[PATCH] Documentation: rw_lock lessons learned
William Allen Simpson
strayed down the rw_lock path. Update the Documentation
based upon comments in those threads.
---
Documentation/spinlocks.txt | 14 ++++++++++++++
1 files changed, 14 insertions(+), 0 deletions(-)
William Allen Simpson
strayed down the rw_lock path. Update the Documentation
based upon comments in those threads.
Signed-off-by: William.Al...@gmail.com
Paul E. McKenney
> diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt
> index 619699d..c112052 100644
> --- a/Documentation/spinlocks.txt
> +++ b/Documentation/spinlocks.txt
> @@ -233,4 +233,18 @@ indeed), while write-locks need to protect themselves against interrupts.
>
> Linus
As you might guess, works for me!!!
Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
> +----
> +
> +The implications of spin_locks on memory are further described in:
> +
> + Documentation/memory-barriers.txt
> + (5) LOCK operations.
> + (6) UNLOCK operations.
> +
> +----
> +
> +We are working hard to remove reader-writer spinlocks (rw_lock) from the
> +network stack, so please don't add a new one. Instead, see:
> +
> + Documentation/RCU/rcu.txt
>
> --
> 1.6.3.3
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majo...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Stephen Hemminger
I would rather see the text in Documentation/spinlocks give an explaination
as to why reader/writer locks are normally not desirable.
The whole document needs work to make it a developer document, rather than
a historical mail thread.. A good document says what should be done today,
and does not have old junk or ask the reader to overly new context
on old information.
--- a/Documentation/spinlocks.txt 2009-11-10 17:47:03.801984416 -0800
+++ b/Documentation/spinlocks.txt 2009-11-10 18:01:00.779749888 -0800
@@ -1,73 +1,8 @@
-SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED defeat lockdep state tracking and
-are hence deprecated.
+Lesson 1: Spin locks
-Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
-__SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate for static
-initialization.
-
-Most of the time, you can simply turn:
-
- static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
-
-into:
-
- static DEFINE_SPINLOCK(xxx_lock);
-
-Static structure member variables go from:
-
- struct foo bar {
- .lock = SPIN_LOCK_UNLOCKED;
- };
-
-to:
-
- struct foo bar {
- .lock = __SPIN_LOCK_UNLOCKED(bar.lock);
- };
-
-Declaration of static rw_locks undergo a similar transformation.
-
-Dynamic initialization, when necessary, may be performed as
-demonstrated below.
-
- spinlock_t xxx_lock;
- rwlock_t xxx_rw_lock;
-
- static int __init xxx_init(void)
- {
- spin_lock_init(&xxx_lock);
- rwlock_init(&xxx_rw_lock);
- ...
- }
-
- module_init(xxx_init);
-
-The following discussion is still valid, however, with the dynamic
-initialization of spinlocks or with DEFINE_SPINLOCK, etc., used
-instead of SPIN_LOCK_UNLOCKED.
-
------------------------
-
-On Fri, 2 Jan 1998, Doug Ledford wrote:
->
-> I'm working on making the aic7xxx driver more SMP friendly (as well as
-> importing the latest FreeBSD sequencer code to have 7895 support) and wanted
-> to get some info from you. The goal here is to make the various routines
-> SMP safe as well as UP safe during interrupts and other manipulating
-> routines. So far, I've added a spin_lock variable to things like my queue
-> structs. Now, from what I recall, there are some spin lock functions I can
-> use to lock these spin locks from other use as opposed to a (nasty)
-> save_flags(); cli(); stuff; restore_flags(); construct. Where do I find
-> these routines and go about making use of them? Do they only lock on a
-> per-processor basis or can they also lock say an interrupt routine from
-> mucking with a queue if the queue routine was manipulating it when the
-> interrupt occurred, or should I still use a cli(); based construct on that
-> one?
-
-See <asm/spinlock.h>. The basic version is:
-
- spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
+The most basic primitive for locking is spinlock.
+static DEFINE_SPINLOCK(xxx_lock);
unsigned long flags;
@@ -141,13 +76,17 @@ Lesson 2: reader-writer spinlocks.
