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Filename : whythefail.stp
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#!/usr/bin/stap

/* Print a trace of the the last few statements (file:line numbers,
 along with selected local variables) that were executed within a
 given function, if it ends up returning with a given condition.  (No
 trace is shown for functions that return with the condition false,
 demonstrating use of speculation tapset.)

 Usage:
 stap whythefail.stp PROC_OR_KERNEL FUNCTION_NAME CONDITION_EXPRESSION [$VARIABLE$]

 PROC_OR_KERNEL:   the first part of the probe point, e.g.:
                           'kernel' 'module("foo")' or 'process("bar")'
 FUNCTION_NAME:    the function name (assumed non-recursive)
 CONDITION:        expression to evaluate at function return, e.g.:
                           '$return != 0' or '1' or 'randint(100)<20'
 VARIABLE:         optional context-variable string to print at each statement, e.g.:
                           '$a_var$' or '@var("global")$' or (default) '$$vars'

 If the script is invoked with -c CMD or -x PID, probes are filtered
 for that process target-set (it and its descendants).

 e.g.:
 stap whythefail.stp kernel do_mlock '$return < 0'
*/


global specs%
global counts

probe $1 . function(@2) . call {
  if (target() && !target_set_pid(pid())) next;
  specs[tid()] = speculation()
  counts["entry"] <<< 1
}

probe $1 . statement(@2 "@*:*") {
  if (target() && !target_set_pid(pid())) next;
  tokenize(pp(),"@");
  fileline=tokenize("","@");
  file=tokenize(fileline,":\"");
  line=tokenize("",":\"");
  speculate(specs[tid()], sprintf("%s[%d] %s:%s %s\n",
                                  execname(), tid(), file, line,
                                  %( $# >= 4
                                     %? @4.": ".@choose_defined($4,"?")
                                     %: "$$vars: ".$$vars %) ))
  counts["statement"] <<< 1
}

probe $1 . function(@2) . return {
  if (target() && !target_set_pid(pid())) next;
  if ($3) {
      printf("\n%s[%d] %s function(%s) $return: %s\n", execname(), tid(), @1, @2, $return$)
      commit(specs[tid()])
      counts["exit-hit"] <<< 1
    } else {
      discard(specs[tid()])
      counts["exit-miss"] <<< 1
    }
}

probe end,error {
  printf("\nstatistics:\n")
  foreach (reason+ in counts) {
    printf("%s count: %d\n", reason, @count(counts[reason]))
  }
}