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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

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/core.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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