linux-kernel - Re: [PATCH AUTOSEL for 3.18 36/40] powerpc/xmon: Avoid tripping SMP hardlockup watchdog

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Date:   Wed, 13 Dec 2017 13:44:55 +0000
From:   alexander.levin@...izon.com
To:     Michael Ellerman <mpe@...erman.id.au>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH AUTOSEL for 3.18 36/40] powerpc/xmon: Avoid tripping SMP
 hardlockup watchdog

On Thu, Dec 14, 2017 at 12:10:39AM +1100, Michael Ellerman wrote:
>alexander.levin@...izon.com writes:
>
>> From: Nicholas Piggin <npiggin@...il.com>
>>
>> [ Upstream commit 064996d62a33ffe10264b5af5dca92d54f60f806 ]
>>
>> The SMP hardlockup watchdog cross-checks other CPUs for lockups, which
>> causes xmon headaches because it's assuming interrupts hard disabled
>> means no watchdog troubles. Try to improve that by calling
>> touch_nmi_watchdog() in obvious places where secondaries are spinning.
>>
>> Also annotate these spin loops with spin_begin/end calls.
>
>These macros didn't exist until 4.13, and haven't been backported AFAIK.

But the touch_nmi_watchdog() bits are something we want in stable, right?

>> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
>> index e23f559faa47..6cec1081cb67 100644
>> --- a/arch/powerpc/xmon/xmon.c
>> +++ b/arch/powerpc/xmon/xmon.c
>> @@ -436,14 +436,19 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>>
>>   waiting:
>>  	secondary = 1;
>> +	spin_begin();
>>  	while (secondary && !xmon_gate) {
>>  		if (in_xmon == 0) {
>> -			if (fromipi)
>> +			if (fromipi) {
>> +				spin_end();
>>  				goto leave;
>> +			}
>>  			secondary = test_and_set_bit(0, &in_xmon);
>>  		}
>> -		barrier();
>> +		spin_cpu_relax();
>> +		touch_nmi_watchdog();
>>  	}
>> +	spin_end();
>>
>>  	if (!secondary && !xmon_gate) {
>>  		/* we are the first cpu to come in */
>> @@ -470,21 +475,25 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>>  		mb();
>>  		xmon_gate = 1;
>>  		barrier();
>> +		touch_nmi_watchdog();
>>  	}
>>
>>   cmdloop:
>>  	while (in_xmon) {
>>  		if (secondary) {
>> +			spin_begin();
>>  			if (cpu == xmon_owner) {
>>  				if (!test_and_set_bit(0, &xmon_taken)) {
>>  					secondary = 0;
>> +					spin_end();
>>  					continue;
>>  				}
>>  				/* missed it */
>>  				while (cpu == xmon_owner)
>> -					barrier();
>> +					spin_cpu_relax();
>>  			}
>> -			barrier();
>> +			spin_cpu_relax();
>> +			touch_nmi_watchdog();
>>  		} else {
>>  			cmd = cmds(regs);
>>  			if (cmd != 0) {
>> --
>> 2.11.0

-- 

Thanks,
Sasha