There is an asynchronous aspect to smp_send_nmi_ipi. The caller waits
for all CPUs to call in to the handler, but it does not wait for
completion of the handler. This is a needless complication, so remove
it and always wait synchronously.
The synchronous wait allows the caller to easily time out and clear
the wait for completion (zero nmi_ipi_busy_count) in the case of badly
behaved handlers. This would have prevented the recent smp_send_stop
NMI IPI bug from causing the system to hang.
Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
Changes since v2:
- Fixed a locking bug in the timeout path
- Move all the shared memory accesses under the lock to avoid worrys
about memory ordering.
arch/powerpc/include/asm/smp.h | 1 -
arch/powerpc/kernel/smp.c | 52 ++++++++++++++++++----------------
arch/powerpc/kernel/watchdog.c | 1 -
3 files changed, 28 insertions(+), 26 deletions(-)
diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index cfecfee1194b..8b844435cbd5 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/arch/powerpc/include/asm/smp.h
@@ -56,7 +56,6 @@ struct smp_ops_t {
int (*cpu_bootable)(unsigned int nr);
};
-extern void smp_flush_nmi_ipi(u64 delay_us);
extern int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64
delay_us);
extern void smp_send_debugger_break(void);
extern void start_secondary_resume(void);
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 250fccf04c6e..8ad9288a72a0 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -412,7 +412,8 @@ int smp_handle_nmi_ipi(struct pt_regs *regs)
fn(regs);
nmi_ipi_lock();
- nmi_ipi_busy_count--;
+ if (nmi_ipi_busy_count > 1) /* Can race with caller time-out */
+ nmi_ipi_busy_count--;
out:
nmi_ipi_unlock_end(&flags);
@@ -437,29 +438,11 @@ static void do_smp_send_nmi_ipi(int cpu)
}
}
-void smp_flush_nmi_ipi(u64 delay_us)
-{
- unsigned long flags;
-
- nmi_ipi_lock_start(&flags);
- while (nmi_ipi_busy_count) {
- nmi_ipi_unlock_end(&flags);
- udelay(1);
- if (delay_us) {
- delay_us--;
- if (!delay_us)
- return;
- }
- nmi_ipi_lock_start(&flags);
- }
- nmi_ipi_unlock_end(&flags);
-}
-
/*
* - cpu is the target CPU (must not be this CPU), or NMI_IPI_ALL_OTHERS.
* - fn is the target callback function.
* - delay_us > 0 is the delay before giving up waiting for targets to
- * enter the handler, == 0 specifies indefinite delay.
+ * complete executing the handler, == 0 specifies indefinite delay.
*/
int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64 delay_us)
{
@@ -496,8 +479,23 @@ int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs
*), u64 delay_us)
do_smp_send_nmi_ipi(cpu);
+ nmi_ipi_lock();
+ /* nmi_ipi_busy_count is held here, so unlock/lock is okay */
while (!cpumask_empty(&nmi_ipi_pending_mask)) {
+ nmi_ipi_unlock();
udelay(1);
+ nmi_ipi_lock();
+ if (delay_us) {
+ delay_us--;
+ if (!delay_us)
+ break;
+ }
+ }
+
+ while (nmi_ipi_busy_count > 1) {
+ nmi_ipi_unlock();
+ udelay(1);
+ nmi_ipi_lock();
if (delay_us) {
delay_us--;
if (!delay_us)
@@ -505,12 +503,17 @@ int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs
*), u64 delay_us)
}
}
- nmi_ipi_lock();
if (!cpumask_empty(&nmi_ipi_pending_mask)) {
- /* Could not gather all CPUs */
+ /* Timeout waiting for CPUs to call smp_handle_nmi_ipi */
ret = 0;
cpumask_clear(&nmi_ipi_pending_mask);
}
+ if (nmi_ipi_busy_count > 1) {
+ /* Timeout waiting for CPUs to execute fn */
+ ret = 0;
+ nmi_ipi_busy_count = 1;
+ }
+
nmi_ipi_busy_count--;
nmi_ipi_unlock_end(&flags);
@@ -574,7 +577,8 @@ static void stop_this_cpu(struct pt_regs *regs)
* smp_send_nmi_ipi() call spin forever. Mark it done now.
*/
nmi_ipi_lock();
- nmi_ipi_busy_count--;
+ if (nmi_ipi_busy_count > 1)
+ nmi_ipi_busy_count--;
nmi_ipi_unlock();
#else
static void stop_this_cpu(void *dummy)
diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 6256dc3b0087..8cf048c6c1eb 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -165,7 +165,6 @@ static void watchdog_smp_panic(int cpu, u64 tb)
continue;
smp_send_nmi_ipi(c, wd_lockup_ipi, 1000000);
}
- smp_flush_nmi_ipi(1000000);
}
/* Take the stuck CPUs out of the watch group */
--
2.17.0
