kcsan: Make KCSAN compatible with new IRQ state tracking

The new IRQ state tracking code does not honor lockdep_off(), and as
such we should again permit tracing by using non-raw functions in
core.c. Update the lockdep_off() comment in report.c, to reflect the
fact there is still a potential risk of deadlock due to using printk()
from scheduler code.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200624113246.GA170324@elver.google.com

kernel/kcsan/core.c

@@ -397,8 +397,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	}
 
 	if (!kcsan_interrupt_watcher)
-		/* Use raw to avoid lockdep recursion via IRQ flags tracing. */
-		raw_local_irq_save(irq_flags);
+		local_irq_save(irq_flags);
 
 	watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
 	if (watchpoint == NULL) {
@@ -539,7 +538,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
 out_unlock:
 	if (!kcsan_interrupt_watcher)
-		raw_local_irq_restore(irq_flags);
+		local_irq_restore(irq_flags);
 out:
 	user_access_restore(ua_flags);
 }

kernel/kcsan/report.c

@@ -606,10 +606,11 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 		goto out;
 
 	/*
-	 * With TRACE_IRQFLAGS, lockdep's IRQ trace state becomes corrupted if
-	 * we do not turn off lockdep here; this could happen due to recursion
-	 * into lockdep via KCSAN if we detect a race in utilities used by
-	 * lockdep.
+	 * Because we may generate reports when we're in scheduler code, the use
+	 * of printk() could deadlock. Until such time that all printing code
+	 * called in print_report() is scheduler-safe, accept the risk, and just
+	 * get our message out. As such, also disable lockdep to hide the
+	 * warning, and avoid disabling lockdep for the rest of the kernel.
 	 */
 	lockdep_off();