membarrier: Provide register expedited private command

This introduces a "register private expedited" membarrier command which
allows eventual removal of important memory barrier constraints on the
scheduler fast-paths. It changes how the "private expedited" membarrier
command (new to 4.14) is used from user-space.

This new command allows processes to register their intent to use the
private expedited command.  This affects how the expedited private
command introduced in 4.14-rc is meant to be used, and should be merged
before 4.14 final.

Processes are now required to register before using
MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.

This fixes a problem that arose when designing requested extensions to
sys_membarrier() to allow JITs to efficiently flush old code from
instruction caches.  Several potential algorithms are much less painful
if the user register intent to use this functionality early on, for
example, before the process spawns the second thread.  Registering at
this time removes the need to interrupt each and every thread in that
process at the first expedited sys_membarrier() system call.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 46f4ecf5479a..1861ea8dba77 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -445,6 +445,9 @@ struct mm_struct {
 	unsigned long flags; /* Must use atomic bitops to access the bits */
 
 	struct core_state *core_state; /* coredumping support */
+#ifdef CONFIG_MEMBARRIER
+	atomic_t membarrier_state;
+#endif
 #ifdef CONFIG_AIO
 	spinlock_t			ioctx_lock;
 	struct kioctx_table __rcu	*ioctx_table;