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doc: Update rcubarrier.rst

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This commit updates rcubarrier.txt to reflect RCU additions and changes
over the past few years.

[ paulmck: Apply Stephen Rothwell feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Paul E. McKenney 2022-11-04 11:44:45 -07:00
parent 8750dfe6fd
commit 42d689ec00
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192
Documentation/RCU/rcubarrier.rst
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@ -5,37 +5,12 @@ RCU and Unloadable Modules
[Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/]
RCU (read-copy update) is a synchronization mechanism that can be thought
of as a replacement for read-writer locking (among other things), but with
very low-overhead readers that are immune to deadlock, priority inversion,
and unbounded latency. RCU read-side critical sections are delimited
by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPTION
kernels, generate no code whatsoever.
This means that RCU writers are unaware of the presence of concurrent
readers, so that RCU updates to shared data must be undertaken quite
carefully, leaving an old version of the data structure in place until all
pre-existing readers have finished. These old versions are needed because
such readers might hold a reference to them. RCU updates can therefore be
rather expensive, and RCU is thus best suited for read-mostly situations.
How can an RCU writer possibly determine when all readers are finished,
given that readers might well leave absolutely no trace of their
presence? There is a synchronize_rcu() primitive that blocks until all
pre-existing readers have completed. An updater wishing to delete an
element p from a linked list might do the following, while holding an
appropriate lock, of course::
list_del_rcu(p);
synchronize_rcu();
kfree(p);
But the above code cannot be used in IRQ context -- the call_rcu()
primitive must be used instead. This primitive takes a pointer to an
rcu_head struct placed within the RCU-protected data structure and
another pointer to a function that may be invoked later to free that
structure. Code to delete an element p from the linked list from IRQ
context might then be as follows::
RCU updaters sometimes use call_rcu() to initiate an asynchronous wait for
a grace period to elapse. This primitive takes a pointer to an rcu_head
struct placed within the RCU-protected data structure and another pointer
to a function that may be invoked later to free that structure. Code to
delete an element p from the linked list from IRQ context might then be
as follows::
list_del_rcu(p);
call_rcu(&p->rcu, p_callback);
@ -54,7 +29,7 @@ IRQ context. The function p_callback() might be defined as follows::
Unloading Modules That Use call_rcu()
-------------------------------------
But what if p_callback is defined in an unloadable module?
But what if the p_callback() function is defined in an unloadable module?
If we unload the module while some RCU callbacks are pending,
the CPUs executing these callbacks are going to be severely
@ -67,20 +42,21 @@ grace period to elapse, it does not wait for the callbacks to complete.
One might be tempted to try several back-to-back synchronize_rcu()
calls, but this is still not guaranteed to work. If there is a very
heavy RCU-callback load, then some of the callbacks might be deferred
in order to allow other processing to proceed. Such deferral is required
in realtime kernels in order to avoid excessive scheduling latencies.
heavy RCU-callback load, then some of the callbacks might be deferred in
order to allow other processing to proceed. For but one example, such
deferral is required in realtime kernels in order to avoid excessive
scheduling latencies.
rcu_barrier()
-------------
We instead need the rcu_barrier() primitive. Rather than waiting for
a grace period to elapse, rcu_barrier() waits for all outstanding RCU
callbacks to complete. Please note that rcu_barrier() does **not** imply
synchronize_rcu(), in particular, if there are no RCU callbacks queued
anywhere, rcu_barrier() is within its rights to return immediately,
without waiting for a grace period to elapse.
This situation can be handled by the rcu_barrier() primitive. Rather
than waiting for a grace period to elapse, rcu_barrier() waits for all
outstanding RCU callbacks to complete. Please note that rcu_barrier()
does **not** imply synchronize_rcu(), in particular, if there are no RCU
callbacks queued anywhere, rcu_barrier() is within its rights to return
immediately, without waiting for anything, let alone a grace period.
Pseudo-code using rcu_barrier() is as follows:
@ -89,19 +65,22 @@ Pseudo-code using rcu_barrier() is as follows:
3. Allow the module to be unloaded.
There is also an srcu_barrier() function for SRCU, and you of course
must match the flavor of rcu_barrier() with that of call_rcu(). If your
module uses multiple flavors of call_rcu(), then it must also use multiple
flavors of rcu_barrier() when unloading that module. For example, if
it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on
srcu_struct_2, then the following three lines of code will be required
when unloading::
must match the flavor of srcu_barrier() with that of call_srcu().
