Root/Documentation/spinlocks.txt

1SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED defeat lockdep state tracking and
2are hence deprecated.
3
4Please use DEFINE_SPINLOCK()/DEFINE_RWLOCK() or
5__SPIN_LOCK_UNLOCKED()/__RW_LOCK_UNLOCKED() as appropriate for static
6initialization.
7
8Most of the time, you can simply turn:
9
10    static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
11
12into:
13
14    static DEFINE_SPINLOCK(xxx_lock);
15
16Static structure member variables go from:
17
18    struct foo bar {
19        .lock = SPIN_LOCK_UNLOCKED;
20    };
21
22to:
23
24    struct foo bar {
25        .lock = __SPIN_LOCK_UNLOCKED(bar.lock);
26    };
27
28Declaration of static rw_locks undergo a similar transformation.
29
30Dynamic initialization, when necessary, may be performed as
31demonstrated below.
32
33   spinlock_t xxx_lock;
34   rwlock_t xxx_rw_lock;
35
36   static int __init xxx_init(void)
37   {
38       spin_lock_init(&xxx_lock);
39    rwlock_init(&xxx_rw_lock);
40    ...
41   }
42
43   module_init(xxx_init);
44
45The following discussion is still valid, however, with the dynamic
46initialization of spinlocks or with DEFINE_SPINLOCK, etc., used
47instead of SPIN_LOCK_UNLOCKED.
48
49-----------------------
50
51On Fri, 2 Jan 1998, Doug Ledford wrote:
52>
53> I'm working on making the aic7xxx driver more SMP friendly (as well as
54> importing the latest FreeBSD sequencer code to have 7895 support) and wanted
55> to get some info from you. The goal here is to make the various routines
56> SMP safe as well as UP safe during interrupts and other manipulating
57> routines. So far, I've added a spin_lock variable to things like my queue
58> structs. Now, from what I recall, there are some spin lock functions I can
59> use to lock these spin locks from other use as opposed to a (nasty)
60> save_flags(); cli(); stuff; restore_flags(); construct. Where do I find
61> these routines and go about making use of them? Do they only lock on a
62> per-processor basis or can they also lock say an interrupt routine from
63> mucking with a queue if the queue routine was manipulating it when the
64> interrupt occurred, or should I still use a cli(); based construct on that
65> one?
66
67See <asm/spinlock.h>. The basic version is:
68
69   spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
70
71
72    unsigned long flags;
73
74    spin_lock_irqsave(&xxx_lock, flags);
75    ... critical section here ..
76    spin_unlock_irqrestore(&xxx_lock, flags);
77
78and the above is always safe. It will disable interrupts _locally_, but the
79spinlock itself will guarantee the global lock, so it will guarantee that
80there is only one thread-of-control within the region(s) protected by that
81lock.
82
83Note that it works well even under UP - the above sequence under UP
84essentially is just the same as doing a
85
86    unsigned long flags;
87
88    save_flags(flags); cli();
89     ... critical section ...
90    restore_flags(flags);
91
92so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
93work correctly under both (and spinlocks are actually more efficient on
94architectures that allow doing the "save_flags + cli" in one go because I
95don't export that interface normally).
96
97NOTE NOTE NOTE! The reason the spinlock is so much faster than a global
98interrupt lock under SMP is exactly because it disables interrupts only on
99the local CPU. The spin-lock is safe only when you _also_ use the lock
100itself to do locking across CPU's, which implies that EVERYTHING that
101touches a shared variable has to agree about the spinlock they want to
102use.
103
104The above is usually pretty simple (you usually need and want only one
105spinlock for most things - using more than one spinlock can make things a
106lot more complex and even slower and is usually worth it only for
107sequences that you _know_ need to be split up: avoid it at all cost if you
108aren't sure). HOWEVER, it _does_ mean that if you have some code that does
109
110    cli();
111    .. critical section ..
112    sti();
113
114and another sequence that does
115
116    spin_lock_irqsave(flags);
117    .. critical section ..
118    spin_unlock_irqrestore(flags);
119
120then they are NOT mutually exclusive, and the critical regions can happen
121at the same time on two different CPU's. That's fine per se, but the
122critical regions had better be critical for different things (ie they
123can't stomp on each other).
