Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Nov 3, 2016, at 1:49 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Thu, Nov 03, 2016 at 10:29:28AM -0600, Mathieu Desnoyers wrote:
>> Userspace applications should be allowed to expect the membarrier system
>> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
>> nohz_full CPUs, but synchronize_sched() does not take those into
>> account.
>> 
>> Given that we do not want unrelated processes to be able to affect
>> real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier
>> is invoked on a kernel with enabled nohz_full CPUs.
>> 
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Hi Paul,

Do you plan to pick it up through your tree, or I should sent
it directly to Linus ?

Thanks,

Mathieu

> 
>> CC: Josh Triplett <josh@joshtriplett.org>
>> CC: Steven Rostedt <rostedt@goodmis.org>
>> CC: Lai Jiangshan <jiangshanlai@gmail.com>
>> CC: <stable@vger.kernel.org>	[3.10+]
>> ---
>>  kernel/membarrier.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
>> index 536c727..9f9284f 100644
>> --- a/kernel/membarrier.c
>> +++ b/kernel/membarrier.c
>> @@ -16,6 +16,7 @@
>> 
>>  #include <linux/syscalls.h>
>>  #include <linux/membarrier.h>
>> +#include <linux/tick.h>
>> 
>>  /*
>>   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
>> @@ -51,6 +52,9 @@
>>   */
>>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
>>  {
>> +	/* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
>> +	if (tick_nohz_full_enabled())
>> +		return -ENOSYS;
>>  	if (unlikely(flags))
>>  		return -EINVAL;
>>  	switch (cmd) {
>> --
>> 2.1.4

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com