Hi all, After merging the rcu tree, today's linux-next build (htmldocs) produced these warnings: Documentation/RCU/Design/Requirements/Requirements.rst:119: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Wait a minute! You said that updaters can make useful forward | | progress concurrently with readers, but pre-existing readers will | | block synchronize_rcu()!!! | | Just who are you trying to fool??? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | First, if updaters do not wish to be blocked by readers, they can use | | call_rcu() or kfree_rcu(), which will be discussed later. | | Second, even when using synchronize_rcu(), the other update-side | | code does run concurrently with readers, whether pre-existing or not. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:178: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Why is the synchronize_rcu() on line 28 needed? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Without that extra grace period, memory reordering could result in | | do_something_dlm() executing do_something() concurrently with | | the last bits of recovery(). | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:289: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | But rcu_assign_pointer() does nothing to prevent the two | | assignments to ``p->a`` and ``p->b`` from being reordered. Can't that | | also cause problems? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | No, it cannot. The readers cannot see either of these two fields | | until the assignment to ``gp``, by which time both fields are fully | | initialized. So reordering the assignments to ``p->a`` and ``p->b`` | | cannot possibly cause any problems. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:430: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Without the rcu_dereference() or the rcu_access_pointer(), | | what destructive optimizations might the compiler make use of? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Let's start with what happens to do_something_gp() if it fails to | | use rcu_dereference(). It could reuse a value formerly fetched | | from this same pointer. It could also fetch the pointer from ``gp`` | | in a byte-at-a-time manner, resulting in *load tearing*, in turn | | resulting a bytewise mash-up of two distinct pointer values. It might | | even use value-speculation optimizations, where it makes a wrong | | guess, but by the time it gets around to checking the value, an | | update has changed the pointer to match the wrong guess. Too bad | | about any dereferences that returned pre-initialization garbage in | | the meantime! | | For remove_gp_synchronous(), as long as all modifications to | | ``gp`` are carried out while holding ``gp_lock``, the above | | optimizations are harmless. However, ``sparse`` will complain if you | | define ``gp`` with ``__rcu`` and then access it without using either | | rcu_access_pointer() or rcu_dereference(). | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:513: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Given that multiple CPUs can start RCU read-side critical sections at | | any time without any ordering whatsoever, how can RCU possibly tell | | whether or not a given RCU read-side critical section starts before a | | given instance of synchronize_rcu()? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | If RCU cannot tell whether or not a given RCU read-side critical | | section starts before a given instance of synchronize_rcu(), then | | it must assume that the RCU read-side critical section started first. | | In other words, a given instance of synchronize_rcu() can avoid | | waiting on a given RCU read-side critical section only if it can | | prove that synchronize_rcu() started first. | | A related question is “When rcu_read_lock() doesn't generate any | | code, why does it matter how it relates to a grace period?” The | | answer is that it is not the relationship of rcu_read_lock() | | itself that is important, but rather the relationship of the code | | within the enclosed RCU read-side critical section to the code | | preceding and following the grace period. If we take this viewpoint, | | then a given RCU read-side critical section begins before a given | | grace period when some access preceding the grace period observes the | | effect of some access within the critical section, in which case none | | of the accesses within the critical section may observe the effects | | of any access following the grace period. | | | | As of late 2016, mathematical models of RCU take this viewpoint, for | | example, see slides 62 and 63 of the `2016 LinuxCon | | EU <http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.201 | | 6.10.04c.LCE.pdf>`__ | | presentation. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:548: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | The first and second guarantees require unbelievably strict ordering! | | Are all these memory barriers *really* required? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Yes, they really are required. To see why the first guarantee is | | required, consider the following sequence of events: | | | | #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp); /* Very likely to return p. */`` | | #. CPU 0: ``list_del_rcu(p);`` | | #. CPU 0: synchronize_rcu() starts. | | #. CPU 1: ``do_something_with(q->a);`` | | ``/* No smp_mb(), so might happen after kfree(). */`` | | #. CPU 1: rcu_read_unlock() | | #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | | | Therefore, there absolutely must be a full memory barrier between the | | end of the RCU read-side critical section and the end of the grace | | period. | | | | The sequence of events demonstrating the necessity of the second rule | | is roughly similar: | | | | #. CPU 0: ``list_del_rcu(p);`` | | #. CPU 0: synchronize_rcu() starts. | | #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp);`` | | ``/* Might return p if no memory barrier. */`` | | #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | #. CPU 1: ``do_something_with(q->a); /* Boom!!! */`` | | #. CPU 1: rcu_read_unlock() | | | | And similarly, without a memory barrier between the beginning of the | | grace period and the beginning of the RCU read-side critical section, | | CPU 1 might end up accessing the freelist. | | | | The “as if” rule of course applies, so that any implementation that | | acts as if the appropriate memory barriers were in place is a correct | | implementation. That said, it is much easier to fool yourself into | | believing that you have adhered to the as-if rule than it is to | | actually adhere to it! | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:597: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | You claim that rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code in some kernel builds. This means that the | | compiler might arbitrarily rearrange consecutive RCU read-side | | critical sections. Given such rearrangement, if a given RCU read-side | | critical section is done, how can you be sure that all prior RCU | | read-side critical sections are done? Won't the compiler | | rearrangements make that impossible to determine? