On (02/16/17 07:10), Tony Lindgren wrote:
[..]
> > > > [..]
> > > > > Below is another issue I noticed caused by commit f975237b7682 that
> > > > > I noticed during booting.
> > > >
> > > > do you mean that with f975237b7682 you _always_ see that illegal RCU
> > > > usage warning?
> > >
> > > Yeah on every boot on devices using cpuidle_coupled.
> >
> > does this mean that with the printk-safe patches reverted
> > (so, basically, the same conditions module 4 printk patches)
> > you don't see illegal RCU usage reports? at the moment I can't
> > see any connection between f975237b7682 and RCU usage from idle CPU.
>
> Yes reverting those four patches I listed earlier also makes it go
> away.
aha... so, the previous RCU warning was simply suppressed by lockdep_off()
that we used to have in printk().
RCU dereference check
#define __rcu_dereference_check(p, c, space) \
({ \
/* Dependency order vs. p above. */ \
typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
rcu_dereference_sparse(p, space); \
((typeof(*p) __force __kernel *)(________p1)); \
})
where RCU_LOCKDEP_WARN() that prints "suspicious rcu_dereference_check() usage"
is
#define RCU_LOCKDEP_WARN(c, s) \
do { \
static bool __section(.data.unlikely) __warned; \
if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \
__warned = true; \
lockdep_rcu_suspicious(__FILE__, __LINE__, s); \
} \
} while (0)
where debug_lockdep_rcu_enabled()
int notrace debug_lockdep_rcu_enabled(void)
{
return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
current->lockdep_recursion == 0;
}
depends on lockdep state. and we just used to have
'current->lockdep_recursion != 0' here, because of lockdep_off()
in printk() around console_unlock(), which increments ->lockdep_recursion.
now we have lockdep enabled and the ->lockdep_recursion == 0.
so the RCU warning is valid and I need to Cc stable on that _rcuidle
patch, the tracepoint is pretty old. it's from 3.4
-ss
