FreeBSD/Linux Kernel Cross Reference
sys/Documentation/spinlocks.txt

     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.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.

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