-UPDATE March 21 2005 Amit Gud <gud@eth.net>
+Lesson 1: Spin locks
-Macros SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED are deprecated and will be
-removed soon. So for any new code dynamic initialization should be used:
-
- spinlock_t xxx_lock;
- rwlock_t xxx_rw_lock;
-
- static int __init xxx_init(void)
- {
- spin_lock_init(&xxx_lock);
- rw_lock_init(&xxx_rw_lock);
- ...
- }
-
- module_init(xxx_init);
-
-Reasons for deprecation
- - it hurts automatic lock validators
- - it becomes intrusive for the realtime preemption patches
-
-Following discussion is still valid, however, with the dynamic initialization
-of spinlocks instead of static.
-
------------------------
-
-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;
... critical section here ..
spin_unlock_irqrestore(&xxx_lock, flags);
-and the above is always safe. It will disable interrupts _locally_, but the
+The above is always safe. It will disable interrupts _locally_, but the
spinlock itself will guarantee the global lock, so it will guarantee that
there is only one thread-of-control within the region(s) protected by that
-lock.
+lock. This works well even under UP also, so the code does _not_ need to
+worry about UP vs SMP issues: the spinlocks work correctly under both.
-Note that it works well even under UP - the above sequence under UP
-essentially is just the same as doing a
+ NOTE! Implications of spin_locks for memory are further described in:
- unsigned long flags;
-
- save_flags(flags); cli();
- ... critical section ...
- restore_flags(flags);
-
-so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
-work correctly under both (and spinlocks are actually more efficient on
-architectures that allow doing the "save_flags + cli" in one go because I
-don't export that interface normally).
-
-NOTE NOTE NOTE! The reason the spinlock is so much faster than a global
-interrupt lock under SMP is exactly because it disables interrupts only on
-the local CPU. The spin-lock is safe only when you _also_ use the lock
-itself to do locking across CPU's, which implies that EVERYTHING that
-touches a shared variable has to agree about the spinlock they want to
-use.
+ Documentation/memory-barriers.txt
+ (5) LOCK operations.
+ (6) UNLOCK operations.
The above is usually pretty simple (you usually need and want only one
spinlock for most things - using more than one spinlock can make things a
lot more complex and even slower and is usually worth it only for
sequences that you _know_ need to be split up: avoid it at all cost if you
-aren't sure). HOWEVER, it _does_ mean that if you have some code that does
-
- cli();
- .. critical section ..
- sti();
-
-and another sequence that does
-
- spin_lock_irqsave(flags);
- .. critical section ..
- spin_unlock_irqrestore(flags);
-
-then they are NOT mutually exclusive, and the critical regions can happen
-at the same time on two different CPU's. That's fine per se, but the
-critical regions had better be critical for different things (ie they
-can't stomp on each other).
-
-The above is a problem mainly if you end up mixing code - for example the
-routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
-their actions, and if a driver uses spinlocks instead then you should
-think about issues like the above..
+aren't sure).
This is really the only really hard part about spinlocks: once you start
using spinlocks they tend to expand to areas you might not have noticed
before, because you have to make sure the spinlocks correctly protect the
shared data structures _everywhere_ they are used. The spinlocks are most
-easily added to places that are completely independent of other code (ie
-internal driver data structures that nobody else ever touches, for
-example).
+easily added to places that are completely independent of other code (for
+example, internal driver data structures that nobody else ever touches).
+
+ NOTE! The spin-lock is safe only when you _also_ use the lock itself
+ to do locking across CPU's, which implies that EVERYTHING that
+ touches a shared variable has to agree about the spinlock they want
+ to use.
----
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. Unless the reader critical section is long, you
+ are better off just using spinlocks.
- rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
+The routines look the same as above:
+ rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
unsigned long flags;
.. 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
-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 above kind of lock may be useful for complex data structures like
+linked lists, especially searching for entries without changing the list
+itself. The read lock allows many concurrent readers. Anything that
+_changes_ the list will have to get the write lock.
-Note: you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
+ NOTE! RCU is better for list traversal, but requires careful
+ attention to design detail (see Documentation/RCU/listRCU.txt).
+
+Also, 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
-to get the write-lock at the very beginning. I could fairly easily add a
-primitive to create a "upgradeable" read-lock, but it hasn't been an issue
-yet. Tell me if you'd want one.
+to get the write-lock at the very beginning.
+
+ NOTE! We are working hard to remove reader-writer spinlocks in most
+ cases, so please don't add a new one without consensus. (Instead, see
+ Documentation/RCU/rcu.txt for complete information.)
----
The single spin-lock primitives above are by no means the only ones. They
are the most safe ones, and the ones that work under all circumstances,
-but partly _because_ they are safe they are also fairly slow. They are
-much faster than a generic global cli/sti pair, but slower than they'd
-need to be, because they do have to disable interrupts (which is just a
-single instruction on a x86, but it's an expensive one - and on other
-architectures it can be worse).
+but partly _because_ they are safe they are also fairly slow. They are slower
+than they'd need to be, because they do have to disable interrupts
+(which is just a single instruction on a x86, but it's an expensive one -
+and on other architectures it can be worse).
If you have a case where you have to protect a data structure across
several CPU's and you want to use spinlocks you can potentially use
Linus
+----
+
+Reference information:
+
+For dynamic initialization, use spin_lock_init() or rwlock_init() as
+appropriate:
+
+ 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);
+For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
+__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
Code Review / linux-2.6.git / blobdiff