If your module uses multiple srcu_struct structures, then it must also
use multiple invocations of srcu_barrier() when unloading that module.
For example, if it uses call_rcu(), call_srcu() on srcu_struct_1, and
call_srcu() on srcu_struct_2, then the following three lines of code
will be required when unloading::
1 rcu_barrier();
2 srcu_barrier(&srcu_struct_1);
3 srcu_barrier(&srcu_struct_2);
The rcutorture module makes use of rcu_barrier() in its exit function
as follows::
If latency is of the essence, workqueues could be used to run these
three functions concurrently.
An ancient version of the rcutorture module makes use of rcu_barrier()
in its exit function as follows::
1 static void
2 rcu_torture_cleanup(void)
@ -190,16 +169,17 @@ Quick Quiz #1:
:ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>`
Your module might have additional complications. For example, if your
module invokes call_rcu() from timers, you will need to first cancel all
the timers, and only then invoke rcu_barrier() to wait for any remaining
module invokes call_rcu() from timers, you will need to first refrain
from posting new timers, cancel (or wait for) all the already-posted
timers, and only then invoke rcu_barrier() to wait for any remaining
RCU callbacks to complete.
Of course, if you module uses call_rcu(), you will need to invoke
Of course, if your module uses call_rcu(), you will need to invoke
rcu_barrier() before unloading. Similarly, if your module uses
call_srcu(), you will need to invoke srcu_barrier() before unloading,
and on the same srcu_struct structure. If your module uses call_rcu()
**and** call_srcu(), then you will need to invoke rcu_barrier() **and**
srcu_barrier().
**and** call_srcu(), then (as noted above) you will need to invoke
rcu_barrier() **and** srcu_barrier().
Implementing rcu_barrier()
@ -211,27 +191,40 @@ queues. His implementation queues an RCU callback on each of the per-CPU
callback queues, and then waits until they have all started executing, at
which point, all earlier RCU callbacks are guaranteed to have completed.
The original code for rcu_barrier() was as follows::
The original code for rcu_barrier() was roughly as follows::
1 void rcu_barrier(void)
2 {
3 BUG_ON(in_interrupt());
4 /* Take cpucontrol mutex to protect against CPU hotplug */
5 mutex_lock(&rcu_barrier_mutex);
6 init_completion(&rcu_barrier_completion);
7 atomic_set(&rcu_barrier_cpu_count, 0);
8 on_each_cpu(rcu_barrier_func, NULL, 0, 1);
9 wait_for_completion(&rcu_barrier_completion);
10 mutex_unlock(&rcu_barrier_mutex);
11 }
1 void rcu_barrier(void)
2 {
3 BUG_ON(in_interrupt());
4 /* Take cpucontrol mutex to protect against CPU hotplug */
5 mutex_lock(&rcu_barrier_mutex);
6 init_completion(&rcu_barrier_completion);
7 atomic_set(&rcu_barrier_cpu_count, 1);
8 on_each_cpu(rcu_barrier_func, NULL, 0, 1);
9 if (atomic_dec_and_test(&rcu_barrier_cpu_count))
10 complete(&rcu_barrier_completion);
11 wait_for_completion(&rcu_barrier_completion);
12 mutex_unlock(&rcu_barrier_mutex);
13 }
Line 3 verifies that the caller is in process context, and lines 5 and 10
Line 3 verifies that the caller is in process context, and lines 5 and 12
use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the
global completion and counters at a time, which are initialized on lines
6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is
shown below. Note that the final "1" in on_each_cpu()'s argument list
ensures that all the calls to rcu_barrier_func() will have completed
before on_each_cpu() returns. Line 9 then waits for the completion.
before on_each_cpu() returns. Line 9 removes the initial count from
rcu_barrier_cpu_count, and if this count is now zero, line 10 finalizes
the completion, which prevents line 11 from blocking. Either way,
line 11 then waits (if needed) for the completion.
.. _rcubarrier_quiz_2:
Quick Quiz #2:
Why doesn't line 8 initialize rcu_barrier_cpu_count to zero,
thereby avoiding the need for lines 9 and 10?
:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
This code was rewritten in 2008 and several times thereafter, but this
still gives the general idea.
