![Paul E. McKenney](/assets/img/avatar_default.png)
Updated patch adding a variant of RCU that permits sleeping in read-side critical sections. SRCU is as follows: o Each use of SRCU creates its own srcu_struct, and each srcu_struct has its own set of grace periods. This is critical, as it prevents one subsystem with a blocking reader from holding up SRCU grace periods for other subsystems. o The SRCU primitives (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu()) all take a pointer to a srcu_struct. o The SRCU primitives must be called from process context. o srcu_read_lock() returns an int that must be passed to the matching srcu_read_unlock(). Realtime RCU avoids the need for this by storing the state in the task struct, but SRCU needs to allow a given code path to pass through multiple SRCU domains -- storing state in the task struct would therefore require either arbitrary space in the task struct or arbitrary limits on SRCU nesting. So I kicked the state-storage problem up to the caller. Of course, it is not permitted to call synchronize_srcu() while in an SRCU read-side critical section. o There is no call_srcu(). It would not be hard to implement one, but it seems like too easy a way to OOM the system. (Hey, we have enough trouble with call_rcu(), which does -not- permit readers to sleep!!!) So, if you want it, please tell me why... [josht@us.ibm.com: sparse notation] Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Josh Triplett <josh@freedesktop.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
262 lines
12 KiB
Text
262 lines
12 KiB
Text
Review Checklist for RCU Patches
|
|
|
|
|
|
This document contains a checklist for producing and reviewing patches
|
|
that make use of RCU. Violating any of the rules listed below will
|
|
result in the same sorts of problems that leaving out a locking primitive
|
|
would cause. This list is based on experiences reviewing such patches
|
|
over a rather long period of time, but improvements are always welcome!
|
|
|
|
0. Is RCU being applied to a read-mostly situation? If the data
|
|
structure is updated more than about 10% of the time, then
|
|
you should strongly consider some other approach, unless
|
|
detailed performance measurements show that RCU is nonetheless
|
|
the right tool for the job.
|
|
|
|
The other exception would be where performance is not an issue,
|
|
and RCU provides a simpler implementation. An example of this
|
|
situation is the dynamic NMI code in the Linux 2.6 kernel,
|
|
at least on architectures where NMIs are rare.
|
|
|
|
1. Does the update code have proper mutual exclusion?
|
|
|
|
RCU does allow -readers- to run (almost) naked, but -writers- must
|
|
still use some sort of mutual exclusion, such as:
|
|
|
|
a. locking,
|
|
b. atomic operations, or
|
|
c. restricting updates to a single task.
|
|
|
|
If you choose #b, be prepared to describe how you have handled
|
|
memory barriers on weakly ordered machines (pretty much all of
|
|
them -- even x86 allows reads to be reordered), and be prepared
|
|
to explain why this added complexity is worthwhile. If you
|
|
choose #c, be prepared to explain how this single task does not
|
|
become a major bottleneck on big multiprocessor machines (for
|
|
example, if the task is updating information relating to itself
|
|
that other tasks can read, there by definition can be no
|
|
bottleneck).
|
|
|
|
2. Do the RCU read-side critical sections make proper use of
|
|
rcu_read_lock() and friends? These primitives are needed
|
|
to suppress preemption (or bottom halves, in the case of
|
|
rcu_read_lock_bh()) in the read-side critical sections,
|
|
and are also an excellent aid to readability.
|
|
|
|
As a rough rule of thumb, any dereference of an RCU-protected
|
|
pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
|
|
or by the appropriate update-side lock.
|
|
|
|
3. Does the update code tolerate concurrent accesses?
|
|
|
|
The whole point of RCU is to permit readers to run without
|
|
any locks or atomic operations. This means that readers will
|
|
be running while updates are in progress. There are a number
|
|
of ways to handle this concurrency, depending on the situation:
|
|
|
|
a. Make updates appear atomic to readers. For example,
|
|
pointer updates to properly aligned fields will appear
|
|
atomic, as will individual atomic primitives. Operations
|
|
performed under a lock and sequences of multiple atomic
|
|
primitives will -not- appear to be atomic.
|
|
|
|
This is almost always the best approach.
|
|
|
|
b. Carefully order the updates and the reads so that
|
|
readers see valid data at all phases of the update.
|
|
This is often more difficult than it sounds, especially
|
|
given modern CPUs' tendency to reorder memory references.
|
|
One must usually liberally sprinkle memory barriers
|
|
(smp_wmb(), smp_rmb(), smp_mb()) through the code,
|
|
making it difficult to understand and to test.
|
|
|
|
It is usually better to group the changing data into
|
|
a separate structure, so that the change may be made
|
|
to appear atomic by updating a pointer to reference
|
|
a new structure containing updated values.
