membarrier: Provide GLOBAL_EXPEDITED command

Allow expedited membarrier to be used for data shared between processes
through shared memory.

Processes wishing to receive the membarriers register with
MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. Those which want to issue
membarrier invoke MEMBARRIER_CMD_GLOBAL_EXPEDITED.

This allows extremely simple kernel-level implementation: we have almost
everything we need with the PRIVATE_EXPEDITED barrier code. All we need
to do is to add a flag in the mm_struct that will be used to check
whether we need to send the IPI to the current thread of each CPU.

There is a slight downside to this approach compared to targeting
specific shared memory users: when performing a membarrier operation,
all registered "global" receivers will get the barrier, even if they
don't share a memory mapping with the sender issuing
MEMBARRIER_CMD_GLOBAL_EXPEDITED.

This registration approach seems to fit the requirement of not
disturbing processes that really deeply care about real-time: they
simply should not register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED.

In order to align the membarrier command names, the "MEMBARRIER_CMD_SHARED"
command is renamed to "MEMBARRIER_CMD_GLOBAL", keeping an alias of
MEMBARRIER_CMD_SHARED to MEMBARRIER_CMD_GLOBAL for UAPI header backward
compatibility.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[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: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Maged Michael <[email protected]>
Cc: Michael Ellerman <[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]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
index 98ff4f1fcf2b..6e20bb5c74ea 100644
--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -13,7 +13,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 	 * store to rq->curr.
 	 */
 	if (likely(!(atomic_read(&next->membarrier_state) &
-		     MEMBARRIER_STATE_PRIVATE_EXPEDITED) || !prev))
+		     (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
+		      MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
 		return;
 
 	/*