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | In cases where rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code, RCU infers quiescent states only at special | | locations, for example, within the scheduler. Because calls to | | schedule() had better prevent calling-code accesses to shared | | variables from being rearranged across the call to schedule(), if | | RCU detects the end of a given RCU read-side critical section, it | | will necessarily detect the end of all prior RCU read-side critical | | sections, no matter how aggressively the compiler scrambles the code. | | Again, this all assumes that the compiler cannot scramble code across | | calls to the scheduler, out of interrupt handlers, into the idle | | loop, into user-mode code, and so on. But if your kernel build allows | | that sort of scrambling, you have broken far more than just RCU! | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:738: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Can't the compiler also reorder this code? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | No, the volatile casts in READ_ONCE() and WRITE_ONCE() | | prevent the compiler from reordering in this particular case. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:793: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Suppose that synchronize_rcu() did wait until *all* readers had | | completed instead of waiting only on pre-existing readers. For how | | long would the updater be able to rely on there being no readers? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | For no time at all. Even if synchronize_rcu() were to wait until | | all readers had completed, a new reader might start immediately after | | synchronize_rcu() completed. Therefore, the code following | | synchronize_rcu() can *never* rely on there being no readers. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:1087: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | What about sleeping locks? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | These are forbidden within Linux-kernel RCU read-side critical | | sections because it is not legal to place a quiescent state (in this | | case, voluntary context switch) within an RCU read-side critical | | section. However, sleeping locks may be used within userspace RCU | | read-side critical sections, and also within Linux-kernel sleepable | | RCU `(SRCU) <#Sleepable%20RCU>`__ read-side critical sections. In | | addition, the -rt patchset turns spinlocks into a sleeping locks so | | that the corresponding critical sections can be preempted, which also | | means that these sleeplockified spinlocks (but not other sleeping | | locks!) may be acquire within -rt-Linux-kernel RCU read-side critical | | sections. | | Note that it *is* legal for a normal RCU read-side critical section | | to conditionally acquire a sleeping locks (as in | | mutex_trylock()), but only as long as it does not loop | | indefinitely attempting to conditionally acquire that sleeping locks. | | The key point is that things like mutex_trylock() either return | | with the mutex held, or return an error indication if the mutex was | | not immediately available. Either way, mutex_trylock() returns | | immediately without sleeping. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:1295: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Why does line 19 use rcu_access_pointer()? After all, | | call_rcu() on line 25 stores into the structure, which would | | interact badly with concurrent insertions. Doesn't this mean that | | rcu_dereference() is required? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Presumably the ``->gp_lock`` acquired on line 18 excludes any | | changes, including any insertions that rcu_dereference() would | | protect against. Therefore, any insertions will be delayed until | | after ``->gp_lock`` is released on line 25, which in turn means that | | rcu_access_pointer() suffices. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:1351: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Earlier it was claimed that call_rcu() and kfree_rcu() | | allowed updaters to avoid being blocked by readers. But how can that | | be correct, given that the invocation of the callback and the freeing | | of the memory (respectively) must still wait for a grace period to | | elapse? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | We could define things this way, but keep in mind that this sort of | | definition would say that updates in garbage-collected languages | | cannot complete until the next time the garbage collector runs, which | | does not seem at all reasonable. The key point is that in most cases, | | an updater using either call_rcu() or kfree_rcu() can proceed | | to the next update as soon as it has invoked call_rcu() or | | kfree_rcu(), without having to wait for a subsequent grace | | period. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:1893: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Wait a minute! Each RCU callbacks must wait for a grace period to | | complete, and rcu_barrier() must wait for each pre-existing | | callback to be invoked. Doesn't rcu_barrier() therefore need to | | wait for a full grace period if there is even one callback posted | | anywhere in the system? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Absolutely not!!! | | Yes, each RCU callbacks must wait for a grace period to complete, but | | it might well be partly (or even completely) finished waiting by the | | time rcu_barrier() is invoked. In that case, rcu_barrier() | | need only wait for the remaining portion of the grace period to | | elapse. So even if there are quite a few callbacks posted, | | rcu_barrier() might well return quite quickly. | | | | So if you need to wait for a grace period as well as for all | | pre-existing callbacks, you will need to invoke both | | synchronize_rcu() and rcu_barrier(). If latency is a concern, | | you can always use workqueues to invoke them concurrently. | +-----------------------------------------------------------------------+ Documentation/RCU/Design/Requirements/Requirements.rst:2220: WARNING: Malformed table. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | But what if my driver has a hardware interrupt handler that can run | | for many seconds? I cannot invoke schedule() from an hardware | | interrupt handler, after all! | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | One approach is to do ``rcu_irq_exit();rcu_irq_enter();`` every so | | often. But given that long-running interrupt handlers can cause other | | problems, not least for response time, shouldn't you work to keep | | your interrupt handler's runtime within reasonable bounds? | +-----------------------------------------------------------------------+ Introduced by commit c0a41bf9dbc7 ("docs: Remove redundant "``" from Requirements.rst") -- Cheers, Stephen Rothwell
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