membarrier/x86: Provide core serializing command

There are two places where core serialization is needed by membarrier:

1) When returning from the membarrier IPI,
2) After scheduler updates curr to a thread with a different mm, before
   going back to user-space, since the curr->mm is used by membarrier to
   check whether it needs to send an IPI to that CPU.

x86-32 uses IRET as return from interrupt, and both IRET and SYSEXIT to go
back to user-space. The IRET instruction is core serializing, but not
SYSEXIT.

x86-64 uses IRET as return from interrupt, which takes care of the IPI.
However, it can return to user-space through either SYSRETL (compat
code), SYSRETQ, or IRET. Given that SYSRET{L,Q} is not core serializing,
we rely instead on write_cr3() performed by switch_mm() to provide core
serialization after changing the current mm, and deal with the special
case of kthread -> uthread (temporarily keeping current mm into
active_mm) by adding a sync_core() in that specific case.

Use the new sync_core_before_usermode() to guarantee this.

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]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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