124
125The above is a problem mainly if you end up mixing code - for example the
126routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
127their actions, and if a driver uses spinlocks instead then you should
128think about issues like the above..
129
130This is really the only really hard part about spinlocks: once you start
131using spinlocks they tend to expand to areas you might not have noticed
132before, because you have to make sure the spinlocks correctly protect the
133shared data structures _everywhere_ they are used. The spinlocks are most
134easily added to places that are completely independent of other code (ie
135internal driver data structures that nobody else ever touches, for
136example).
137
138----
139
140Lesson 2: reader-writer spinlocks.
141
142If your data accesses have a very natural pattern where you usually tend
143to mostly read from the shared variables, the reader-writer locks
144(rw_lock) versions of the spinlocks are often nicer. They allow multiple
145readers to be in the same critical region at once, but if somebody wants
146to change the variables it has to get an exclusive write lock. The
147routines look the same as above:
148
149   rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
150
151
152    unsigned long flags;
153
154    read_lock_irqsave(&xxx_lock, flags);
155    .. critical section that only reads the info ...
156    read_unlock_irqrestore(&xxx_lock, flags);
157
158    write_lock_irqsave(&xxx_lock, flags);
159    .. read and write exclusive access to the info ...
160    write_unlock_irqrestore(&xxx_lock, flags);
161
162The above kind of lock is useful for complex data structures like linked
163lists etc, especially when you know that most of the work is to just
164traverse the list searching for entries without changing the list itself,
165for example. Then you can use the read lock for that kind of list
166traversal, which allows many concurrent readers. Anything that _changes_
167the list will have to get the write lock.
168
169Note: you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
170time need to do any changes (even if you don't do it every time), you have
171to get the write-lock at the very beginning. I could fairly easily add a
172primitive to create a "upgradeable" read-lock, but it hasn't been an issue
173yet. Tell me if you'd want one.
174
175----
176
177Lesson 3: spinlocks revisited.
178
179The single spin-lock primitives above are by no means the only ones. They
180are the most safe ones, and the ones that work under all circumstances,
181but partly _because_ they are safe they are also fairly slow. They are
182much faster than a generic global cli/sti pair, but slower than they'd
183need to be, because they do have to disable interrupts (which is just a
184single instruction on a x86, but it's an expensive one - and on other
185architectures it can be worse).
186
187If you have a case where you have to protect a data structure across
188several CPU's and you want to use spinlocks you can potentially use
189cheaper versions of the spinlocks. IFF you know that the spinlocks are
190never used in interrupt handlers, you can use the non-irq versions:
191
192    spin_lock(&lock);
193    ...
194    spin_unlock(&lock);
195
196(and the equivalent read-write versions too, of course). The spinlock will
197guarantee the same kind of exclusive access, and it will be much faster.
198This is useful if you know that the data in question is only ever
199manipulated from a "process context", ie no interrupts involved.
200
201The reasons you mustn't use these versions if you have interrupts that
202play with the spinlock is that you can get deadlocks:
203
204    spin_lock(&lock);
205    ...
206        <- interrupt comes in:
207            spin_lock(&lock);
208
209where an interrupt tries to lock an already locked variable. This is ok if
210the other interrupt happens on another CPU, but it is _not_ ok if the
211interrupt happens on the same CPU that already holds the lock, because the
212lock will obviously never be released (because the interrupt is waiting
213for the lock, and the lock-holder is interrupted by the interrupt and will
214not continue until the interrupt has been processed).
215
216(This is also the reason why the irq-versions of the spinlocks only need
217to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
218on other CPU's, because an interrupt on another CPU doesn't interrupt the
219CPU that holds the lock, so the lock-holder can continue and eventually
220releases the lock).
221
222Note that you can be clever with read-write locks and interrupts. For
223example, if you know that the interrupt only ever gets a read-lock, then
224you can use a non-irq version of read locks everywhere - because they
225don't block on each other (and thus there is no dead-lock wrt interrupts.
226But when you do the write-lock, you have to use the irq-safe version.
227
228For an example of being clever with rw-locks, see the "waitqueue_lock"
229handling in kernel/sched.c - nothing ever _changes_ a wait-queue from
230within an interrupt, they only read the queue in order to know whom to
231wake up. So read-locks are safe (which is good: they are very common
232indeed), while write-locks need to protect themselves against interrupts.
233
234        Linus
235
236
237

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