|
|
|
|
4. Weakly ordered CPUs pose special challenges. Almost all CPUs
|
|
are weakly ordered -- even i386 CPUs allow reads to be reordered.
|
|
RCU code must take all of the following measures to prevent
|
|
memory-corruption problems:
|
|
|
|
a. Readers must maintain proper ordering of their memory
|
|
accesses. The rcu_dereference() primitive ensures that
|
|
the CPU picks up the pointer before it picks up the data
|
|
that the pointer points to. This really is necessary
|
|
on Alpha CPUs. If you don't believe me, see:
|
|
|
|
http://www.openvms.compaq.com/wizard/wiz_2637.html
|
|
|
|
The rcu_dereference() primitive is also an excellent
|
|
documentation aid, letting the person reading the code
|
|
know exactly which pointers are protected by RCU.
|
|
|
|
The rcu_dereference() primitive is used by the various
|
|
"_rcu()" list-traversal primitives, such as the
|
|
list_for_each_entry_rcu(). Note that it is perfectly
|
|
legal (if redundant) for update-side code to use
|
|
rcu_dereference() and the "_rcu()" list-traversal
|
|
primitives. This is particularly useful in code
|
|
that is common to readers and updaters.
|
|
|
|
b. If the list macros are being used, the list_add_tail_rcu()
|
|
and list_add_rcu() primitives must be used in order
|
|
to prevent weakly ordered machines from misordering
|
|
structure initialization and pointer planting.
|
|
Similarly, if the hlist macros are being used, the
|
|
hlist_add_head_rcu() primitive is required.
|
|
|
|
c. If the list macros are being used, the list_del_rcu()
|
|
primitive must be used to keep list_del()'s pointer
|
|
poisoning from inflicting toxic effects on concurrent
|
|
readers. Similarly, if the hlist macros are being used,
|
|
the hlist_del_rcu() primitive is required.
|
|
|
|
The list_replace_rcu() primitive may be used to
|
|
replace an old structure with a new one in an
|
|
RCU-protected list.
|
|
|
|
d. Updates must ensure that initialization of a given
|
|
structure happens before pointers to that structure are
|
|
publicized. Use the rcu_assign_pointer() primitive
|
|
when publicizing a pointer to a structure that can
|
|
be traversed by an RCU read-side critical section.
|
|
|
|
5. If call_rcu(), or a related primitive such as call_rcu_bh(),
|
|
is used, the callback function must be written to be called
|
|
from softirq context. In particular, it cannot block.
|
|
|
|
6. Since synchronize_rcu() can block, it cannot be called from
|
|
any sort of irq context.
|
|
|
|
7. If the updater uses call_rcu(), then the corresponding readers
|
|
must use rcu_read_lock() and rcu_read_unlock(). If the updater
|
|
uses call_rcu_bh(), then the corresponding readers must use
|
|
rcu_read_lock_bh() and rcu_read_unlock_bh(). Mixing things up
|
|
will result in confusion and broken kernels.
|
|
|
|
One exception to this rule: rcu_read_lock() and rcu_read_unlock()
|
|
may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
|
|
in cases where local bottom halves are already known to be
|
|
disabled, for example, in irq or softirq context. Commenting
|
|
such cases is a must, of course! And the jury is still out on
|
|
whether the increased speed is worth it.
|
|
|
|
8. Although synchronize_rcu() is a bit slower than is call_rcu(),
|
|
it usually results in simpler code. So, unless update
|
|
performance is critically important or the updaters cannot block,
|
|
synchronize_rcu() should be used in preference to call_rcu().
|
|
|
|
An especially important property of the synchronize_rcu()
|
|
primitive is that it automatically self-limits: if grace periods
|
|
are delayed for whatever reason, then the synchronize_rcu()
|
|
primitive will correspondingly delay updates. In contrast,
|
|
code using call_rcu() should explicitly limit update rate in
|
|
cases where grace periods are delayed, as failing to do so can
|
|
result in excessive realtime latencies or even OOM conditions.
|
|
|
|
Ways of gaining this self-limiting property when using call_rcu()
|
|
include:
|
|
|
|
a. Keeping a count of the number of data-structure elements
|
|
used by the RCU-protected data structure, including those
|
|
waiting for a grace period to elapse. Enforce a limit
|
|
on this number, stalling updates as needed to allow
|
|
previously deferred frees to complete.
|
|
|
|
Alternatively, limit only the number awaiting deferred
|
|
free rather than the total number of elements.
