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kernel/smp: Fix races in SMP initialization

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This has bitrotten a bit.  Early implementations had a synchronous
arch_start_cpu(), but then we started allowing that to be an async
operation.  But that means that CPU start now becomes surprisingly
reentrant to the arch layer (cpu 0 can get a call to start cpu 2 while
cpu 1's initialization code is still running).  That's just error
prone; we never documented the requirements cleanly (the window is
very small, but not so small to a slow simulator!).

Add an extra flag so we don't issue the next start until the last is
out of the arch layer and running in smp_init_top().

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
This commit is contained in:
Andy Ross 2022-01-07 17:08:20 -08:00 committed by Carles Cufí
parent 97ada8bc04
commit 3457118540
1 changed files with 21 additions and 3 deletions
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24
kernel/smp.c
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@ -13,6 +13,10 @@
static atomic_t global_lock; static atomic_t global_lock;
static atomic_t start_flag; static atomic_t start_flag;
#if CONFIG_MP_NUM_CPUS > 1
static atomic_t ready_flag;
#endif
unsigned int z_smp_global_lock(void) unsigned int z_smp_global_lock(void)
{ {
unsigned int key = arch_irq_lock(); unsigned int key = arch_irq_lock();
@ -48,6 +52,15 @@ void z_smp_release_global_lock(struct k_thread *thread)
} }
} }
/* Tiny delay that relaxes bus traffic to avoid spamming a shared
* memory bus looking at an atomic variable
*/
static inline void local_delay(void)
{
for (volatile int i = 0; i < 1000; i++) {
}
}
#if CONFIG_MP_NUM_CPUS > 1 #if CONFIG_MP_NUM_CPUS > 1
void z_smp_thread_init(void *arg, struct k_thread *thread) void z_smp_thread_init(void *arg, struct k_thread *thread)
@ -56,6 +69,7 @@ void z_smp_thread_init(void *arg, struct k_thread *thread)
/* Wait for the signal to begin scheduling */ /* Wait for the signal to begin scheduling */
while (!atomic_get(cpu_start_flag)) { while (!atomic_get(cpu_start_flag)) {
local_delay();
} }
z_dummy_thread_init(thread); z_dummy_thread_init(thread);
@ -70,9 +84,9 @@ static inline FUNC_NORETURN void smp_init_top(void *arg)
{ {
struct k_thread dummy_thread; struct k_thread dummy_thread;
z_smp_thread_init(arg, &dummy_thread);
smp_timer_init(); smp_timer_init();
(void)atomic_set(&ready_flag, 1);
z_smp_thread_init(arg, &dummy_thread);
z_swap_unlocked(); z_swap_unlocked();
CODE_UNREACHABLE; /* LCOV_EXCL_LINE */ CODE_UNREACHABLE; /* LCOV_EXCL_LINE */
@ -92,10 +106,14 @@ void z_smp_init(void)
{ {
(void)atomic_clear(&start_flag); (void)atomic_clear(&start_flag);
#if CONFIG_MP_NUM_CPUS > 1 && !defined(CONFIG_SMP_BOOT_DELAY) #if !defined(CONFIG_SMP_BOOT_DELAY) && (CONFIG_MP_NUM_CPUS > 1)
for (int i = 1; i < CONFIG_MP_NUM_CPUS; i++) { for (int i = 1; i < CONFIG_MP_NUM_CPUS; i++) {
(void)atomic_clear(&ready_flag);
arch_start_cpu(i, z_interrupt_stacks[i], CONFIG_ISR_STACK_SIZE, arch_start_cpu(i, z_interrupt_stacks[i], CONFIG_ISR_STACK_SIZE,
smp_init_top, &start_flag); smp_init_top, &start_flag);
while (!atomic_get(&ready_flag)) {
local_delay();
}
} }
#endif #endif