SMP: Clean-up and simplication of logic that I implemented late last night.
This commit is contained in:
Gregory Nutt
parent
465e297b5b
commit
1d06e786e1
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@ -88,7 +88,8 @@ static uint8_t g_cpu_nestcount[CONFIG_SMP_NCPUS];
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* Description:
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* Spin to get g_irq_waitlock, handling a known deadlock condition:
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*
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* Suppose this situation:
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* A deadlock may occur if enter_critical_section is called from an
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* interrupt handler. Suppose:
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*
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* - CPUn is in a critical section and has the g_cpu_irqlock spinlock.
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* - CPUm takes an interrupt and attempts to enter the critical section.
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@ -98,83 +99,34 @@ static uint8_t g_cpu_nestcount[CONFIG_SMP_NCPUS];
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* - But interrupts are disabled on CPUm so the up_cpu_pause() is never
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* handled, causing the deadlock.
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*
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* This function detects this deadlock condition while spinning in an
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* interrupt and calls up_cpu_pause() handler, breaking the deadlock.
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* This same deadlock can occur in the normal tasking case:
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*
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****************************************************************************/
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#ifdef CONFIG_SMP
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static inline void irq_waitlock(int cpu)
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{
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/* Duplicate the spin_lock() logic from spinlock.c, but adding the check
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* for the deadlock condition.
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*/
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while (spin_trylock(&g_cpu_irqlock) == SP_LOCKED)
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{
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/* A deadlock condition would occur if CPUn:
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*
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* 1. Holds the g_cpu_irqlock, and
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* 2. Is trying to interrupt CPUm, but
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* 3. CPUm is spinning trying acquire the g_cpu_irqlock.
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*
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* The protocol for CPUn to pause CPUm is as follows
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*
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* 1. The up_cpu_pause() implementation on CPUn locks both
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* g_cpu_wait[m] and g_cpu_paused[m]. CPUn then waits
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* spinning on g_cpu_wait[m].
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* 2. When CPUm receives the interrupt it (1) unlocks
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* g_cpu_paused[m] and (2) locks g_cpu_wait[m]. The
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* first unblocks CPUn and the second blocks CPUm in the
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* interrupt handler.
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*
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* The problem in the deadlock case is that CPUm cannot receive
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* the interrupt because it is locked up spinning. Here we break
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* the deadlock here be handling the pause interrupt request
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* while waiting.
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*/
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if (up_cpu_pausereq(cpu))
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{
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/* Yes.. some other CPU is requesting to pause this CPU! Handle
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* the pause interrupt now.
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*/
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DEBUGVERIFY(up_cpu_paused(cpu));
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}
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SP_DSB();
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}
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/* We have g_cpu_irqlock! */
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SP_DMB();
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}
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#endif
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/****************************************************************************
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* Name: task_waitlock
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*
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* Description:
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* Spin to get g_irq_waitlock, handling a known deadlock condition:
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*
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* Suppose this situation:
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*
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* - CPUn enters a critical section and has the g_cpu_irqlock spinlock.
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* - CPUn causes a task to become ready to run and scheduler selects
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* CPUm. CPUm is requested to pause.
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* - But CPUm is in a normal task and is also attempting to enter a
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* critical section. So it is also spinning to get g_cpu_irqlock
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* with interrupts disabled and cannot respond to the pause request,
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* causing the deadlock.
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* - A task on CPUn enters a critical section and has the g_cpu_irqlock
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* spinlock.
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* - Another task on CPUm attempts to enter the critical section but has
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* to wait, spinning to get g_cpu_irqlock with interrupts disabled.
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* - The task on CPUn causes a new task to become ready-torun and the
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* scheduler selects CPUm. CPUm is requested to pause via a pause
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* interrupt.
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* - But the task on CPUm is also attempting to enter the critical
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* section. Since it is spinning with interrupts disabled, CPUm cannot
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* process the pending pause interrupt, causing the deadlock.
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*
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* This function detects this deadlock condition while spinning with \
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* interrupts disabled.
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*
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* Input Parameters:
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* cpu - The index of CPU that is trying to enter the critical section.
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*
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* Returned Value:
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* True: The g_cpu_irqlock spinlock has been taken.
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* False: The g_cpu_irqlock spinlock has not been taken yet, but there is
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* a pending pause interrupt request.
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*
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****************************************************************************/
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#ifdef CONFIG_SMP
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static inline bool task_waitlock(int cpu)
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static inline bool irq_waitlock(int cpu)
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{
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/* Duplicate the spin_lock() logic from spinlock.c, but adding the check
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* for the deadlock condition.
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@ -317,7 +269,15 @@ try_again:
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* no longer blocked by the critical section).
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*/
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irq_waitlock(cpu);
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if (!irq_waitlock(cpu))
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{
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/* We are in a deadlock condition due to a pending
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* pause request interrupt request. Break the
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* deadlock by handling the pause interrupt now.
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*/
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DEBUGVERIFY(up_cpu_paused(cpu));
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}
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}
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/* In any event, the nesting count is now one */
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@ -359,13 +319,13 @@ try_again:
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cpu = this_cpu();
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DEBUGASSERT((g_cpu_irqset & (1 << cpu)) == 0);
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if (!task_waitlock(cpu))
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if (!irq_waitlock(cpu))
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{
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/* We are in a potential deadlock condition due to a
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* pending pause request interrupt. Re-enable interrupts
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* on this CPU and try again. Briefly re-enabling
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* interrupts should be sufficient to permit processing
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* the pending pause request.
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/* We are in a deadlock condition due to a pending pause
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* request interrupt. Re-enable interrupts on this CPU
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* and try again. Briefly re-enabling interrupts should
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* be sufficient to permit processing the pending pause
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* request.
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*/
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up_irq_restore(ret);
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