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

[Avi Kivity]

On 10/05/2017 07:23 AM, Nicholas Piggin wrote:
> On Wed,  4 Oct 2017 14:37:53 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
> > From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >
> > 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.

As a future satisfied user of the expedited private membarrier syscall, 
I am happy with the change.