Re: [PATCH 4/5] tracing: Handle the trace array ref counter in new functions

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 23 Oct 2019 15:57:49 -0700
Divya Indi <divya.indi@oracle.com> wrote:

> Hi Steven,                                                                         
>                                                                                  
> A few clarifications on this discussion on reference counter -                   
>                                                                                  
> 1) We will still need to export trace_array_put() to be used for every           
> trace_array_get_by_name() OR trace_array_create() + trace_array_get().        

I'm fine with exporting trace_array_put, and even trace_array_get.
      
>                                                                                  
> How else will we reduce the reference counter [For eg: When multiple modules     
> lookup the same trace array (say, reference counter = 4)]?                       
>                                                                                  
> 2) tr = trace_array_create("my_tr");                                             
>    trace_array_get(tr);                                                          
>                                                                                  
> Both of these functions will iterate through the list of trace arrays to verify  
> whether the trace array exists (redundant, but more intuitive? Does this seem    
> acceptable?)                                                                     
>                                                                                  
> To avoid iterating twice, we went with increasing ref_ctr in trace_array_create. 
> This necessitated a trace_array_put() in instance_mkdir (Or as suggested below,
> we can do this trace_array_put() in instance_rmdir().)                        
>                                                                                                                
>                                                                                  
> 3) A summary of suggested changes (Let me know if this looks good) -                                              
>                                                                                  
> tr = trace_array_get_by_name("foo-bar"); // ref_ctr++.                           
>                                                                                  
> if (!tr)                                                                         
> {                                                                                
>         // instance_mkdir also causes ref_ctr = 1                                

You'll need locking for anyone who does this, and check the return
status below for "foo-bar" existing already (due to another thread
jumping in here).

-- Steve

>         tr = trace_array_create("foo-bar"); // ref_ctr = 1                       
>         trace_array_get(tr); // ref_ctr++                                        
> }                                                                                
>                                                                                  
> trace_array_printk(.....);                                                       
> trace_array_set_clr_event(......);                                               
> ...                                                                              
> ...                                                                              
> ...                                                                              
> // Done using the trace array.                                                   
> trace_array_put(tr); // ref_ctr--                                                
> ...                                                                              
> ...                                                                              
> ...                                                                              
> // We can now remove the trace array via trace_array_destroy or instance_rmdir()
> trace_array_destroy(tr); // ref_ctr > 1 returns -EBUSY.