Subject: Re: [PATCH v4 7/8] lockdep: Change hardirq{s_enabled,_context} to per-cpu variables

From: Peter Zijlstra <[email-masked]>
Date: Tue, 23 Jun 2020 22:24:04 +0200

On Tue, Jun 23, 2020 at 08:12:32PM +0200, Peter Zijlstra wrote:
> Fair enough; I'll rip it all up and boot a KCSAN kernel, see what if
> anything happens.

OK, so the below patch doesn't seem to have any nasty recursion issues
here. The only 'problem' is that lockdep now sees report_lock can cause
deadlocks.

It is completely right about it too, but I don't suspect there's much we
can do about it, it's pretty much the standard printk() with scheduler
locks held report.

---
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 15f67949d11e..732623c30359 100644
--- a/kernel/kcsan/core.c
+++ b/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);
 }
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index ac5f8345bae9..ef31c1d2dac3 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -605,14 +605,6 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 	if (WARN_ON(watchpoint_idx < 0 || watchpoint_idx >= ARRAY_SIZE(other_infos)))
 		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.
-	 */
-	lockdep_off();
-
 	if (prepare_report(&flags, type, &ai, other_info)) {
 		/*
 		 * Never report if value_change is FALSE, only if we it is
@@ -628,7 +620,6 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
 		release_report(&flags, other_info);
 	}
 
-	lockdep_on();
 out:
 	kcsan_enable_current();
 }