If your data accesses have a very natural pattern where you usually tend
to mostly read from the shared variables, the reader-writer locks
-(rw_lock) versions of the spinlocks are often nicer. They allow multiple
+(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
readers to be in the same critical region at once, but if somebody wants
-to change the variables it has to get an exclusive write lock. The
-routines look the same as above:
+to change the variables it has to get an exclusive write lock.
+
+NOTE! reader-writer locks require more atomic memory operations than
+simple spinlocks, so unless the reader critical secition is long you
+are better off just using spinlocks.
- rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
+The routines look the same as above:
+static DEFINE_RWLOCK(xxx_lock);
unsigned long flags;
@@ -159,12 +98,15 @@ routines look the same as above:
.. read and write exclusive access to the info ...
write_unlock_irqrestore(&xxx_lock, flags);
-The above kind of lock is useful for complex data structures like linked
+The above kind of lock might useful for complex data structures like linked
lists etc, especially when you know that most of the work is to just
traverse the list searching for entries without changing the list itself,
for example. Then you can use the read lock for that kind of list
traversal, which allows many concurrent readers. Anything that _changes_
-the list will have to get the write lock.
+the list will have to get the write lock.
+
+NOTE! RCU is better for that most read only access, but requires
+correct operations (see Documentation/RCU/listRCU.txt)
Note: you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
time need to do any changes (even if you don't do it every time), you have
@@ -233,4 +175,45 @@ indeed), while write-locks need to prote
Linus
+Reference information:
+
+* Older code used SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED to initialize
+ locks, but this is now deprecated because it interferes with the
+ lockdep state tracking. Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
+ __SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate for static
+ initialization.
+
+ Most of the time, you can simply turn:
+ static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
+ into:
+ static DEFINE_SPINLOCK(xxx_lock);
+
+ Static structure member variables go from:
+
+ struct foo bar {
+ .lock = SPIN_LOCK_UNLOCKED;
+ };
+
+ to:
+
+ struct foo bar {
+ .lock = __SPIN_LOCK_UNLOCKED(bar.lock);
+ };
+
+ Declaration of static rw_locks undergo a similar transformation.
+
+ Dynamic initialization, when necessary, may be performed as
+ demonstrated below.
+
+ spinlock_t xxx_lock;
+ rwlock_t xxx_rw_lock;
+
+ static int __init xxx_init(void)
+ {
+ spin_lock_init(&xxx_lock);
+ rwlock_init(&xxx_rw_lock);
+ ...
+ }
+
+ module_init(xxx_init);
William Allen Simpson
> I would rather see the text in Documentation/spinlocks give an explaination
> as to why reader/writer locks are normally not desirable.
>
> The whole document needs work to make it a developer document, rather than
> a historical mail thread.. A good document says what should be done today,
> and does not have old junk or ask the reader to overly new context
> on old information.
>
Or this is a second patch in a proposed series?
Stephen Hemminger
William Allen Simpson <william.al...@gmail.com> wrote:
> Stephen Hemminger wrote:
> > I would rather see the text in Documentation/spinlocks give an explaination
> > as to why reader/writer locks are normally not desirable.
> >
> > The whole document needs work to make it a developer document, rather than
> > a historical mail thread.. A good document says what should be done today,
> > and does not have old junk or ask the reader to overly new context
> > on old information.
> >
> You wish me to merge our patches?
Sure, I am more concerned about document structure being readable than
preserving my sloppy prose.
> Or this is a second patch in a proposed series?
No. But taking more input from others (maybe Randy will help he is a good
editor) would get this back in shape.
--
William Allen Simpson
strayed down the rw_lock path. Update the Documentation
those threads.
Merged with editorial changes by Stephen Hemminger.
Signed-off-by: William.Al...@gmail.com
Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
---
Documentation/spinlocks.txt | 186 ++++++++++++++++++++-----------------------
1 files changed, 86 insertions(+), 100 deletions(-)
Linus Torvalds
On Thu, 12 Nov 2009, William Allen Simpson wrote:
>
> In recent weeks, two different network projects erroneously
> strayed down the rw_lock path. Update the Documentation
> based upon comments by Eric Dumazet and Paul E. McKenney in
> those threads.