@ -253,7 +246,7 @@ to post an RCU callback, as follows::
Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure,
which contains the struct rcu_head that needed for the later call to
call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line
8 increments a global counter. This counter will later be decremented
8 increments the global counter. This counter will later be decremented
by the callback. Line 9 then registers the rcu_barrier_callback() on
the current CPU's queue.
@ -267,27 +260,28 @@ reaches zero, as follows::
4 complete(&rcu_barrier_completion);
5 }
.. _rcubarrier_quiz_2:
.. _rcubarrier_quiz_3:
Quick Quiz #2:
Quick Quiz #3:
What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
are delayed for a full grace period? Couldn't this result in
rcu_barrier() returning prematurely?
:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>`
:ref:`Answer to Quick Quiz #3 <answer_rcubarrier_quiz_3>`
The current rcu_barrier() implementation is more complex, due to the need
to avoid disturbing idle CPUs (especially on battery-powered systems)
and the need to minimally disturb non-idle CPUs in real-time systems.
However, the code above illustrates the concepts.
In addition, a great many optimizations have been applied. However,
the code above illustrates the concepts.
rcu_barrier() Summary
---------------------
The rcu_barrier() primitive has seen relatively little use, since most
The rcu_barrier() primitive is used relatively infrequently, since most
code using RCU is in the core kernel rather than in modules. However, if
you are using RCU from an unloadable module, you need to use rcu_barrier()
so that your module may be safely unloaded.
@ -318,6 +312,39 @@ Answer: Interestingly enough, rcu_barrier() was not originally
.. _answer_rcubarrier_quiz_2:
Quick Quiz #2:
Why doesn't line 8 initialize rcu_barrier_cpu_count to zero,
thereby avoiding the need for lines 9 and 10?
Answer: Suppose that the on_each_cpu() function shown on line 8 was
delayed, so that CPU 0's rcu_barrier_func() executed and
the corresponding grace period elapsed, all before CPU 1's
rcu_barrier_func() started executing. This would result in
rcu_barrier_cpu_count being decremented to zero, so that line
11's wait_for_completion() would return immediately, failing to
wait for CPU 1's callbacks to be invoked.
Note that this was not a problem when the rcu_barrier() code
was first added back in 2005. This is because on_each_cpu()
disables preemption, which acted as an RCU read-side critical
section, thus preventing CPU 0's grace period from completing
until on_each_cpu() had dealt with all of the CPUs. However,
with the advent of preemptible RCU, rcu_barrier() no longer
waited on nonpreemptible regions of code in preemptible kernels,
that being the job of the new rcu_barrier_sched() function.
However, with the RCU flavor consolidation around v4.20, this
possibility was once again ruled out, because the consolidated
RCU once again waits on nonpreemptible regions of code.
Nevertheless, that extra count might still be a good idea.
Relying on these sort of accidents of implementation can result
in later surprise bugs when the implementation changes.
:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
.. _answer_rcubarrier_quiz_3:
Quick Quiz #3:
What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
@ -336,18 +363,15 @@ Answer: This cannot happen. The reason is that on_each_cpu() has its last
Therefore, on_each_cpu() disables preemption across its call
to smp_call_function() and also across the local call to
rcu_barrier_func(). This prevents the local CPU from context
switching, again preventing grace periods from completing. This
rcu_barrier_func(). Because recent RCU implementations treat
preemption-disabled regions of code as RCU read-side critical
sections, this prevents grace periods from completing. This
means that all CPUs have executed rcu_barrier_func() before
the first rcu_barrier_callback() can possibly execute, in turn
preventing rcu_barrier_cpu_count from prematurely reaching zero.
Currently, -rt implementations of RCU keep but a single global
queue for RCU callbacks, and thus do not suffer from this
problem. However, when the -rt RCU eventually does have per-CPU
callback queues, things will have to change. One simple change
is to add an rcu_read_lock() before line 8 of rcu_barrier()
and an rcu_read_unlock() after line 8 of this same function. If
you can think of a better change, please let me know!
But if on_each_cpu() ever decides to forgo disabling preemption,
as might well happen due to real-time latency considerations,
initializing rcu_barrier_cpu_count to one will save the day.
:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>`
:ref:`Back to Quick Quiz #3 <rcubarrier_quiz_3>`