* Sergey Senozhatsky <sergey.senozhat...@gmail.com> [170216 08:33]:
> 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
OK thanks for checking why it changed.
Regards,
Tony