This still retains some pretty old and bogus language. For example, that
file still talks about spinlocks being "faster than a global interrupt
lock", which is kind of amusing - because we've not done that global
interrupt lock thing for the last ten years or so.
So I certainly agree with discouraging rwlocks - I don't think we've ever
really found a situation where they are useful except for some really
special cases - but I suspect the thing needs a bigger overhaul.
I also suspect somebody should actually take a look at our _users_ of
rwlocks. We have a few fairly central ones like 'tasklist_lock', and it
may be an example of a _good_ case of rwlocks, but for a very non-obvious
reason.
In the case of 'tasklist_lock', the magic subtle reason that makes it a
good idea is that that lock is commonly taken for reading in _interrupts_.
And the way rwlocks are defined, that means that you can take it for
reading without doing the *_irq() or *_irqsave() versions, because rwlocks
are not fair, so an active reader will never block a new reader even if
there is a blocked writer pending.
So for tasklist_lock, raw rwlocks are still slower than raw spinlocks, but
because of the rwlock semantics the common case doesn't need to disable
and enable interrupts, so for the common case the comparison is not "raw
rwlock vs raw spinlock", but "raw rwlock vs interrupt-disabling spinlock",
and then it turns out rwlocks tend to win again.
(Of course, lock_write() needs to disable interrupts for tasklist_lock,
but that tends to be the uncommon case).
Ho humm..
Linus
Stephen Hemminger
William Allen Simpson <william.al...@gmail.com> wrote:
> + NOTE! We are working hard to remove reader-writer spinlocks from the
> + network stack, so please don't add a new one. (Instead, see
> + Documentation/RCU/rcu.txt for complete information.)
It is not just networking, so don't single that out.
Also, should mention -RT kernel and locking here (Appendix?)
Stephen Clark
How up to date is this doc?
http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/index.html
Should it be in the Documentation directory?
--
"They that give up essential liberty to obtain temporary safety,
deserve neither liberty nor safety." (Ben Franklin)
"The course of history shows that as a government grows, liberty
decreases." (Thomas Jefferson)
Stephen Hemminger
Stephen Clark <scla...@earthlink.net> wrote:
> William Allen Simpson wrote:
> > In recent weeks, two different network projects erroneously
> > strayed down the rw_lock path. Update the Documentation
> > based upon comments by Eric Dumazet and Paul E. McKenney in
> > those threads.
> >
> > Merged with editorial changes by Stephen Hemminger.
> >
> > Signed-off-by: William.Al...@gmail.com
> > Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
> > ---
> > Documentation/spinlocks.txt | 186
> > ++++++++++++++++++++-----------------------
> > 1 files changed, 86 insertions(+), 100 deletions(-)
> >
>
> How up to date is this doc?
>
> http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/index.html
>
Out of date.
1. Missing mutex's which have largely replaced semaphores.
2. Missing change to lock initialization in later kernels.
3. Missing description of lock dependency checker which should be in same guide.
Stefan Richter
> On Thu, 12 Nov 2009 14:13:03 -0500
> Stephen Clark <scla...@earthlink.net> wrote:
>> How up to date is this doc?
>>
>> http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/index.html
>>
>
> Out of date.
> 1. Missing mutex's which have largely replaced semaphores.
>
> 2. Missing change to lock initialization in later kernels.
>
> 3. Missing description of lock dependency checker which should be in same guide.
4. The section on atomic reference counting should refer to <linux/kref.h>.
--
Stefan Richter
-=====-==--= =-== -==-=
http://arcgraph.de/sr/
William Allen Simpson
> Stephen Hemminger wrote:
>> On Thu, 12 Nov 2009 14:13:03 -0500
>> Stephen Clark <scla...@earthlink.net> wrote:
>>> How up to date is this doc?
>>>
>>> http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/index.html
>>>
>> Out of date.
>> 1. Missing mutex's which have largely replaced semaphores.
>>
>> 2. Missing change to lock initialization in later kernels.
>>
>> 3. Missing description of lock dependency checker which should be in same guide.
>
> 4. The section on atomic reference counting should refer to <linux/kref.h>.
