[PATCH 1/6] powerpc: NMI IPI improve lock primitive

[Nicholas Piggin]

When the NMI IPI lock is contended, spin at low SMT priority, using
loads only, and with interrupts enabled (where possible). This
improves behaviour under high contention (e.g., a system crash when
a number of CPUs are trying to enter the debugger).

Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
 arch/powerpc/kernel/smp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index cf0e1245b8cc..8d3320562c70 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -351,7 +351,7 @@ static void nmi_ipi_lock_start(unsigned long *flags)
        hard_irq_disable();
        while (atomic_cmpxchg(&__nmi_ipi_lock, 0, 1) == 1) {
                raw_local_irq_restore(*flags);
-               cpu_relax();
+               spin_until_cond(atomic_read(&__nmi_ipi_lock) == 0);
                raw_local_irq_save(*flags);
                hard_irq_disable();
        }
@@ -360,7 +360,7 @@ static void nmi_ipi_lock_start(unsigned long *flags)
 static void nmi_ipi_lock(void)
 {
        while (atomic_cmpxchg(&__nmi_ipi_lock, 0, 1) == 1)
-               cpu_relax();
+               spin_until_cond(atomic_read(&__nmi_ipi_lock) == 0);
 }
 
 static void nmi_ipi_unlock(void)
@@ -475,7 +475,7 @@ int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), 
u64 delay_us)
        nmi_ipi_lock_start(&flags);
        while (nmi_ipi_busy_count) {
                nmi_ipi_unlock_end(&flags);
-               cpu_relax();
+               spin_until_cond(nmi_ipi_busy_count == 0);
                nmi_ipi_lock_start(&flags);
        }
 
-- 
2.13.3