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SMP: Clean-up and simplication of logic that I implemented late last night.

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This commit is contained in:
Gregory Nutt 2016-11-26 07:05:27 -06:00
parent 465e297b5b
commit 1d06e786e1
1 changed files with 37 additions and 77 deletions
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114
sched/irq/irq_csection.c
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@ -88,7 +88,8 @@ static uint8_t g_cpu_nestcount[CONFIG_SMP_NCPUS];
* Description:
* Spin to get g_irq_waitlock, handling a known deadlock condition:
*
* Suppose this situation:
* A deadlock may occur if enter_critical_section is called from an
* interrupt handler. Suppose:
*
* - CPUn is in a critical section and has the g_cpu_irqlock spinlock.
* - CPUm takes an interrupt and attempts to enter the critical section.
@ -98,83 +99,34 @@ static uint8_t g_cpu_nestcount[CONFIG_SMP_NCPUS];
* - But interrupts are disabled on CPUm so the up_cpu_pause() is never
* handled, causing the deadlock.
*
* This function detects this deadlock condition while spinning in an
* interrupt and calls up_cpu_pause() handler, breaking the deadlock.
* This same deadlock can occur in the normal tasking case:
*
****************************************************************************/
#ifdef CONFIG_SMP
static inline void irq_waitlock(int cpu)
{
/* Duplicate the spin_lock() logic from spinlock.c, but adding the check
* for the deadlock condition.
*/
while (spin_trylock(&g_cpu_irqlock) == SP_LOCKED)
{
/* A deadlock condition would occur if CPUn:
*
* 1. Holds the g_cpu_irqlock, and
* 2. Is trying to interrupt CPUm, but
* 3. CPUm is spinning trying acquire the g_cpu_irqlock.
*
* The protocol for CPUn to pause CPUm is as follows
*
* 1. The up_cpu_pause() implementation on CPUn locks both
* g_cpu_wait[m] and g_cpu_paused[m]. CPUn then waits
* spinning on g_cpu_wait[m].
* 2. When CPUm receives the interrupt it (1) unlocks
* g_cpu_paused[m] and (2) locks g_cpu_wait[m]. The
* first unblocks CPUn and the second blocks CPUm in the
* interrupt handler.
*
* The problem in the deadlock case is that CPUm cannot receive
* the interrupt because it is locked up spinning. Here we break
* the deadlock here be handling the pause interrupt request
* while waiting.
*/
if (up_cpu_pausereq(cpu))
{
/* Yes.. some other CPU is requesting to pause this CPU! Handle
* the pause interrupt now.
*/
DEBUGVERIFY(up_cpu_paused(cpu));
}
SP_DSB();
}
/* We have g_cpu_irqlock! */
SP_DMB();
}
#endif
/****************************************************************************
* Name: task_waitlock
*
* Description:
* Spin to get g_irq_waitlock, handling a known deadlock condition:
*
* Suppose this situation:
*
* - CPUn enters a critical section and has the g_cpu_irqlock spinlock.
* - CPUn causes a task to become ready to run and scheduler selects
* CPUm. CPUm is requested to pause.
* - But CPUm is in a normal task and is also attempting to enter a
* critical section. So it is also spinning to get g_cpu_irqlock
* with interrupts disabled and cannot respond to the pause request,
* causing the deadlock.
* - A task on CPUn enters a critical section and has the g_cpu_irqlock
* spinlock.
* - Another task on CPUm attempts to enter the critical section but has
* to wait, spinning to get g_cpu_irqlock with interrupts disabled.
* - The task on CPUn causes a new task to become ready-torun and the
* scheduler selects CPUm. CPUm is requested to pause via a pause
* interrupt.
* - But the task on CPUm is also attempting to enter the critical
* section. Since it is spinning with interrupts disabled, CPUm cannot
* process the pending pause interrupt, causing the deadlock.
*
* This function detects this deadlock condition while spinning with \
* interrupts disabled.
*
* Input Parameters:
* cpu - The index of CPU that is trying to enter the critical section.
*
* Returned Value:
* True: The g_cpu_irqlock spinlock has been taken.
* False: The g_cpu_irqlock spinlock has not been taken yet, but there is
* a pending pause interrupt request.
*
****************************************************************************/
#ifdef CONFIG_SMP
static inline bool task_waitlock(int cpu)
static inline bool irq_waitlock(int cpu)
{
/* Duplicate the spin_lock() logic from spinlock.c, but adding the check
* for the deadlock condition.
@ -317,7 +269,15 @@ try_again:
* no longer blocked by the critical section).
*/
irq_waitlock(cpu);
if (!irq_waitlock(cpu))
{
/* We are in a deadlock condition due to a pending
* pause request interrupt request. Break the
* deadlock by handling the pause interrupt now.
*/
DEBUGVERIFY(up_cpu_paused(cpu));
}
}
/* In any event, the nesting count is now one */
@ -359,13 +319,13 @@ try_again:
cpu = this_cpu();
DEBUGASSERT((g_cpu_irqset & (1 << cpu)) == 0);
if (!task_waitlock(cpu))
if (!irq_waitlock(cpu))
{
/* We are in a potential deadlock condition due to a
* pending pause request interrupt. Re-enable interrupts
* on this CPU and try again. Briefly re-enabling
* interrupts should be sufficient to permit processing
* the pending pause request.
/* We are in a deadlock condition due to a pending pause
* request interrupt. Re-enable interrupts on this CPU
* and try again. Briefly re-enabling interrupts should
* be sufficient to permit processing the pending pause
* request.
*/
up_irq_restore(ret);