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Linux/Documentation/spinlocks.txt

  1 Lesson 1: Spin locks
  2 
  3 The most basic primitive for locking is spinlock.
  4 
  5 static DEFINE_SPINLOCK(xxx_lock);
  6 
  7         unsigned long flags;
  8 
  9         spin_lock_irqsave(&xxx_lock, flags);
 10         ... critical section here ..
 11         spin_unlock_irqrestore(&xxx_lock, flags);
 12 
 13 The above is always safe. It will disable interrupts _locally_, but the
 14 spinlock itself will guarantee the global lock, so it will guarantee that
 15 there is only one thread-of-control within the region(s) protected by that
 16 lock. This works well even under UP also, so the code does _not_ need to
 17 worry about UP vs SMP issues: the spinlocks work correctly under both.
 18 
 19    NOTE! Implications of spin_locks for memory are further described in:
 20 
 21      Documentation/memory-barriers.txt
 22        (5) LOCK operations.
 23        (6) UNLOCK operations.
 24 
 25 The above is usually pretty simple (you usually need and want only one
 26 spinlock for most things - using more than one spinlock can make things a
 27 lot more complex and even slower and is usually worth it only for
 28 sequences that you _know_ need to be split up: avoid it at all cost if you
 29 aren't sure).
 30 
 31 This is really the only really hard part about spinlocks: once you start
 32 using spinlocks they tend to expand to areas you might not have noticed
 33 before, because you have to make sure the spinlocks correctly protect the
 34 shared data structures _everywhere_ they are used. The spinlocks are most
 35 easily added to places that are completely independent of other code (for
 36 example, internal driver data structures that nobody else ever touches).
 37 
 38    NOTE! The spin-lock is safe only when you _also_ use the lock itself
 39    to do locking across CPU's, which implies that EVERYTHING that
 40    touches a shared variable has to agree about the spinlock they want
 41    to use.
 42 
 43 ----
 44 
 45 Lesson 2: reader-writer spinlocks.
 46 
 47 If your data accesses have a very natural pattern where you usually tend
 48 to mostly read from the shared variables, the reader-writer locks
 49 (rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
 50 readers to be in the same critical region at once, but if somebody wants
 51 to change the variables it has to get an exclusive write lock.
 52 
 53    NOTE! reader-writer locks require more atomic memory operations than
 54    simple spinlocks.  Unless the reader critical section is long, you
 55    are better off just using spinlocks.
 56 
 57 The routines look the same as above:
 58 
 59    rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
 60 
 61         unsigned long flags;
 62 
 63         read_lock_irqsave(&xxx_lock, flags);
 64         .. critical section that only reads the info ...
 65         read_unlock_irqrestore(&xxx_lock, flags);
 66 
 67         write_lock_irqsave(&xxx_lock, flags);
 68         .. read and write exclusive access to the info ...
 69         write_unlock_irqrestore(&xxx_lock, flags);
 70 
 71 The above kind of lock may be useful for complex data structures like
 72 linked lists, especially searching for entries without changing the list
 73 itself.  The read lock allows many concurrent readers.  Anything that
 74 _changes_ the list will have to get the write lock.
 75 
 76    NOTE! RCU is better for list traversal, but requires careful
 77    attention to design detail (see Documentation/RCU/listRCU.txt).
 78 
 79 Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
 80 time need to do any changes (even if you don't do it every time), you have
 81 to get the write-lock at the very beginning.
 82 
 83    NOTE! We are working hard to remove reader-writer spinlocks in most
 84    cases, so please don't add a new one without consensus.  (Instead, see
 85    Documentation/RCU/rcu.txt for complete information.)
 86 
 87 ----
 88 
 89 Lesson 3: spinlocks revisited.
 90 
 91 The single spin-lock primitives above are by no means the only ones. They
 92 are the most safe ones, and the ones that work under all circumstances,
 93 but partly _because_ they are safe they are also fairly slow. They are slower
 94 than they'd need to be, because they do have to disable interrupts
 95 (which is just a single instruction on a x86, but it's an expensive one -
 96 and on other architectures it can be worse).
 97 
 98 If you have a case where you have to protect a data structure across
 99 several CPU's and you want to use spinlocks you can potentially use
100 cheaper versions of the spinlocks. IFF you know that the spinlocks are
101 never used in interrupt handlers, you can use the non-irq versions:
102 
103         spin_lock(&lock);
104         ...
105         spin_unlock(&lock);
106 
107 (and the equivalent read-write versions too, of course). The spinlock will
108 guarantee the same kind of exclusive access, and it will be much faster. 
109 This is useful if you know that the data in question is only ever
110 manipulated from a "process context", ie no interrupts involved. 
111 
112 The reasons you mustn't use these versions if you have interrupts that
113 play with the spinlock is that you can get deadlocks:
114 
115         spin_lock(&lock);
116         ...
117                 <- interrupt comes in:
118                         spin_lock(&lock);
119 
120 where an interrupt tries to lock an already locked variable. This is ok if
121 the other interrupt happens on another CPU, but it is _not_ ok if the
122 interrupt happens on the same CPU that already holds the lock, because the
123 lock will obviously never be released (because the interrupt is waiting
124 for the lock, and the lock-holder is interrupted by the interrupt and will
125 not continue until the interrupt has been processed). 
126 
127 (This is also the reason why the irq-versions of the spinlocks only need
128 to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
129 on other CPU's, because an interrupt on another CPU doesn't interrupt the
130 CPU that holds the lock, so the lock-holder can continue and eventually
131 releases the lock). 
132 
133 Note that you can be clever with read-write locks and interrupts. For
134 example, if you know that the interrupt only ever gets a read-lock, then
135 you can use a non-irq version of read locks everywhere - because they
136 don't block on each other (and thus there is no dead-lock wrt interrupts. 
137 But when you do the write-lock, you have to use the irq-safe version. 
138 
139 For an example of being clever with rw-locks, see the "waitqueue_lock" 
140 handling in kernel/sched.c - nothing ever _changes_ a wait-queue from
141 within an interrupt, they only read the queue in order to know whom to
142 wake up. So read-locks are safe (which is good: they are very common
143 indeed), while write-locks need to protect themselves against interrupts.
144 
145                 Linus
146 
147 ----
148 
149 Reference information:
150 
151 For dynamic initialization, use spin_lock_init() or rwlock_init() as
152 appropriate:
153 
154    spinlock_t xxx_lock;
155    rwlock_t xxx_rw_lock;
156 
157    static int __init xxx_init(void)
158    {
159         spin_lock_init(&xxx_lock);
160         rwlock_init(&xxx_rw_lock);
161         ...
162    }
163 
164    module_init(xxx_init);
165 
166 For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
167 __SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.

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