Re: [PATCH v2] epoll: Support for disabling items, and a self-testapp.

[Matt Helsley]

On Thu, Oct 25, 2012 at 12:23:24PM +0200, Michael Kerrisk (man-pages) wrote:
> Hi Pat,
> 
> 
> >> I suppose that I have a concern that goes in the other direction. Is
> >> there not some other solution possible that doesn't require the use of
> >> EPOLLONESHOT? It seems overly restrictive to require that the caller
> >> must employ this flag, and imposes the burden that the caller must
> >> re-enable monitoring after each event.
> >>
> >> Does a solution like the following (with no requirement for EPOLLONESHOT)
> >> work?
> >>
> >> 0. Implement an epoll_ctl() operation EPOLL_CTL_XXX
> >>    where the name XXX might be chosen based on the decision
> >>    in 4(a).
> >> 1. EPOLL_CTL_XXX employs a private flag, EPOLLUSED, in the
> >>    per-fd events mask in the ready list. By default,
> >>    that flag is off.
> >> 2. epoll_wait() always clears the EPOLLUSED flag if a
> >>    file descriptor is found to be ready.
> >> 3. If an epoll_ctl(EPOLL_CTL_XXX) discovers that the EPOLLUSED
> >>    flag is NOT set, then
> >>         a) it sets the EPOLLUSED flag
> >>         b) It disables I/O events (as per EPOLL_CTL_DISABLE)
> >>            (I'm not 100% sure if this is necesary).
> >>         c) it returns EBUSY to the caller
> >> 4. If an epoll_ctl(EPOLL_CTL_XXX) discovers that the EPOLLUSED
> >>    flag IS set, then it
> >>         a) either deletes the fd or disables events for the fd
> >>            (the choice here is a matter of design taste, I think;
> >>            deletion has the virtue of simplicity; disabling provides
> >>            the option to re-enable the fd later, if desired)
> >>         b) returns 0 to the caller.
> >>
> >> All of the above with suitable locking around the user-space cache.
> >>
> >> Cheers,
> >>
> >> Michael
> >
> >
> > I don't believe that proposal will solve the problem. Consider the case
> > where a worker thread has just executed epoll_wait and is about to execute
> > the next line of code (which will access the data associated with the fd
> > receiving the event). If the deletion thread manages to call
> > epoll_ctl(EPOLL_CTL_XXX) for that fd twice in a row before the worker thread
> > is able to execute the next statement, then the deletion thread will
> > mistakenly conclude that it is safe to destroy the data that the worker
> > thread is about to access.
> 
> Okay -- I had the idea there might be a hole in my proposal ;-).
> 
> By the way, have you been reading the comments in the two LWN articles
> on EPOLL_CTL_DISABLE?
> https://lwn.net/Articles/520012/
> http://lwn.net/SubscriberLink/520198/fd81ba0ecb1858a2/
> 
> There's some interesting proposals there--some suggesting that an
> entirely user-space solution might be possible. I haven't looked
> deeply into the ideas though.

Yeah, I became quite interested so I wrote a crude epoll + urcu test.
Since it's RCU review to ensure I've not made any serious mistakes could
be quite helpful:

#define _LGPL_SOURCE 1
#define _GNU_SOURCE 1

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>
#include <fcntl.h>
#include <time.h>

#include <sys/epoll.h>

/*
 * Locking Voodoo:
 *
 * The globabls prefixed by _ require special care because they will be
 * accessed from multiple threads.
 *
 * The precise locking scheme we use varies whether READERS_USE_MUTEX is defined
 * When we're using userspace RCU the mutex only gets acquired for writes
 *     to _-prefixed globals. Reads are done inside RCU read side critical
 *     sections.
 * Otherwise the epmutex covers reads and writes to them all and the test
 * is not very scalable.
 */
static pthread_mutex_t epmutex = PTHREAD_MUTEX_INITIALIZER;
static int _p[2]; /* Send dummy data from one thread to another */
static int _epfd; /* Threads wait to read/write on epfd */
static int _nepitems = 0;

#ifdef READERS_USE_MUTEX
#define init_lock() do {} while(0)
#define init_thread() do {} while(0)
#define read_lock pthread_mutex_lock
#define read_unlock pthread_mutex_unlock
#define fini_thread() do {} while(0)
/* Because readers use the mutex synchronize_rcu() is a no-op */
#define synchronize_rcu() do {} while(0)
#else
#include <urcu.h>
#define init_lock rcu_init
#define init_thread rcu_register_thread
#define read_lock(m) rcu_read_lock()
#define read_unlock(m) rcu_read_unlock()
#define fini_thread() do { rcu_unregister_thread(); } while(0)
#endif
#define write_lock pthread_mutex_lock
#define write_unlock pthread_mutex_unlock

