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

[Peter Zijlstra]

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();
 }