Version:  2.0.40 2.2.26 2.4.37 3.13 3.14 3.15 3.16 3.17 3.18 3.19 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Linux/Documentation/preempt-locking.txt

  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, fpu__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.

This page was automatically generated by LXR 0.3.1 (source).  •  Linux is a registered trademark of Linus Torvalds  •  Contact us