kcov: simplify interrupt check

in_interrupt() semantics are confusing and wrong for most users as it
also returns true when bh is disabled.  Thus we open coded a proper
check for interrupts in __sanitizer_cov_trace_pc() with a lengthy
explanatory comment.

Use the new in_task() predicate instead.

Link: http://lkml.kernel.org/r/20170321091026.139655-1-dvyukov@google.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: James Morse <james.morse@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kcov.c

@@ -60,15 +60,8 @@ void notrace __sanitizer_cov_trace_pc(void)
 	/*
 	 * We are interested in code coverage as a function of a syscall inputs,
 	 * so we ignore code executed in interrupts.
-	 * The checks for whether we are in an interrupt are open-coded, because
-	 * 1. We can't use in_interrupt() here, since it also returns true
-	 *    when we are inside local_bh_disable() section.
-	 * 2. We don't want to use (in_irq() | in_serving_softirq() | in_nmi()),
-	 *    since that leads to slower generated code (three separate tests,
-	 *    one for each of the flags).
 	 */
-	if (!t || (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET
-							| NMI_MASK)))
+	if (!t || !in_task())
 		return;
 	mode = READ_ONCE(t->kcov_mode);
 	if (mode == KCOV_MODE_TRACE) {