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

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Message-ID: <74a1af20-5f99-bb64-1e0f-c8405bd68014@oracle.com>
Date:   Thu, 24 Oct 2019 14:31:51 -0700
From:   Divya Indi <divya.indi@...cle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Joe Jin <joe.jin@...cle.com>,
        Srinivas Eeda <srinivas.eeda@...cle.com>,
        Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
Subject: Re: [PATCH 4/5] tracing: Handle the trace array ref counter in new
 functions

Hi Steven,

On 10/24/19 6:00 AM, Steven Rostedt wrote:
> On Wed, 23 Oct 2019 15:57:49 -0700
> Divya Indi <divya.indi@...cle.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).

Right, Noted! Thanks for the pointer. 

>
> -- 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.