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SubjectRe: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command
----- On Oct 5, 2017, at 12:23 AM, Nicholas Piggin npiggin@gmail.com wrote:

> On Wed, 4 Oct 2017 14:37:53 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
>> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>
>> Provide a new command allowing processes to register their intent to use
>> the private expedited command.
>>
>> This allows PowerPC to skip the full memory barrier in switch_mm(), and
>> only issue the barrier when scheduling into a task belonging to a
>> process that has registered to use expedited private.
>>
>> Processes are now required to register before using
>> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>>
>> Changes since v1:
>> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>> powerpc membarrier_arch_sched_in(), given that we want to specifically
>> check the next thread state.
>> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
>> - Use task_thread_info() to pass thread_info from task to
>> *_ti_thread_flag().
>>
>> Changes since v2:
>> - Move membarrier_arch_sched_in() call to finish_task_switch().
>> - Check for NULL t->mm in membarrier_arch_fork().
>> - Use membarrier_sched_in() in generic code, which invokes the
>> arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>> build on PowerPC.
>> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>> allnoconfig build on PowerPC.
>> - Build and runtime tested on PowerPC.
>>
>> Changes since v3:
>> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>> field on fork.
>> - powerpc: test thread flag instead of reading
>> membarrier_private_expedited in membarrier_arch_fork().
>> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>> from kernel thread, since mmdrop() implies a full barrier.
>> - Set membarrier_private_expedited to 1 only after arch registration
>> code, thus eliminating a race where concurrent commands could succeed
>> when they should fail if issued concurrently with process
>> registration.
>> - Use READ_ONCE() for membarrier_private_expedited field access in
>> membarrier_private_expedited. Matches WRITE_ONCE() performed in
>> process registration.
>>
>> Changes since v4:
>> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>> from Nicholas Piggin.
>
> For now, the powerpc approach is okay by me. I plan to test
> others (e.g., taking runqueue locks) on larger systems, but that can
> be sent as an incremental patch at a later time.
>
> The main thing I would like is for people to review the userspace API.
>
>
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 3a19c253bdb1..4af1b719c65f 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -205,4 +205,54 @@ static inline void memalloc_noreclaim_restore(unsigned int
>> flags)
>> current->flags = (current->flags & ~PF_MEMALLOC) | flags;
>> }
>>
>> +#ifdef CONFIG_MEMBARRIER
>> +
>> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
>> +#include <asm/membarrier.h>
>> +#else
>> +static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
>> + struct mm_struct *next, struct task_struct *tsk)
>> +{
>> +}
>
> This is no longer required in architecture independent code, is it?

Yes, good point! I'll remove this unused code in a follow up patch.

Thanks,

Mathieu

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