Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with
or
.
Download ZIP
Newer
Older
100644 136 lines (95 sloc) 5.346 kb
1da177e Linux-2.6.12-rc2
Linus Torvalds authored
1 Proper Locking Under a Preemptible Kernel:
2 Keeping Kernel Code Preempt-Safe
3 Robert Love <rml@tech9.net>
4 Last Updated: 28 Aug 2002
5
6
7 INTRODUCTION
8
9
10 A preemptible kernel creates new locking issues. The issues are the same as
11 those under SMP: concurrency and reentrancy. Thankfully, the Linux preemptible
12 kernel model leverages existing SMP locking mechanisms. Thus, the kernel
13 requires explicit additional locking for very few additional situations.
14
15 This document is for all kernel hackers. Developing code in the kernel
16 requires protecting these situations.
17
18
19 RULE #1: Per-CPU data structures need explicit protection
20
21
22 Two similar problems arise. An example code snippet:
23
24 struct this_needs_locking tux[NR_CPUS];
25 tux[smp_processor_id()] = some_value;
26 /* task is preempted here... */
27 something = tux[smp_processor_id()];
28
29 First, since the data is per-CPU, it may not have explicit SMP locking, but
30 require it otherwise. Second, when a preempted task is finally rescheduled,
31 the previous value of smp_processor_id may not equal the current. You must
32 protect these situations by disabling preemption around them.
33
34 You can also use put_cpu() and get_cpu(), which will disable preemption.
35
36
37 RULE #2: CPU state must be protected.
38
39
40 Under preemption, the state of the CPU must be protected. This is arch-
41 dependent, but includes CPU structures and state not preserved over a context
42 switch. For example, on x86, entering and exiting FPU mode is now a critical
43 section that must occur while preemption is disabled. Think what would happen
44 if the kernel is executing a floating-point instruction and is then preempted.
45 Remember, the kernel does not save FPU state except for user tasks. Therefore,
46 upon preemption, the FPU registers will be sold to the lowest bidder. Thus,
47 preemption must be disabled around such regions.
48
49 Note, some FPU functions are already explicitly preempt safe. For example,
50 kernel_fpu_begin and kernel_fpu_end will disable and enable preemption.
51 However, math_state_restore must be called with preemption disabled.
52
53
54 RULE #3: Lock acquire and release must be performed by same task
55
56
57 A lock acquired in one task must be released by the same task. This
58 means you can't do oddball things like acquire a lock and go off to
59 play while another task releases it. If you want to do something
60 like this, acquire and release the task in the same code path and
61 have the caller wait on an event by the other task.
62
63
64 SOLUTION
65
66
67 Data protection under preemption is achieved by disabling preemption for the
68 duration of the critical region.
69
70 preempt_enable() decrement the preempt counter
71 preempt_disable() increment the preempt counter
72 preempt_enable_no_resched() decrement, but do not immediately preempt
73 preempt_check_resched() if needed, reschedule
74 preempt_count() return the preempt counter
75
76 The functions are nestable. In other words, you can call preempt_disable
77 n-times in a code path, and preemption will not be reenabled until the n-th
78 call to preempt_enable. The preempt statements define to nothing if
79 preemption is not enabled.
80
81 Note that you do not need to explicitly prevent preemption if you are holding
82 any locks or interrupts are disabled, since preemption is implicitly disabled
83 in those cases.
84
85 But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
86 disabling preemption - any spin_unlock() decreasing the preemption count
87 to 0 might trigger a reschedule. A simple printk() might trigger a reschedule.
88 So use this implicit preemption-disabling property only if you know that the
89 affected codepath does not do any of this. Best policy is to use this only for
90 small, atomic code that you wrote and which calls no complex functions.
91
92 Example:
93
94 cpucache_t *cc; /* this is per-CPU */
95 preempt_disable();
96 cc = cc_data(searchp);
97 if (cc && cc->avail) {
98 __free_block(searchp, cc_entry(cc), cc->avail);
99 cc->avail = 0;
100 }
101 preempt_enable();
102 return 0;
103
104 Notice how the preemption statements must encompass every reference of the
105 critical variables. Another example:
106
107 int buf[NR_CPUS];
108 set_cpu_val(buf);
109 if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n");
110 spin_lock(&buf_lock);
111 /* ... */
112
113 This code is not preempt-safe, but see how easily we can fix it by simply
114 moving the spin_lock up two lines.
115
116
117 PREVENTING PREEMPTION USING INTERRUPT DISABLING
118
119
120 It is possible to prevent a preemption event using local_irq_disable and
121 local_irq_save. Note, when doing so, you must be very careful to not cause
122 an event that would set need_resched and result in a preemption check. When
123 in doubt, rely on locking or explicit preemption disabling.
124
125 Note in 2.5 interrupt disabling is now only per-CPU (e.g. local).
126
127 An additional concern is proper usage of local_irq_disable and local_irq_save.
128 These may be used to protect from preemption, however, on exit, if preemption
129 may be enabled, a test to see if preemption is required should be done. If
130 these are called from the spin_lock and read/write lock macros, the right thing
131 is done. They may also be called within a spin-lock protected region, however,
132 if they are ever called outside of this context, a test for preemption should
133 be made. Do note that calls from interrupt context or bottom half/ tasklets
134 are also protected by preemption locks and so may use the versions which do
135 not check preemption.
Something went wrong with that request. Please try again.