qemu/docs/spin/aio_notify_bug.promela

/*
 * This model describes a bug in aio_notify.  If ctx->notifier is
 * cleared too late, a wakeup could be lost.
 *
 * Author: Paolo Bonzini <pbonzini@redhat.com>
 *
 * This file is in the public domain.  If you really want a license,
 * the WTFPL will do.
 *
 * To verify the buggy version:
 *     spin -a -DBUG docs/aio_notify_bug.promela
 *     gcc -O2 pan.c
 *     ./a.out -a -f
 *
 * To verify the fixed version:
 *     spin -a docs/aio_notify_bug.promela
 *     gcc -O2 pan.c
 *     ./a.out -a -f
 *
 * Add -DCHECK_REQ to test an alternative invariant and the
 * "notify_me" optimization.
 */

int notify_me;
bool event;
bool req;
bool notifier_done;

#ifdef CHECK_REQ
#define USE_NOTIFY_ME 1
#else
#define USE_NOTIFY_ME 0
#endif

active proctype notifier()
{
    do
        :: true -> {
            req = 1;
            if
               :: !USE_NOTIFY_ME || notify_me -> event = 1;
               :: else -> skip;
            fi
        }
        :: true -> break;
    od;
    notifier_done = 1;
}

#ifdef BUG
#define AIO_POLL                                                    \
    notify_me++;                                                    \
    if                                                              \
        :: !req -> {                                                \
            if                                                      \
                :: event -> skip;                                   \
            fi;                                                     \
        }                                                           \
        :: else -> skip;                                            \
    fi;                                                             \
    notify_me--;                                                    \
                                                                    \
    req = 0;                                                        \
    event = 0;
#else
#define AIO_POLL                                                    \
    notify_me++;                                                    \
    if                                                              \
        :: !req -> {                                                \
            if                                                      \
                :: event -> skip;                                   \
            fi;                                                     \
        }                                                           \
        :: else -> skip;                                            \
    fi;                                                             \
    notify_me--;                                                    \
                                                                    \
    event = 0;                                                      \
    req = 0;
#endif

active proctype waiter()
{
    do
       :: true -> AIO_POLL;
    od;
}

/* Same as waiter(), but disappears after a while.  */
active proctype temporary_waiter()
{
    do
       :: true -> AIO_POLL;
       :: true -> break;
    od;
}

#ifdef CHECK_REQ
never {
    do
        :: req -> goto accept_if_req_not_eventually_false;
        :: true -> skip;
    od;

accept_if_req_not_eventually_false:
    if
        :: req -> goto accept_if_req_not_eventually_false;
    fi;
    assert(0);
}

#else
/* There must be infinitely many transitions of event as long
 * as the notifier does not exit.
 *
 * If event stayed always true, the waiters would be busy looping.
 * If event stayed always false, the waiters would be sleeping
 * forever.
 */
never {
    do
        :: !event    -> goto accept_if_event_not_eventually_true;
        :: event     -> goto accept_if_event_not_eventually_false;
        :: true      -> skip;
    od;

accept_if_event_not_eventually_true:
    if
        :: !event && notifier_done  -> do :: true -> skip; od;
        :: !event && !notifier_done -> goto accept_if_event_not_eventually_true;
    fi;
    assert(0);

accept_if_event_not_eventually_false:
    if
        :: event     -> goto accept_if_event_not_eventually_false;
    fi;
    assert(0);
}
#endif