sched: 1Q08 RCU doc update, add call_rcu_sched()
Long-delayed update to the RCU documentation, including adding the new call_rcu_sched() and rcu_barrier_sched() APIs. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Paul E. McKenney
Ingo Molnar
parent
2326974df2
commit
32300751b4
|
|
@ -93,6 +93,9 @@ Since NMI handlers disable preemption, synchronize_sched() is guaranteed
|
|||
not to return until all ongoing NMI handlers exit. It is therefore safe
|
||||
to free up the handler's data as soon as synchronize_sched() returns.
|
||||
|
||||
Important note: for this to work, the architecture in question must
|
||||
invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.
|
||||
|
||||
|
||||
Answer to Quick Quiz
|
||||
|
||||
|
|
|
|||
|
|
@ -52,6 +52,10 @@ of each iteration. Unfortunately, chaotic relaxation requires highly
|
|||
structured data, such as the matrices used in scientific programs, and
|
||||
is thus inapplicable to most data structures in operating-system kernels.
|
||||
|
||||
In 1992, Henry (now Alexia) Massalin completed a dissertation advising
|
||||
parallel programmers to defer processing when feasible to simplify
|
||||
synchronization. RCU makes extremely heavy use of this advice.
|
||||
|
||||
In 1993, Jacobson [Jacobson93] verbally described what is perhaps the
|
||||
simplest deferred-free technique: simply waiting a fixed amount of time
|
||||
before freeing blocks awaiting deferred free. Jacobson did not describe
|
||||
|
|
@ -138,6 +142,13 @@ blocking in read-side critical sections appeared [PaulEMcKenney2006c],
|
|||
Robert Olsson described an RCU-protected trie-hash combination
|
||||
[RobertOlsson2006a].
|
||||
|
||||
2007 saw the journal version of the award-winning RCU paper from 2006
|
||||
[ThomasEHart2007a], as well as a paper demonstrating use of Promela
|
||||
and Spin to mechanically verify an optimization to Oleg Nesterov's
|
||||
QRCU [PaulEMcKenney2007QRCUspin], a design document describing
|
||||
preemptible RCU [PaulEMcKenney2007PreemptibleRCU], and the three-part
|
||||
LWN "What is RCU?" series [PaulEMcKenney2007WhatIsRCUFundamentally,
|
||||
PaulEMcKenney2008WhatIsRCUUsage, and PaulEMcKenney2008WhatIsRCUAPI].
|
||||
|
||||
Bibtex Entries
|
||||
|
||||
|
|
@ -202,6 +213,20 @@ Bibtex Entries
|
|||
,Year="1991"
|
||||
}
|
||||
|
||||
@phdthesis{HMassalinPhD
|
||||
,author="H. Massalin"
|
||||
,title="Synthesis: An Efficient Implementation of Fundamental Operating
|
||||
System Services"
|
||||
,school="Columbia University"
|
||||
,address="New York, NY"
|
||||
,year="1992"
|
||||
,annotation="
|
||||
Mondo optimizing compiler.
|
||||
Wait-free stuff.
|
||||
Good advice: defer work to avoid synchronization.
|
||||
"
|
||||
}
|
||||
|
||||
@unpublished{Jacobson93
|
||||
,author="Van Jacobson"
|
||||
,title="Avoid Read-Side Locking Via Delayed Free"
|
||||
|
|
@ -635,3 +660,86 @@ Revised:
|
|||
"
|
||||
}
|
||||
|
||||
@unpublished{PaulEMcKenney2007PreemptibleRCU
|
||||
,Author="Paul E. McKenney"
|
||||
,Title="The design of preemptible read-copy-update"
|
||||
,month="October"
|
||||
,day="8"
|
||||
,year="2007"
|
||||
,note="Available:
|
||||
\url{http://lwn.net/Articles/253651/}
|
||||
[Viewed October 25, 2007]"
|
||||
,annotation="
|
||||
LWN article describing the design of preemptible RCU.
|
||||
"
|
||||
}
|
||||
|
||||
########################################################################
|
||||
#
|
||||
# "What is RCU?" LWN series.
|
||||
#
|
||||
|
||||
@unpublished{PaulEMcKenney2007WhatIsRCUFundamentally
|
||||
,Author="Paul E. McKenney and Jonathan Walpole"
|
||||
,Title="What is {RCU}, Fundamentally?"
|
||||
,month="December"
|
||||
,day="17"
|
||||
,year="2007"
|
||||
,note="Available:
|
||||
\url{http://lwn.net/Articles/262464/}
|
||||
[Viewed December 27, 2007]"
|
||||
,annotation="
|
||||
Lays out the three basic components of RCU: (1) publish-subscribe,
|
||||
(2) wait for pre-existing readers to complete, and (2) maintain
|
||||
multiple versions.
