sched/vtime: Fix guest/system mis-accounting on task switch

vtime_account_system() assumes that the target task to account cputime
to is always the current task. This is most often true indeed except on
task switch where we call:

	vtime_common_task_switch(prev)
		vtime_account_system(prev)

Here prev is the scheduling-out task where we account the cputime to. It
doesn't match current that is already the scheduling-in task at this
stage of the context switch.

So we end up checking the wrong task flags to determine if we are
accounting guest or system time to the previous task.

As a result the wrong task is used to check if the target is running in
guest mode. We may then spuriously account or leak either system or
guest time on task switch.

Fix this assumption and also turn vtime_guest_enter/exit() to use the
task passed in parameter as well to avoid future similar issues.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Wanpeng Li <[email protected]>
Fixes: 2a42eb9 ("sched/cputime: Accumulate vtime on top of nsec clocksource")
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Frederic Weisbecker

kernel/sched/cputime.c

@@ -740,7 +740,7 @@ void vtime_account_system(struct task_struct *tsk)
 
 	write_seqcount_begin(&vtime->seqcount);
 	/* We might have scheduled out from guest path */
-	if (current->flags & PF_VCPU)
+	if (tsk->flags & PF_VCPU)
 		vtime_account_guest(tsk, vtime);
 	else
 		__vtime_account_system(tsk, vtime);
@@ -783,7 +783,7 @@ void vtime_guest_enter(struct task_struct *tsk)
 	 */
 	write_seqcount_begin(&vtime->seqcount);
 	__vtime_account_system(tsk, vtime);
-	current->flags |= PF_VCPU;
+	tsk->flags |= PF_VCPU;
 	write_seqcount_end(&vtime->seqcount);
 }
 EXPORT_SYMBOL_GPL(vtime_guest_enter);
@@ -794,7 +794,7 @@ void vtime_guest_exit(struct task_struct *tsk)
 
 	write_seqcount_begin(&vtime->seqcount);
 	vtime_account_guest(tsk, vtime);
-	current->flags &= ~PF_VCPU;
+	tsk->flags &= ~PF_VCPU;
 	write_seqcount_end(&vtime->seqcount);
 }
 EXPORT_SYMBOL_GPL(vtime_guest_exit);