Re: Regression in next with use printk_safe buffers in printk

[Tony Lindgren]

* Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> [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