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