I'd also read that, and that's where I got some of my wrong thinking. But
that does point to Documentation/spin_locks.txt, which I took to be
authoritative (and followed). That's the reason spin_locks.txt should be
updated, as others are having the same problems....
Anybody have answers/updates to Linus's concerns about "pretty old and
bogus language"?
William Allen Simpson
> In recent weeks, two different network projects erroneously
> strayed down the rw_lock path. Update the Documentation
> based upon comments by Eric Dumazet and Paul E. McKenney in
> those threads.
>
> Merged with editorial changes by Stephen Hemminger.
>
> Signed-off-by: William.Al...@gmail.com
> Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
>
response to mean he wasn't interested. But at the local
discussion yesterday, I'm told that's just a typical Linusism.
The thread diverged into discussion of another document entirely.
I'm not the person to update this document with any of the other
information about global locks and tasklists and such. But surely
somebody else could handle that in another patch.
Anybody have answers/updates to Linus's concerns about "pretty old
and bogus language"? Would folks be interested in the update?
Does anybody know which list(s) would be better for discussion?
Jarek Poplawski
> William Allen Simpson wrote:
>> In recent weeks, two different network projects erroneously
>> strayed down the rw_lock path. Update the Documentation
>> based upon comments by Eric Dumazet and Paul E. McKenney in
>> those threads.
>>
>> Merged with editorial changes by Stephen Hemminger.
>>
>> Signed-off-by: William.Al...@gmail.com
>> Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
>>
> A month ago, I'd taken the final line "Ho humm.." of Linus'
> response to mean he wasn't interested. But at the local
> discussion yesterday, I'm told that's just a typical Linusism.
Why would he write 6 paragraphs if he wasn't interested?
>
> The thread diverged into discussion of another document entirely.
>
> I'm not the person to update this document with any of the other
> information about global locks and tasklists and such. But surely
> somebody else could handle that in another patch.
>
> Anybody have answers/updates to Linus's concerns about "pretty old
> and bogus language"? Would folks be interested in the update?
> Does anybody know which list(s) would be better for discussion?
I guess, you could literally start with removing this "global
interrupt lock", adding "the example of a _good_ case of rwlocks",
plus Stephen's "it is not just networking" fix in v3.
Jarek P.
William Allen Simpson
> William Allen Simpson wrote, On 12/11/2009 06:01 PM:
>> A month ago, I'd taken the final line "Ho humm.." of Linus'
>> response to mean he wasn't interested. But at the local
>> discussion yesterday, I'm told that's just a typical Linusism.
>
> Why would he write 6 paragraphs if he wasn't interested?
>
decade or so ago, it wasn't obvious to me that it wasn't just a rant.
>> The thread diverged into discussion of another document entirely.
>>
>> I'm not the person to update this document with any of the other
>> information about global locks and tasklists and such. But surely
>> somebody else could handle that in another patch.
>>
>> Anybody have answers/updates to Linus's concerns about "pretty old
>> and bogus language"? Would folks be interested in the update?
>> Does anybody know which list(s) would be better for discussion?
>
> I guess, you could literally start with removing this "global
> interrupt lock", adding "the example of a _good_ case of rwlocks",
> plus Stephen's "it is not just networking" fix in v3.
>
As I mentioned, I'm not the person to do either of the former -- I'm
simply not conversant with the details. If anybody has more specific
information, I'd be happy to edit it together with mine. Or it could
be another patch entirely.
I'll do the latter later today. Thanks for your interest.
William Allen Simpson
strayed down the rw_lock path. Update the Documentation
based upon comments by Eric Dumazet and Paul E. McKenney in
those threads.
Further updates await somebody else with more expertise.
Changes:
- Merged with extensive content by Stephen Hemminger.
- Fix one of the comments by Linus Torvalds.
Signed-off-by: William.Al...@gmail.com
Acked-by: Paul E. McKenney <pau...@linux.vnet.ibm.com>
---
Documentation/spinlocks.txt | 184 ++++++++++++++++++++-----------------------
1 files changed, 84 insertions(+), 100 deletions(-)
Stephen Hemminger
William Allen Simpson <william.al...@gmail.com> wrote:
The whole document needs a writer to go over it and make it readable.
Something like an updated chapter on locking from LDD3 book.
John? Randy?