Speculation Example

One potential use for speculations is to highlight a particular code path. The following example shows the entire code path under the open(2) system call only when the open() fails:

Example 13-1 specopen.d: Code Flow for Failed open(2)

#!/usr/sbin/dtrace -Fs

syscall::open:entry,
syscall::open64:entry
{
    /*
     * The call to speculation() creates a new speculation.  If this fails,
     * dtrace(1M) will generate an error message indicating the reason for
     * the failed speculation(), but subsequent speculative tracing will be
     * silently discarded.
     */
    self->spec = speculation();
    speculate(self->spec);

    /*
     * Because this printf() follows the speculate(), it is being 
     * speculatively traced; it will only appear in the data buffer if the
     * speculation is subsequently commited.
     */
    printf("%s", stringof(copyinstr(arg0)));
}

fbt:::
/self->spec/
{
    /*
     * A speculate() with no other actions speculates the default action:
     * tracing the EPID.
     */
    speculate(self->spec);
}

syscall::open:return,
syscall::open64:return
/self->spec/
{
    /*
     * To balance the output with the -F option, we want to be sure that
     * every entry has a matching return.  Because we speculated the
     * open entry above, we want to also speculate the open return.
     * This is also a convenient time to trace the errno value.
     */
    speculate(self->spec);
    trace(errno);
}

syscall::open:return,
syscall::open64:return
/self->spec && errno != 0/
{
    /*
     * If errno is non-zero, we want to commit the speculation.
     */
    commit(self->spec);
    self->spec = 0;
}

syscall::open:return,
syscall::open64:return
/self->spec && errno == 0/
{
    /*
     * If errno is not set, we discard the speculation.
     */
    discard(self->spec);
    self->spec = 0;
}

Running the above script produces output similar to the following example:

# ./specopen.d
dtrace: script './specopen.d' matched 24282 probes
CPU FUNCTION                                 
  1  => open                                  /var/ld/ld.config
  1    -> open                                
  1      -> copen                             
  1        -> falloc                          
  1          -> ufalloc                       
  1            -> fd_find                     
  1              -> mutex_owned               
  1              <- mutex_owned               
  1            <- fd_find                     
  1            -> fd_reserve                  
  1              -> mutex_owned               
  1              <- mutex_owned               
  1              -> mutex_owned               
  1              <- mutex_owned               
  1            <- fd_reserve                  
  1          <- ufalloc                       
  1          -> kmem_cache_alloc              
  1            -> kmem_cache_alloc_debug      
  1              -> verify_and_copy_pattern   
  1              <- verify_and_copy_pattern   
  1              -> file_cache_constructor    
  1                -> mutex_init              
  1                <- mutex_init              
  1              <- file_cache_constructor    
  1              -> tsc_gethrtime             
  1              <- tsc_gethrtime             
  1              -> getpcstack                
  1              <- getpcstack                
  1              -> kmem_log_enter            
  1              <- kmem_log_enter            
  1            <- kmem_cache_alloc_debug      
  1          <- kmem_cache_alloc              
  1          -> crhold                        
  1          <- crhold                        
  1        <- falloc                          
  1        -> vn_openat                       
  1          -> lookupnameat                  
  1            -> copyinstr                   
  1            <- copyinstr                   
  1            -> lookuppnat                  
  1              -> lookuppnvp                
  1                -> pn_fixslash             
  1                <- pn_fixslash             
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                    -> groupmember         
  1                      -> supgroupmember    
  1                      <- supgroupmember
  1                    <- groupmember         
  1                  <- ufs_iaccess           
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                  <- ufs_iaccess           
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                  <- ufs_iaccess           
  1                  -> vn_rele               
  1                  <- vn_rele               
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1              <- lookuppnvp                
  1            <- lookuppnat                  
  1          <- lookupnameat                  
  1        <- vn_openat                       
  1        -> setf                            
  1          -> fd_reserve                    
  1            -> mutex_owned                 
  1            <- mutex_owned                 
  1            -> mutex_owned                 
  1            <- mutex_owned                 
  1          <- fd_reserve                    
  1          -> cv_broadcast                  
  1          <- cv_broadcast                  
  1        <- setf                            
  1        -> unfalloc                        
  1          -> mutex_owned                   
  1          <- mutex_owned                   
  1          -> crfree                        
  1          <- crfree                        
  1          -> kmem_cache_free               
  1            -> kmem_cache_free_debug       
  1              -> kmem_log_enter            
  1              <- kmem_log_enter            
  1              -> tsc_gethrtime             
  1              <- tsc_gethrtime             
  1              -> getpcstack                
  1              <- getpcstack                
  1              -> kmem_log_enter            
  1              <- kmem_log_enter
  1              -> file_cache_destructor     
  1                -> mutex_destroy           
  1                <- mutex_destroy           
  1              <- file_cache_destructor     
  1              -> copy_pattern              
  1              <- copy_pattern              
  1            <- kmem_cache_free_debug       
  1          <- kmem_cache_free               
  1        <- unfalloc                        
  1        -> set_errno                       
  1        <- set_errno                       
  1      <- copen                             
  1    <- open                                
  1  <= open                                          2