FreeBSD/Linux Kernel Cross Reference
sys/Documentation/preempt-locking.txt

                       Proper Locking Under a Preemptible Kernel:
                            Keeping Kernel Code Preempt-Safe
                              Robert Love <rml@tech9.net>
                               Last Updated: 28 Aug 2002
     
     
     INTRODUCTION
     
     
    A preemptible kernel creates new locking issues.  The issues are the same as
    those under SMP: concurrency and reentrancy.  Thankfully, the Linux preemptible
    kernel model leverages existing SMP locking mechanisms.  Thus, the kernel
    requires explicit additional locking for very few additional situations.
    
    This document is for all kernel hackers.  Developing code in the kernel
    requires protecting these situations.
     
    
    RULE #1: Per-CPU data structures need explicit protection
    
    
    Two similar problems arise. An example code snippet:
    
            struct this_needs_locking tux[NR_CPUS];
            tux[smp_processor_id()] = some_value;
            /* task is preempted here... */
            something = tux[smp_processor_id()];
    
    First, since the data is per-CPU, it may not have explicit SMP locking, but
    require it otherwise.  Second, when a preempted task is finally rescheduled,
    the previous value of smp_processor_id may not equal the current.  You must
    protect these situations by disabling preemption around them.
    
    You can also use put_cpu() and get_cpu(), which will disable preemption.
    
    
    RULE #2: CPU state must be protected.
    
    
    Under preemption, the state of the CPU must be protected.  This is arch-
    dependent, but includes CPU structures and state not preserved over a context
    switch.  For example, on x86, entering and exiting FPU mode is now a critical
    section that must occur while preemption is disabled.  Think what would happen
    if the kernel is executing a floating-point instruction and is then preempted.
    Remember, the kernel does not save FPU state except for user tasks.  Therefore,
    upon preemption, the FPU registers will be sold to the lowest bidder.  Thus,
    preemption must be disabled around such regions.
    
    Note, some FPU functions are already explicitly preempt safe.  For example,
    kernel_fpu_begin and kernel_fpu_end will disable and enable preemption.
    However, math_state_restore must be called with preemption disabled.
    
    
    RULE #3: Lock acquire and release must be performed by same task
    
    
    A lock acquired in one task must be released by the same task.  This
    means you can't do oddball things like acquire a lock and go off to
    play while another task releases it.  If you want to do something
    like this, acquire and release the task in the same code path and
    have the caller wait on an event by the other task.
    
    
    SOLUTION
    
    
    Data protection under preemption is achieved by disabling preemption for the
    duration of the critical region.
    
    preempt_enable()                decrement the preempt counter
    preempt_disable()               increment the preempt counter
    preempt_enable_no_resched()     decrement, but do not immediately preempt
    preempt_check_resched()         if needed, reschedule
    preempt_count()                 return the preempt counter
    
    The functions are nestable.  In other words, you can call preempt_disable
    n-times in a code path, and preemption will not be reenabled until the n-th
    call to preempt_enable.  The preempt statements define to nothing if
    preemption is not enabled.
    
    Note that you do not need to explicitly prevent preemption if you are holding
    any locks or interrupts are disabled, since preemption is implicitly disabled
    in those cases.
    
    But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
    disabling preemption - any spin_unlock() decreasing the preemption count
    to 0 might trigger a reschedule. A simple printk() might trigger a reschedule.
    So use this implicit preemption-disabling property only if you know that the
    affected codepath does not do any of this. Best policy is to use this only for
    small, atomic code that you wrote and which calls no complex functions.
    
    Example:
    
            cpucache_t *cc; /* this is per-CPU */
            preempt_disable();
            cc = cc_data(searchp);
            if (cc && cc->avail) {
                    __free_block(searchp, cc_entry(cc), cc->avail);
                    cc->avail = 0;
           }
           preempt_enable();
           return 0;
   
   Notice how the preemption statements must encompass every reference of the
   critical variables.  Another example:
   
           int buf[NR_CPUS];
           set_cpu_val(buf);
           if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n");
           spin_lock(&buf_lock);
           /* ... */
   
   This code is not preempt-safe, but see how easily we can fix it by simply
   moving the spin_lock up two lines.
   
   
   PREVENTING PREEMPTION USING INTERRUPT DISABLING
   
   
   It is possible to prevent a preemption event using local_irq_disable and
   local_irq_save.  Note, when doing so, you must be very careful to not cause
   an event that would set need_resched and result in a preemption check.  When
   in doubt, rely on locking or explicit preemption disabling.
   
   Note in 2.5 interrupt disabling is now only per-CPU (e.g. local).
   
   An additional concern is proper usage of local_irq_disable and local_irq_save.
   These may be used to protect from preemption, however, on exit, if preemption
   may be enabled, a test to see if preemption is required should be done.  If
   these are called from the spin_lock and read/write lock macros, the right thing
   is done.  They may also be called within a spin-lock protected region, however,
   if they are ever called outside of this context, a test for preemption should
   be made. Do note that calls from interrupt context or bottom half/ tasklets
   are also protected by preemption locks and so may use the versions which do
   not check preemption.

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