locks.c

#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include "debug.h"
#include "locks.h"
#include "log.h"
#include "pids.h"
#include "trinity.h"
#include "utils.h"

/*
 * Check that the processes holding locks are still alive.
 * And if they are, ensure they haven't held them for an
 * excessive length of time.
 */
#define STEAL_THRESHOLD 1000000

static void check_lock(lock_t *_lock)
{
	pid_t pid;

	if (_lock->lock != LOCKED)
		return;

	/* First the easy case. If it's held by a dead pid, release it. */
	pid = _lock->owner;
	if (pid_alive(pid) == -1) {
		if (errno != ESRCH)
			return;

		debugf("Found a lock held by dead pid %d. Freeing.\n", pid);
		unlock(_lock);
		return;
	}

	/* If a pid has had a lock a long time, something is up. */
	if (_lock->contention > STEAL_THRESHOLD) {
		debugf("pid %d has held lock for too long. Releasing, and killing.\n", pid);
		kill_pid(pid);
		unlock(_lock);
		return;
	}
	return;
}

void check_all_locks(void)
{
	unsigned int i;

	check_lock(&shm->reaper_lock);
	check_lock(&shm->syscalltable_lock);

	for_each_child(i)
		check_lock(&shm->children[i]->syscall.lock);
}

void lock(lock_t *_lock)
{
	pid_t pid = getpid();

	while (_lock->lock == LOCKED) {
		if (_lock->owner == pid) {
			debugf("lol, already have lock!\n");
			show_backtrace();
			panic(EXIT_LOCKING_CATASTROPHE);
			_exit(EXIT_FAILURE);
		}

		/* This is pretty horrible. But if we call lock()
		 * from the watchdog code, and a child is hogging a lock
		 * (or worse, a dead child), we'll never call check_lock()
		 * from the watchdog loop because we'll be stuck here.
		 */
		if (pid == watchdog_pid) {
			check_lock(_lock);
		} else {
			/* Ok, we're a child pid.
			 * if something bad happened, like main/watchdog crashed,
			 * we don't want to spin forever, so just get out.
			 */
			if ((shm->exit_reason != STILL_RUNNING) &&
			    (shm->exit_reason != EXIT_REACHED_COUNT)) {
				_exit(EXIT_FAILURE);
			}
		}

		_lock->contention++;
		usleep(1);
	}

	_lock->lock = LOCKED;
	_lock->contention = 0;
	_lock->owner = pid;
}

void unlock(lock_t *_lock)
{
	asm volatile("" ::: "memory");
	_lock->contention = 0;
	_lock->owner = 0;
	_lock->lock = UNLOCKED;
}

/*
 * Release a lock we already hold.
 *
 * This function should be used sparingly. It's pretty much never something
 * that you'll need, just for rare occasions like when we return from a
 * signal handler with a lock held.
 */
void bust_lock(lock_t *_lock)
{
	if (_lock->lock != LOCKED)
		return;
	if (getpid() != _lock->owner)
		return;
	unlock(_lock);
}