1===============
2Locking lessons
3===============
4
5Lesson 1: Spin locks
6====================
7
8The most basic primitive for locking is spinlock::
9
10  static DEFINE_SPINLOCK(xxx_lock);
11
12	unsigned long flags;
13
14	spin_lock_irqsave(&xxx_lock, flags);
15	... critical section here ..
16	spin_unlock_irqrestore(&xxx_lock, flags);
17
18The above is always safe. It will disable interrupts _locally_, but the
19spinlock itself will guarantee the global lock, so it will guarantee that
20there is only one thread-of-control within the region(s) protected by that
21lock. This works well even under UP also, so the code does _not_ need to
22worry about UP vs SMP issues: the spinlocks work correctly under both.
23
24   NOTE! Implications of spin_locks for memory are further described in:
25
26     Documentation/memory-barriers.txt
27
28       (5) LOCK operations.
29
30       (6) UNLOCK operations.
31
32The above is usually pretty simple (you usually need and want only one
33spinlock for most things - using more than one spinlock can make things a
34lot more complex and even slower and is usually worth it only for
35sequences that you **know** need to be split up: avoid it at all cost if you
36aren't sure).
37
38This is really the only really hard part about spinlocks: once you start
39using spinlocks they tend to expand to areas you might not have noticed
40before, because you have to make sure the spinlocks correctly protect the
41shared data structures **everywhere** they are used. The spinlocks are most
42easily added to places that are completely independent of other code (for
43example, internal driver data structures that nobody else ever touches).
44
45   NOTE! The spin-lock is safe only when you **also** use the lock itself
46   to do locking across CPU's, which implies that EVERYTHING that
47   touches a shared variable has to agree about the spinlock they want
48   to use.
49
50----
51
52Lesson 2: reader-writer spinlocks.
53==================================
54
55If your data accesses have a very natural pattern where you usually tend
56to mostly read from the shared variables, the reader-writer locks
57(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
58readers to be in the same critical region at once, but if somebody wants
59to change the variables it has to get an exclusive write lock.
60
61   NOTE! reader-writer locks require more atomic memory operations than
62   simple spinlocks.  Unless the reader critical section is long, you
63   are better off just using spinlocks.
64
65The routines look the same as above::
66
67   rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
68
69	unsigned long flags;
70
71	read_lock_irqsave(&xxx_lock, flags);
72	.. critical section that only reads the info ...
73	read_unlock_irqrestore(&xxx_lock, flags);
74
75	write_lock_irqsave(&xxx_lock, flags);
76	.. read and write exclusive access to the info ...
77	write_unlock_irqrestore(&xxx_lock, flags);
78
79The above kind of lock may be useful for complex data structures like
80linked lists, especially searching for entries without changing the list
81itself.  The read lock allows many concurrent readers.  Anything that
82**changes** the list will have to get the write lock.
83
84   NOTE! RCU is better for list traversal, but requires careful
85   attention to design detail (see Documentation/RCU/listRCU.rst).
86
87Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
88time need to do any changes (even if you don't do it every time), you have
89to get the write-lock at the very beginning.
90
91   NOTE! We are working hard to remove reader-writer spinlocks in most
92   cases, so please don't add a new one without consensus.  (Instead, see
93   Documentation/RCU/rcu.rst for complete information.)
94
95----
96
97Lesson 3: spinlocks revisited.
98==============================
99
100The single spin-lock primitives above are by no means the only ones. They
101are the most safe ones, and the ones that work under all circumstances,
102but partly **because** they are safe they are also fairly slow. They are slower
103than they'd need to be, because they do have to disable interrupts
104(which is just a single instruction on a x86, but it's an expensive one -
105and on other architectures it can be worse).
106
107If you have a case where you have to protect a data structure across
108several CPU's and you want to use spinlocks you can potentially use
109cheaper versions of the spinlocks. IFF you know that the spinlocks are
110never used in interrupt handlers, you can use the non-irq versions::
111
112	spin_lock(&lock);
113	...
114	spin_unlock(&lock);
115
116(and the equivalent read-write versions too, of course). The spinlock will
117guarantee the same kind of exclusive access, and it will be much faster.
118This is useful if you know that the data in question is only ever
119manipulated from a "process context", ie no interrupts involved.
120
121The reasons you mustn't use these versions if you have interrupts that
122play with the spinlock is that you can get deadlocks::
123
124	spin_lock(&lock);
125	...
126		<- interrupt comes in:
127			spin_lock(&lock);
128
129where an interrupt tries to lock an already locked variable. This is ok if
130the other interrupt happens on another CPU, but it is _not_ ok if the
131interrupt happens on the same CPU that already holds the lock, because the
132lock will obviously never be released (because the interrupt is waiting
133for the lock, and the lock-holder is interrupted by the interrupt and will
134not continue until the interrupt has been processed).
135
136(This is also the reason why the irq-versions of the spinlocks only need
137to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
138on other CPU's, because an interrupt on another CPU doesn't interrupt the
139CPU that holds the lock, so the lock-holder can continue and eventually
140releases the lock).
141
142Note that you can be clever with read-write locks and interrupts. For
143example, if you know that the interrupt only ever gets a read-lock, then
144you can use a non-irq version of read locks everywhere - because they
145don't block on each other (and thus there is no dead-lock wrt interrupts.
146But when you do the write-lock, you have to use the irq-safe version.
147
148For an example of being clever with rw-locks, see the "waitqueue_lock"
149handling in kernel/sched/core.c - nothing ever _changes_ a wait-queue from
150within an interrupt, they only read the queue in order to know whom to
151wake up. So read-locks are safe (which is good: they are very common
152indeed), while write-locks need to protect themselves against interrupts.
153
154		Linus
155
156----
157
158Reference information:
159======================
160
161For dynamic initialization, use spin_lock_init() or rwlock_init() as
162appropriate::
163
164   spinlock_t xxx_lock;
165   rwlock_t xxx_rw_lock;
166
167   static int __init xxx_init(void)
168   {
169	spin_lock_init(&xxx_lock);
170	rwlock_init(&xxx_rw_lock);
171	...
172   }
173
174   module_init(xxx_init);
175
176For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
177__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
178