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