168 lines
6.5 KiB
Plaintext
168 lines
6.5 KiB
Plaintext
Lesson 1: Spin locks
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The most basic primitive for locking is spinlock.
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static DEFINE_SPINLOCK(xxx_lock);
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unsigned long flags;
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spin_lock_irqsave(&xxx_lock, flags);
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... critical section here ..
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spin_unlock_irqrestore(&xxx_lock, flags);
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The above is always safe. It will disable interrupts _locally_, but the
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spinlock itself will guarantee the global lock, so it will guarantee that
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there is only one thread-of-control within the region(s) protected by that
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lock. This works well even under UP also, so the code does _not_ need to
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worry about UP vs SMP issues: the spinlocks work correctly under both.
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NOTE! Implications of spin_locks for memory are further described in:
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Documentation/memory-barriers.txt
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(5) LOCK operations.
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(6) UNLOCK operations.
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The above is usually pretty simple (you usually need and want only one
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spinlock for most things - using more than one spinlock can make things a
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lot more complex and even slower and is usually worth it only for
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sequences that you _know_ need to be split up: avoid it at all cost if you
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aren't sure).
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This is really the only really hard part about spinlocks: once you start
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using spinlocks they tend to expand to areas you might not have noticed
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before, because you have to make sure the spinlocks correctly protect the
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shared data structures _everywhere_ they are used. The spinlocks are most
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easily added to places that are completely independent of other code (for
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example, internal driver data structures that nobody else ever touches).
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NOTE! The spin-lock is safe only when you _also_ use the lock itself
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to do locking across CPU's, which implies that EVERYTHING that
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touches a shared variable has to agree about the spinlock they want
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to use.
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----
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Lesson 2: reader-writer spinlocks.
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If your data accesses have a very natural pattern where you usually tend
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to mostly read from the shared variables, the reader-writer locks
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(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
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readers to be in the same critical region at once, but if somebody wants
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to change the variables it has to get an exclusive write lock.
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NOTE! reader-writer locks require more atomic memory operations than
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simple spinlocks. Unless the reader critical section is long, you
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are better off just using spinlocks.
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The routines look the same as above:
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rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
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unsigned long flags;
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read_lock_irqsave(&xxx_lock, flags);
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.. critical section that only reads the info ...
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read_unlock_irqrestore(&xxx_lock, flags);
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write_lock_irqsave(&xxx_lock, flags);
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.. read and write exclusive access to the info ...
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write_unlock_irqrestore(&xxx_lock, flags);
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The above kind of lock may be useful for complex data structures like
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linked lists, especially searching for entries without changing the list
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itself. The read lock allows many concurrent readers. Anything that
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_changes_ the list will have to get the write lock.
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NOTE! RCU is better for list traversal, but requires careful
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attention to design detail (see Documentation/RCU/listRCU.txt).
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Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
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time need to do any changes (even if you don't do it every time), you have
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to get the write-lock at the very beginning.
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NOTE! We are working hard to remove reader-writer spinlocks in most
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cases, so please don't add a new one without consensus. (Instead, see
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Documentation/RCU/rcu.txt for complete information.)
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----
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Lesson 3: spinlocks revisited.
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The single spin-lock primitives above are by no means the only ones. They
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are the most safe ones, and the ones that work under all circumstances,
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but partly _because_ they are safe they are also fairly slow. They are slower
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than they'd need to be, because they do have to disable interrupts
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(which is just a single instruction on a x86, but it's an expensive one -
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and on other architectures it can be worse).
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If you have a case where you have to protect a data structure across
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several CPU's and you want to use spinlocks you can potentially use
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cheaper versions of the spinlocks. IFF you know that the spinlocks are
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never used in interrupt handlers, you can use the non-irq versions:
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spin_lock(&lock);
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...
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spin_unlock(&lock);
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(and the equivalent read-write versions too, of course). The spinlock will
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guarantee the same kind of exclusive access, and it will be much faster.
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This is useful if you know that the data in question is only ever
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manipulated from a "process context", ie no interrupts involved.
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The reasons you mustn't use these versions if you have interrupts that
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play with the spinlock is that you can get deadlocks:
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spin_lock(&lock);
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...
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<- interrupt comes in:
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spin_lock(&lock);
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where an interrupt tries to lock an already locked variable. This is ok if
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the other interrupt happens on another CPU, but it is _not_ ok if the
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interrupt happens on the same CPU that already holds the lock, because the
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lock will obviously never be released (because the interrupt is waiting
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for the lock, and the lock-holder is interrupted by the interrupt and will
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not continue until the interrupt has been processed).
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(This is also the reason why the irq-versions of the spinlocks only need
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to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
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on other CPU's, because an interrupt on another CPU doesn't interrupt the
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CPU that holds the lock, so the lock-holder can continue and eventually
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releases the lock).
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Note that you can be clever with read-write locks and interrupts. For
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example, if you know that the interrupt only ever gets a read-lock, then
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you can use a non-irq version of read locks everywhere - because they
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don't block on each other (and thus there is no dead-lock wrt interrupts.
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But when you do the write-lock, you have to use the irq-safe version.
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For an example of being clever with rw-locks, see the "waitqueue_lock"
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handling in kernel/sched/core.c - nothing ever _changes_ a wait-queue from
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within an interrupt, they only read the queue in order to know whom to
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wake up. So read-locks are safe (which is good: they are very common
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indeed), while write-locks need to protect themselves against interrupts.
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Linus
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----
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Reference information:
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For dynamic initialization, use spin_lock_init() or rwlock_init() as
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appropriate:
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spinlock_t xxx_lock;
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rwlock_t xxx_rw_lock;
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static int __init xxx_init(void)
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{
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spin_lock_init(&xxx_lock);
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rwlock_init(&xxx_rw_lock);
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...
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}
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module_init(xxx_init);
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For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
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__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
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