|
|
|
|
b. Limiting update rate. For example, if updates occur only
|
|
once per hour, then no explicit rate limiting is required,
|
|
unless your system is already badly broken. The dcache
|
|
subsystem takes this approach -- updates are guarded
|
|
by a global lock, limiting their rate.
|
|
|
|
c. Trusted update -- if updates can only be done manually by
|
|
superuser or some other trusted user, then it might not
|
|
be necessary to automatically limit them. The theory
|
|
here is that superuser already has lots of ways to crash
|
|
the machine.
|
|
|
|
d. Use call_rcu_bh() rather than call_rcu(), in order to take
|
|
advantage of call_rcu_bh()'s faster grace periods.
|
|
|
|
e. Periodically invoke synchronize_rcu(), permitting a limited
|
|
number of updates per grace period.
|
|
|
|
9. All RCU list-traversal primitives, which include
|
|
list_for_each_rcu(), list_for_each_entry_rcu(),
|
|
list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
|
|
must be within an RCU read-side critical section. RCU
|
|
read-side critical sections are delimited by rcu_read_lock()
|
|
and rcu_read_unlock(), or by similar primitives such as
|
|
rcu_read_lock_bh() and rcu_read_unlock_bh().
|
|
|
|
Use of the _rcu() list-traversal primitives outside of an
|
|
RCU read-side critical section causes no harm other than
|
|
a slight performance degradation on Alpha CPUs. It can
|
|
also be quite helpful in reducing code bloat when common
|
|
code is shared between readers and updaters.
|
|
|
|
10. Conversely, if you are in an RCU read-side critical section,
|
|
you -must- use the "_rcu()" variants of the list macros.
|
|
Failing to do so will break Alpha and confuse people reading
|
|
your code.
|
|
|
|
11. Note that synchronize_rcu() -only- guarantees to wait until
|
|
all currently executing rcu_read_lock()-protected RCU read-side
|
|
critical sections complete. It does -not- necessarily guarantee
|
|
that all currently running interrupts, NMIs, preempt_disable()
|
|
code, or idle loops will complete. Therefore, if you do not have
|
|
rcu_read_lock()-protected read-side critical sections, do -not-
|
|
use synchronize_rcu().
|
|
|
|
If you want to wait for some of these other things, you might
|
|
instead need to use synchronize_irq() or synchronize_sched().
|
|
|
|
12. Any lock acquired by an RCU callback must be acquired elsewhere
|
|
with irq disabled, e.g., via spin_lock_irqsave(). Failing to
|
|
disable irq on a given acquisition of that lock will result in
|
|
deadlock as soon as the RCU callback happens to interrupt that
|
|
acquisition's critical section.
|
|
|
|
13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
|
|
may only be invoked from process context. Unlike other forms of
|
|
RCU, it -is- permissible to block in an SRCU read-side critical
|
|
section (demarked by srcu_read_lock() and srcu_read_unlock()),
|
|
hence the "SRCU": "sleepable RCU". Please note that if you
|
|
don't need to sleep in read-side critical sections, you should
|
|
be using RCU rather than SRCU, because RCU is almost always
|
|
faster and easier to use than is SRCU.
|
|
|
|
Also unlike other forms of RCU, explicit initialization
|
|
and cleanup is required via init_srcu_struct() and
|
|
cleanup_srcu_struct(). These are passed a "struct srcu_struct"
|
|
that defines the scope of a given SRCU domain. Once initialized,
|
|
the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
|
|
and synchronize_srcu(). A given synchronize_srcu() waits only
|
|
for SRCU read-side critical sections governed by srcu_read_lock()
|
|
and srcu_read_unlock() calls that have been passd the same
|
|
srcu_struct. This property is what makes sleeping read-side
|
|
critical sections tolerable -- a given subsystem delays only
|
|
its own updates, not those of other subsystems using SRCU.
|
|
Therefore, SRCU is less prone to OOM the system than RCU would
|
|
be if RCU's read-side critical sections were permitted to
|
|
sleep.
|
|
|
|
The ability to sleep in read-side critical sections does not
|
|
come for free. First, corresponding srcu_read_lock() and
|
|
srcu_read_unlock() calls must be passed the same srcu_struct.
|
|
Second, grace-period-detection overhead is amortized only
|
|
over those updates sharing a given srcu_struct, rather than
|
|
being globally amortized as they are for other forms of RCU.
|
|
Therefore, SRCU should be used in preference to rw_semaphore
|
|
only in extremely read-intensive situations, or in situations
|
|
requiring SRCU's read-side deadlock immunity or low read-side
|
|
realtime latency.
|
|
|
|
Note that, rcu_assign_pointer() and rcu_dereference() relate to
|
|
SRCU just as they do to other forms of RCU.
|