NAME

       KFAIL_POINT_CODE,        KFAIL_POINT_CODE_FLAGS,        KFAIL_POINT_CODE_COND,        KFAIL_POINT_RETURN,
       KFAIL_POINT_RETURN_VOID, KFAIL_POINT_ERROR,  KFAIL_POINT_GOTO,  KFAIL_POINT_SLEEP_CALLBACKS,  fail_point,
       DEBUG_FP — fail points

SYNOPSIS

       #include <sys/fail.h>

       KFAIL_POINT_CODE(parent, name, code);

       KFAIL_POINT_CODE_FLAGS(parent, name, flags, code);

       KFAIL_POINT_CODE_COND(parent, name, cond, flags, code);

       KFAIL_POINT_RETURN(parent, name);

       KFAIL_POINT_RETURN_VOID(parent, name);

       KFAIL_POINT_ERROR(parent, name, error_var);

       KFAIL_POINT_GOTO(parent, name, error_var, label);

       KFAIL_POINT_SLEEP_CALLBACKS(parent, name, pre_func, pre_arg, post_func, post_arg, code);

DESCRIPTION

       Fail  points are used to add code points where errors may be injected in a user controlled fashion.  Fail
       points provide a convenient wrapper around user-provided error injection code, providing a sysctl(9) MIB,
       and a parser for that MIB that describes how the error injection code should fire.

       The base fail point macro is KFAIL_POINT_CODE() where parent is a sysctl tree  (frequently  DEBUG_FP  for
       kernel  fail  points, but various subsystems may wish to provide their own fail point trees), and name is
       the name of the MIB in that tree, and code is the error injection  code.   The  code  argument  does  not
       require  braces, but it is considered good style to use braces for any multi-line code arguments.  Inside
       the code argument, the evaluation of RETURN_VALUE is derived from the return() value set  in  the  sysctl
       MIB.

       Additionally,  KFAIL_POINT_CODE_FLAGS()  provides  a  flags  argument  which  controls  the  fail point's
       behaviour.  This can be used to e.g., mark the fail point's context as non-sleepable,  which  causes  the
       sleep action to be coerced to a busy wait.  The supported flags are:

             FAIL_POINT_USE_TIMEOUT_PATH
             Rather than sleeping on a sleep() call, just fire the post-sleep function after a timeout fires.

             FAIL_POINT_NONSLEEPABLE
             Mark  the  fail  point  as being in a non-sleepable context, which coerces sleep() calls to delay()
             calls.

       Likewise, KFAIL_POINT_CODE_COND() supplies a cond argument, which allows you to set the  condition  under
       which the fail point's code may fire.  This is equivalent to:

               if (cond)
                       KFAIL_POINT_CODE_FLAGS(...);

       See “SYSCTL VARIABLES” below.

       The remaining KFAIL_POINT_*() macros are wrappers around common error injection paths:

       KFAIL_POINT_RETURN(parent, name) is the equivalent of KFAIL_POINT_CODE(..., return RETURN_VALUE)

       KFAIL_POINT_RETURN_VOID(parent, name) is the equivalent of KFAIL_POINT_CODE(..., return)

       KFAIL_POINT_ERROR(parent,  name,  error_var)  is  the  equivalent  of  KFAIL_POINT_CODE(...,  error_var =
       RETURN_VALUE)

       KFAIL_POINT_GOTO(parent, name, error_var, label) is the equivalent of KFAIL_POINT_CODE(..., { error_var =
       RETURN_VALUE; goto label;})

SYSCTL VARIABLES

       The KFAIL_POINT_*() macros add sysctl MIBs where specified.  Many base kernel MIBs can be  found  in  the
       debug.fail_point tree (referenced in code by DEBUG_FP).

       The sysctl variable may be set in a number of ways:

         [<pct>%][<cnt>*]<type>[(args...)][-><more terms>]

       The <type> argument specifies which action to take; it can be one of:

       off     Take no action (does not trigger fail point code)

       return  Trigger fail point code with specified argument

       sleep   Sleep the specified number of milliseconds

       panic   Panic

       break   Break into the debugger, or trap if there is no debugger support

       print   Print that the fail point executed

       pause   Threads sleep at the fail point until the fail point is set to off

       yield   Thread yields the cpu when the fail point is evaluated

       delay   Similar to sleep, but busy waits the cpu.  (Useful in non-sleepable contexts.)

       The  <pct>%  and <cnt>* modifiers prior to <type> control when <type> is executed.  The <pct>% form (e.g.
       "1.2%") can be used to specify a probability that <type> will execute.  This is a decimal  in  the  range
       (0, 100] which can specify up to 1/10,000% precision.  The <cnt>* form (e.g. "5*") can be used to specify
       the  number of times <type> should be executed before this <term> is disabled.  Only the last probability
       and the last count are used if multiple are specified, i.e. "1.2%2%" is the same as "2%".   When  both  a
       probability  and  a count are specified, the probability is evaluated before the count, i.e. "2%5*" means
       "2% of the time, but only 5 times total".

       The operator -> can be used to express cascading terms.  If you specify <term1>-><term2>, it  means  that
       if  <term1> does not ‘execute’, <term2> is evaluated.  For the purpose of this operator, the return() and
       print() operators are the only types that cascade.  A return() term only cascades if the  code  executes,
       and  a  print()  term  only cascades when passed a non-zero argument.  A pid can optionally be specified.
       The fail point term is only executed when invoked by a process with a matching p_pid.

EXAMPLES

       sysctl debug.fail_point.foobar="2.1%return(5)"
               21/1000ths of the time, execute code with RETURN_VALUE set to 5.

       sysctl debug.fail_point.foobar="2%return(5)->5%return(22)"
               2/100ths of the time, execute code with RETURN_VALUE set to 5.  If that does not  happen,  5%  of
               the time execute code with RETURN_VALUE set to 22.

       sysctl debug.fail_point.foobar="5*return(5)->0.1%return(22)"
               For 5 times, return 5.  After that, 1/1000th of the time, return 22.

       sysctl debug.fail_point.foobar="0.1%5*return(5)"
               Return 5 for 1 in 1000 executions, but only 5 times total.

       sysctl debug.fail_point.foobar="1%*sleep(50)"
               1/100th of the time, sleep 50ms.

       sysctl debug.fail_point.foobar="1*return(5)[pid 1234]"
               Return 5 once, when pid 1234 executes the fail point.

AUTHORS

       This manual page was written by

       Matthew Bryan <matthew.bryan@isilon.com> and

       Zach Loafman <zml@FreeBSD.org>.

CAVEATS

       It  is  easy to shoot yourself in the foot by setting fail points too aggressively or setting too many in
       combination.  For example, forcing malloc() to fail consistently is potentially harmful to uptime.

       The sleep() sysctl setting may not be appropriate in all situations.  Currently,  fail_point_eval()  does
       not verify whether the context is appropriate for calling msleep().  You can force it to evaluate a sleep
       action  as  a  delay action by specifying the FAIL_POINT_NONSLEEPABLE flag at the point the fail point is
       declared.

Debian                                           March 15, 2016                                          FAIL(9)