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Diffstat (limited to 'Documentation/locking/spinlocks.txt')
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diff --git a/Documentation/locking/spinlocks.txt b/Documentation/locking/spinlocks.txt deleted file mode 100644 index ff35e40bdf5b..000000000000 --- a/Documentation/locking/spinlocks.txt +++ /dev/null @@ -1,167 +0,0 @@ -Lesson 1: Spin locks - -The most basic primitive for locking is spinlock. - -static DEFINE_SPINLOCK(xxx_lock); - - unsigned long flags; - - spin_lock_irqsave(&xxx_lock, flags); - ... critical section here .. - spin_unlock_irqrestore(&xxx_lock, flags); - -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. 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! Implications of spin_locks for memory are further described in: - - 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). - -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 (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. - ----- - -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 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. - - 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. - -The routines look the same as above: - - rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock); - - unsigned long flags; - - read_lock_irqsave(&xxx_lock, flags); - .. critical section that only reads the info ... - read_unlock_irqrestore(&xxx_lock, flags); - - write_lock_irqsave(&xxx_lock, flags); - .. read and write exclusive access to the info ... - write_unlock_irqrestore(&xxx_lock, flags); - -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! 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. - - 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.) - ----- - -Lesson 3: spinlocks revisited. - -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 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 -cheaper versions of the spinlocks. IFF you know that the spinlocks are -never used in interrupt handlers, you can use the non-irq versions: - - spin_lock(&lock); - ... - spin_unlock(&lock); - -(and the equivalent read-write versions too, of course). The spinlock will -guarantee the same kind of exclusive access, and it will be much faster. -This is useful if you know that the data in question is only ever -manipulated from a "process context", ie no interrupts involved. - -The reasons you mustn't use these versions if you have interrupts that -play with the spinlock is that you can get deadlocks: - - spin_lock(&lock); - ... - <- interrupt comes in: - spin_lock(&lock); - -where an interrupt tries to lock an already locked variable. This is ok if -the other interrupt happens on another CPU, but it is _not_ ok if the -interrupt happens on the same CPU that already holds the lock, because the -lock will obviously never be released (because the interrupt is waiting -for the lock, and the lock-holder is interrupted by the interrupt and will -not continue until the interrupt has been processed). - -(This is also the reason why the irq-versions of the spinlocks only need -to disable the _local_ interrupts - it's ok to use spinlocks in interrupts -on other CPU's, because an interrupt on another CPU doesn't interrupt the -CPU that holds the lock, so the lock-holder can continue and eventually -releases the lock). - -Note that you can be clever with read-write locks and interrupts. For -example, if you know that the interrupt only ever gets a read-lock, then -you can use a non-irq version of read locks everywhere - because they -don't block on each other (and thus there is no dead-lock wrt interrupts. -But when you do the write-lock, you have to use the irq-safe version. - -For an example of being clever with rw-locks, see the "waitqueue_lock" -handling in kernel/sched/core.c - nothing ever _changes_ a wait-queue from -within an interrupt, they only read the queue in order to know whom to -wake up. So read-locks are safe (which is good: they are very common -indeed), while write-locks need to protect themselves against interrupts. - - 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. |