|
||||
"
|
||||
}
|
||||
|
||||
@unpublished{PaulEMcKenney2008WhatIsRCUUsage
|
||||
,Author="Paul E. McKenney"
|
||||
,Title="What is {RCU}? Part 2: Usage"
|
||||
,month="January"
|
||||
,day="4"
|
||||
,year="2008"
|
||||
,note="Available:
|
||||
\url{http://lwn.net/Articles/263130/}
|
||||
[Viewed January 4, 2008]"
|
||||
,annotation="
|
||||
Lays out six uses of RCU:
|
||||
1. RCU is a Reader-Writer Lock Replacement
|
||||
2. RCU is a Restricted Reference-Counting Mechanism
|
||||
3. RCU is a Bulk Reference-Counting Mechanism
|
||||
4. RCU is a Poor Man's Garbage Collector
|
||||
5. RCU is a Way of Providing Existence Guarantees
|
||||
6. RCU is a Way of Waiting for Things to Finish
|
||||
"
|
||||
}
|
||||
|
||||
@unpublished{PaulEMcKenney2008WhatIsRCUAPI
|
||||
,Author="Paul E. McKenney"
|
||||
,Title="{RCU} part 3: the {RCU} {API}"
|
||||
,month="January"
|
||||
,day="17"
|
||||
,year="2008"
|
||||
,note="Available:
|
||||
\url{http://lwn.net/Articles/264090/}
|
||||
[Viewed January 10, 2008]"
|
||||
,annotation="
|
||||
Gives an overview of the Linux-kernel RCU API and a brief annotated RCU
|
||||
bibliography.
|
||||
"
|
||||
}
|
||||
|
||||
@article{DinakarGuniguntala2008IBMSysJ
|
||||
,author="D. Guniguntala and P. E. McKenney and J. Triplett and J. Walpole"
|
||||
,title="The read-copy-update mechanism for supporting real-time applications on shared-memory multiprocessor systems with {Linux}"
|
||||
,Year="2008"
|
||||
,Month="April"
|
||||
,journal="IBM Systems Journal"
|
||||
,volume="47"
|
||||
,number="2"
|
||||
,pages="@@-@@"
|
||||
,annotation="
|
||||
RCU, realtime RCU, sleepable RCU, performance.
|
||||
"
|
||||
}
|
||||
|
|
|
|||
|
|
@ -13,10 +13,13 @@ over a rather long period of time, but improvements are always welcome!
|
|||
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.
|
||||
Another exception is 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.
|
||||
|
||||
Yet another exception is where the low real-time latency of RCU's
|
||||
read-side primitives is critically important.
|
||||
|
||||
1. Does the update code have proper mutual exclusion?
|
||||
|
||||
|
|
@ -39,9 +42,10 @@ over a rather long period of time, but improvements are always welcome!
|
|||
|
||||
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.
|
||||
to prevent grace periods from ending prematurely, which
|
||||
could result in data being unceremoniously freed out from
|
||||
under your read-side code, which can greatly increase the
|
||||
actuarial risk of your kernel.
|
||||
|
||||
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()
|
||||
|
|
@ -54,15 +58,30 @@ over a rather long period of time, but improvements are always welcome!
|
|||
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,
|
||||
a. Use the RCU variants of the list and hlist update
|
||||
primitives to add, remove, and replace elements on an
|
||||
RCU-protected list. Alternatively, use the RCU-protected
|
||||
trees that have been added to the Linux kernel.
|
||||
|
||||
This is almost always the best approach.
|
||||
|
||||
b. Proceed as in (a) above, but also maintain per-element
|
||||
locks (that are acquired by both readers and writers)
|
||||
that guard per-element state. Of course, fields that
|
||||
the readers refrain from accessing can be guarded by the
|
||||
update-side lock.
|
||||
|
||||
This works quite well, also.
|
||||
|
||||
c. 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.
|
||||
This can work, but is starting to get a bit tricky.
|
||||
|
||||
b. Carefully order the updates and the reads so that
|
||||
d. 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.
|
||||
|
|
@ -123,18 +142,22 @@ over a rather long period of time, but improvements are always welcome!
|
|||
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.
|
||||
5. If call_rcu(), or a related primitive such as call_rcu_bh() or
|
||||
call_rcu_sched(), 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.
|
||||
any sort of irq context. Ditto for synchronize_sched() and
|
||||
synchronize_srcu().
|
||||
|
||||
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.
|
||||
rcu_read_lock_bh() and rcu_read_unlock_bh(). If the updater
|
||||
uses call_rcu_sched(), then the corresponding readers must
|
||||
disable preemption. 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()
|
||||
|
|
@ -143,9 +166,9 @@ over a rather long period of time, but improvements are always welcome!
