locking/lockdep: Add a comment about crossrelease_hist_end() in lockdep_sys_exit()

In lockdep_sys_exit(), crossrelease_hist_end() is called unconditionally
even when getting here without having started e.g. just after forking.

But it's no problem since it would roll back to an invalid entry anyway.
Add a comment to explain this.

Signed-off-by: Byungchul Park <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
[ Improved the description and the comments. ]
Signed-off-by: Ingo Molnar <[email protected]>

Author: lgebyungchulpark

kernel/locking/lockdep.c

@@ -4623,6 +4623,10 @@ asmlinkage __visible void lockdep_sys_exit(void)
 	/*
 	 * The lock history for each syscall should be independent. So wipe the
 	 * slate clean on return to userspace.
+	 *
+	 * crossrelease_hist_end() works well here even when getting here
+	 * without starting (i.e. just after forking), because it rolls back
+	 * the index to point to the last entry, which is already invalid.
 	 */
 	crossrelease_hist_end(XHLOCK_PROC);
 	crossrelease_hist_start(XHLOCK_PROC);