powerpc, membarrier: Skip memory barrier in switch_mm()

Allow 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.

Threads targeting the same VM but which belong to different thread
groups is a tricky case. It has a few consequences:

It turns out that we cannot rely on get_nr_threads(p) to count the
number of threads using a VM. We can use
(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
instead to skip the synchronize_sched() for cases where the VM only has
a single user, and that user only has a single thread.

It also turns out that we cannot use for_each_thread() to set
thread flags in all threads using a VM, as it only iterates on the
thread group.

Therefore, test the membarrier state variable directly rather than
relying on thread flags. This means
membarrier_register_private_expedited() needs to set the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and
only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows
private expedited membarrier commands to succeed.
membarrier_arch_switch_mm() now tests for the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alan Stern <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Andrea Parri <[email protected]>
Cc: Andrew Hunter <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Dave Watson <[email protected]>
Cc: David Sehr <[email protected]>
Cc: Greg Hackmann <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Maged Michael <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Russell King <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index a9a2e2c86671..837adcf075d9 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1412,6 +1412,9 @@ config USERFAULTFD
 	  Enable the userfaultfd() system call that allows to intercept and
 	  handle page faults in userland.
 
+config ARCH_HAS_MEMBARRIER_CALLBACKS
+	bool
+
 config EMBEDDED
 	bool "Embedded system"
 	option allnoconfig_y