns 0, then CPU 3's load from X must +also return 1. + +However, transitivity is -not- guaranteed for read or write barriers. +For example, suppose that CPU 2's general barrier in the above example +is changed to a read barrier as shown below: + + CPU 1 CPU 2 CPU 3 + ======================= ======================= ======================= + { X = 0, Y = 0 } + STORE X=1 LOAD X STORE Y=1 + <read barrier> <general barrier> + LOAD Y LOAD X + +This substitution destroys transitivity: in this example, it is perfectly +legal for CPU 2's load from X to return 1, its load from Y to return 0, +and CPU 3's load from X to return 0. + +The key point is that although CPU 2's read barrier orders its pair +of loads, it does not guarantee to order CPU 1's store. Therefore, if +this example runs on a system where CPUs 1 and 2 share a store buffer +or a level of cache, CPU 2 might have early access to CPU 1's writes. +General barriers are therefore required to ensure that all CPUs agree +on the combined order of CPU 1's and CPU 2's accesses. + +To reiterate, if your code requires transitivity, use general barriers +throughout. + + ======================== EXPLICIT KERNEL BARRIERS ======================== diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c index a23a57a976d1..f3240e987928 100644 --- a/kernel/rcupdate.c +++ b/kernel/rcupdate.c @@ -214,11 +214,12 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state) * Ensure that queued callbacks are all executed. * If we detect that we are nested in a RCU read-side critical * section, we should simply fail, otherwise we would deadlock. + * Note that the machinery to reliably determine whether + * or not we are in an RCU read-side critical section + * exists only in the preemptible RCU implementations + * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why + * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT. */ -#ifndef CONFIG_PREEMPT - WARN_ON(1); - return 0; -#else if (rcu_preempt_depth() != 0 || preempt_count() != 0 || irqs_disabled()) { WARN_ON(1); @@ -229,7 +230,6 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state) rcu_barrier_bh(); debug_object_free(head, &rcuhead_debug_descr); return 1; -#endif default: return 0; } diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h index 015abaea962a..3cb8e362e883 100644 --- a/kernel/rcutiny_plugin.h +++ b/kernel/rcutiny_plugin.h @@ -852,7 +852,7 @@ void exit_rcu(void) if (t->rcu_read_lock_nesting == 0) return; t->rcu_read_lock_nesting = 1; - rcu_read_unlock(); + __rcu_read_unlock(); } #else /* #ifdef CONFIG_TINY_PREEMPT_RCU */ diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c index 89613f97ff26..c224da41890c 100644 --- a/kernel/rcutorture.c +++ b/kernel/rcutorture.c @@ -47,7 +47,6 @@ #include <linux/srcu.h> #include <linux/slab.h> #include <asm/byteorder.h> -#include <linux/sched.h> MODULE_LICENSE("GPL"); MODULE_AUTHOR("Paul E. McKenney <paulmck@us.ibm.com> and " |