powerpc: NMI IPI make NMI IPIs fully sychronous

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.

---
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(-)

index cfecfee1194b..8b844435cbd5 100644
@@ -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);

index 6256dc3b0087..8cf048c6c1eb 100644
@@ -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

Applied to powerpc next, thanks.

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

cheers