Re: [PATCH 1/2] rcu: Show the real fqs_state

[Petr Mladek]

On Fri 2015-09-04 16:24:22, Paul E. McKenney wrote:
> On Fri, Sep 04, 2015 at 02:11:29PM +0200, Petr Mladek wrote:
> > The value of "fqs_state" in struct rcu_state is always RCU_GP_IDLE.
> > 
> > The real state is stored in a local variable in rcu_gp_kthread().
> > It is modified by rcu_gp_fqs() via parameter and return value.
> > But the actual value is never stored to rsp->fqs_state.
> > 
> > The result is that print_one_rcu_state() does not show the real
> > state.
> > 
> > This code has been added 3 years ago by the commit 4cdfc175c25c89ee
> > ("rcu: Move quiescent-state forcing into kthread"). I guess that it
> > was an overlook or optimization.
> > 
> > Anyway, the value seems to be manipulated only by the thread, except
> > for shoving the status. I do not see any risk in updating it directly
> > in the struct.
> > 
> > Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> 
> Good catch, but how about the following fix instead?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
>     rcu: Finish folding ->fqs_state into ->gp_state
>     
>     Commit commit 4cdfc175c25c89ee ("rcu: Move quiescent-state forcing
>     into kthread") started the process of folding the old ->fqs_state
>     into ->gp_state, but did not complete it.  This situation does not
>     cause any malfunction, but can result in extremely confusing trace
>     output.  This commit completes this task of eliminating ->fqs_state
>     in favor of ->gp_state.

It makes sense but it breaks dynticks handling in rcu_gp_fqs(), see
below.

>     
>     Reported-by: Petr Mladek <pmladek@xxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 69ab7ce2cf7b..04234936d897 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1949,16 +1949,15 @@ static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
>  /*
>   * Do one round of quiescent-state forcing.
>   */
> -static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
> +static void rcu_gp_fqs(struct rcu_state *rsp)
>  {
> -	int fqs_state = fqs_state_in;
>  	bool isidle = false;
>  	unsigned long maxj;
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
>  	WRITE_ONCE(rsp->gp_activity, jiffies);
>  	rsp->n_force_qs++;
> -	if (fqs_state == RCU_SAVE_DYNTICK) {
> +	if (rsp->gp_state == RCU_SAVE_DYNTICK) {

This will never happen because rcu_gp_kthread() modifies rsp->gp_state
many times. The last value before calling rcu_gp_fqs() is
RCU_GP_DOING_FQS.

I think about passing this information via a separate bool.

[...]

> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index d5f58e717c8b..9faad70a8246 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -417,12 +417,11 @@ struct rcu_data {
>  	struct rcu_state *rsp;
>  };
>  
> -/* Values for fqs_state field in struct rcu_state. */
> +/* Values for gp_state field in struct rcu_state. */
>  #define RCU_GP_IDLE		0	/* No grace period in progress. */

This value seems to be used instead of the new RCU_GP_WAIT_INIT.

>  #define RCU_GP_INIT		1	/* Grace period being
>  #initialized. */

This value is unused.

>  #define RCU_SAVE_DYNTICK	2	/* Need to scan dyntick
>  #state. */

This one is not longer preserved when merged with the other state.

>  #define RCU_FORCE_QS		3	/* Need to force quiescent
>  #state. */

The meaning of this one is strange. If I get it correctly,
it is set after the state was forced. But the comment suggests
that it is before.

By other words, these states seems to get obsoleted by

/* Values for rcu_state structure's gp_flags field. */
#define RCU_GP_WAIT_INIT 0	/* Initial state. */
#define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
#define RCU_GP_DONE_GPS  2	/* Wait done for grace-period start. */
#define RCU_GP_WAIT_FQS  3	/* Wait for force-quiescent-state time. */
#define RCU_GP_DOING_FQS 4	/* Wait done for force-quiescent-state time. */
#define RCU_GP_CLEANUP   5	/* Grace-period cleanup started. */
#define RCU_GP_CLEANED   6	/* Grace-period cleanup complete. */


Please, find below your commit updated with my ideas:

	+ used bool save_dyntick instead of RCU_SAVE_DYNTICK
	  and RCU_FORCE_QS states
	+ rename RCU_GP_WAIT_INIT -> RCU_GP_IDLE
	+ remove all the obsolete states

I am sorry if I handled "Signed-off-by" flags a wrong way. It is
basically your patch with few small updates from me. I am not sure
what is the right process in this case. Feel free to use Reviewed-by
instead of Signed-off-by with my name.

Well, I guess that this is not the final state ;-)