Re: [v2] powerpc: NMI IPI make NMI IPIs fully sychronous

[Michael Ellerman]

On Wed, 2018-04-25 at 05:17:59 UTC, Nicholas Piggin wrote:
> 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 

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/5b73151fff63fb019db8171cb81c6c

cheers

[PATCH v2] powerpc: NMI IPI make NMI IPIs fully sychronous

[Nicholas Piggin]

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 
---
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();
 
@@ -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();
-   while (nmi_ipi_busy_count) {
-   nmi_ipi_unlock_end();
-   udelay(1);
-   if (delay_us) {
-   delay_us--;
-   if (!delay_us)
-   return;
-   }
-   nmi_ipi_lock_start();
-   }
-   nmi_ipi_unlock_end();
-}
-
 /*
  * - 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(_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(_ipi_pending_mask)) {
-   /* Could not gather all CPUs */
+   /* Timeout waiting for CPUs to call smp_handle_nmi_ipi */
ret = 0;
cpumask_clear(_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();
 
@@ -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, 100);
}
-