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    SubjectRe: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command
    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?

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