|
|||
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,
|
||||
8. Although synchronize_rcu() is 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()
|
||||
|
|
@ -187,23 +210,23 @@ over a rather long period of time, but improvements are always welcome!
|
|||
number of updates per grace period.
|
||||
|
||||
9. All RCU list-traversal primitives, which include
|
||||
list_for_each_rcu(), list_for_each_entry_rcu(),
|
||||
rcu_dereference(), 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
|
||||
must be either within an RCU read-side critical section or
|
||||
must be protected by appropriate update-side locks. 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.
|
||||
The reason that it is permissible to use RCU list-traversal
|
||||
primitives when the update-side lock is held is that doing so
|
||||
can 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.
|
||||
and you don't hold the appropriate update-side lock, 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
|
||||
|
|
@ -230,6 +253,14 @@ over a rather long period of time, but improvements are always welcome!
|
|||
must use whatever locking or other synchronization is required
|
||||
to safely access and/or modify that data structure.
|
||||
|
||||
RCU callbacks are -usually- executed on the same CPU that executed
|
||||
the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
|
||||
but are by -no- means guaranteed to be. For example, if a given
|
||||
CPU goes offline while having an RCU callback pending, then that
|
||||
RCU callback will execute on some surviving CPU. (If this was
|
||||
not the case, a self-spawning RCU callback would prevent the
|
||||
victim CPU from ever going offline.)
|
||||
|
||||
14. 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
|
||||
|
|
|
|||
|
|
@ -1,3 +1,11 @@
|
|||
Please note that the "What is RCU?" LWN series is an excellent place
|
||||
to start learning about RCU:
|
||||
|
||||
1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
|
||||
2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
|
||||
3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
|
||||
|
||||
|
||||
What is RCU?
|
||||
|
||||
RCU is a synchronization mechanism that was added to the Linux kernel
|
||||
|
|
@ -772,26 +780,18 @@ Linux-kernel source code, but it helps to have a full list of the
|
|||
APIs, since there does not appear to be a way to categorize them
|
||||
in docbook. Here is the list, by category.
|
||||
|
||||
Markers for RCU read-side critical sections:
|
||||
|
||||
rcu_read_lock
|
||||
rcu_read_unlock
|
||||
rcu_read_lock_bh
|
||||
rcu_read_unlock_bh
|
||||
srcu_read_lock
|
||||
srcu_read_unlock
|
||||
|
||||
RCU pointer/list traversal:
|
||||
|
||||
rcu_dereference
|
||||
list_for_each_rcu (to be deprecated in favor of
|
||||
list_for_each_entry_rcu)
|
||||
list_for_each_entry_rcu
|
||||
list_for_each_continue_rcu (to be deprecated in favor of new
|
||||
list_for_each_entry_continue_rcu)
|
||||
hlist_for_each_entry_rcu
|
||||
|
||||
RCU pointer update:
|
||||
list_for_each_rcu (to be deprecated in favor of
|
||||
list_for_each_entry_rcu)
|
||||
list_for_each_continue_rcu (to be deprecated in favor of new
|
||||
list_for_each_entry_continue_rcu)
|
||||
|
||||
RCU pointer/list update:
|
||||
|
||||
rcu_assign_pointer
|
||||
list_add_rcu
|
||||
|
|
@ -799,16 +799,36 @@ RCU pointer update:
|
|||
list_del_rcu
|
||||
list_replace_rcu
|
||||
hlist_del_rcu
|
||||
hlist_add_after_rcu
|
||||
hlist_add_before_rcu
|
||||
hlist_add_head_rcu
|
||||
hlist_replace_rcu
|
||||
list_splice_init_rcu()
|
||||
|
||||
RCU grace period:
|
||||
RCU: Critical sections Grace period Barrier
|
||||
|
||||
rcu_read_lock synchronize_net rcu_barrier
|
||||
rcu_read_unlock synchronize_rcu
|
||||
call_rcu
|
||||
|
||||
|
||||
bh: Critical sections Grace period Barrier
|
||||
|
||||
rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
|
||||
rcu_read_unlock_bh
|
||||
|
||||
|
||||
sched: Critical sections Grace period Barrier
|
||||
|
||||
[preempt_disable] synchronize_sched rcu_barrier_sched
|
||||
[and friends] call_rcu_sched
|
||||
|
||||
|
||||
SRCU: Critical sections Grace period Barrier
|
||||
|
||||
srcu_read_lock synchronize_srcu N/A
|
||||
srcu_read_unlock
|
||||
|
||||
synchronize_net
|
||||
synchronize_sched
|
||||
synchronize_rcu
|
||||
synchronize_srcu
|
||||
call_rcu
|
||||
call_rcu_bh
|
||||
|
||||
See the comment headers in the source code (or the docbook generated
|
||||
from them) for more information.
|
||||
|
|
|
|||
Loading…
Reference in New Issue