/* We send this data through the pipe. */
static const char *data = "test";
const size_t dlen = 5;

static inline int harmless_errno(void)
{
	return ((errno == EWOULDBLOCK) || (errno == EAGAIN) || (errno == EINTR));
}

static void* thread_main(void *thread_nr)
{
	struct epoll_event ev;
	int rc = 0;
	char buffer[dlen];
	unsigned long long _niterations = 0;

	init_thread();
	while (!rc) {
		read_lock(&epmutex);
		if (_nepitems < 1) {
			read_unlock(&epmutex);
			break;
		}
		rc = epoll_wait(_epfd, &ev, 1, 1);
		if (rc < 1) {
			read_unlock(&epmutex);
			if (rc == 0)
				continue;
			if (harmless_errno()) {
				rc = 0;
				continue;
			}
			break;
		}

		if (ev.events & EPOLLOUT) {
			rc = write(_p[1], data, dlen);
			read_unlock(&epmutex);
			if (rc < 0) {
				if (harmless_errno()) {
					rc = 0;
					continue;
				}
				break;
			}
			rc = 0;
		} else if (ev.events & EPOLLIN) {
			rc = read(_p[0], buffer, dlen);
			read_unlock(&epmutex);
			if (rc < 0) {
				if (harmless_errno()) {
					rc = 0;
					continue;
				}
				break;
			}
			rc = 0;
		} else
			read_unlock(&epmutex);
		_niterations++;
	}
	fini_thread();
	return (void *)_niterations;
}

/* Some sample numbers from varying MAX_THREADS on my laptop:
 * With a global mutex:
 * 1 core for the main thread
 * 1 core for epoll_wait()'ing threads
 * The mutex doesn't scale -- increasing the number of threads despite
 * having more real cores just causes performance to go down.
 * 7 threads,  213432.128160 iterations per second
 * 3 threads,  606560.183997 iterations per second
 * 2 threads, 1346006.413404 iterations per second
 * 1 thread , 2148936.348793 iterations per second
 *
 * With URCU:
 * 1 core for the main thread which spins reading niterations.
 * N-1 cores for the epoll_wait()'ing threads.
 * "Hyperthreading" doesn't help here -- I've got 4 cores:
 * 7 threads, 1537304.965009 iterations per second
 * 4 threads, 1912846.753203 iterations per second
 * 3 threads, 2278639.336464 iterations per second
 * 2 threads, 1928805.899146 iterations per second
 * 1 thread , 2007198.066327 iterations per second
 */
#define MAX_THREADS 3

int main (int argc, char **argv)
{
	struct timespec before, req, after;
	unsigned long long niterations = 0;
	pthread_t threads[MAX_THREADS];
	struct epoll_event ev;
	int nthreads = 0, rc;

	init_lock();

	/* Since we haven't made the threads yet we can safely use _ globals */
	rc = pipe2(_p, O_NONBLOCK);
	if (rc < 0)
		goto error;

	_epfd = epoll_create1(EPOLL_CLOEXEC);
	if (_epfd < 0)
		goto error;

	/* Monitor the pipe via epoll */
	ev.events = EPOLLIN;
	ev.data.u32 = 0; /* index in _p[] */
	rc = epoll_ctl(_epfd, EPOLL_CTL_ADD, _p[0], &ev);
	if (rc < 0)
		goto error;
	_nepitems++;
	printf("Added fd %d to epoll set %d\n", _p[0], _epfd);
	ev.events = EPOLLOUT;
	ev.data.u32 = 1;
	rc = epoll_ctl(_epfd, EPOLL_CTL_ADD, _p[1], &ev);
	if (rc < 0)
		goto error;
	_nepitems++;
	printf("Added fd %d to epoll set %d\n", _p[1], _epfd);
	fflush(stdout);

	/* 
	 * After the first pthread_create() we can't safely use _ globals
	 * without adhering to the locking scheme. pthread_create() should
	 * also imply some thorough memory barriers so all our previous
	 * modifications to the _ globals should be visible after this point.
	 */
	for (rc = 0; nthreads < MAX_THREADS; nthreads++) {
		rc = pthread_create(&threads[nthreads], NULL, &thread_main,
				    (void *)(long)nthreads);
		if (rc < 0)
			goto error;
	}

	/* Wait for our child threads to do some "work" */
	req.tv_sec = 30;
	rc = clock_gettime(CLOCK_MONOTONIC_RAW, &before);
	rc = nanosleep(&req, NULL);
	rc = clock_gettime(CLOCK_MONOTONIC_RAW, &after);

	/* 
	 * Modify the epoll interest set. This can leave stale
	 * data in other threads because they may have done an
	 * epoll_wait() with RCU read lock held instead of the
	 * epmutex.
	 */
	write_lock(&epmutex);
	rc = epoll_ctl(_epfd, EPOLL_CTL_DEL, _p[0], &ev);
	if (rc == 0) {
		_nepitems--;
		printf("Removed fd %d from epoll set %d\n", _p[0], _epfd);
		rc = epoll_ctl(_epfd, EPOLL_CTL_DEL, _p[1], &ev);
		if (rc == 0) {
			printf("Removed fd %d from epoll set %d\n", _p[1], _epfd);
			_nepitems--;
		}
	}
	write_unlock(&epmutex);
	if (rc < 0)
		goto error;

	/*
	 * Wait until the stale data are no longer in use.
	 * We could use call_rcu() here too, but let's keep the test simple.
	 */
	printf("synchronize_rcu()\n");
	fflush(stdout);
	synchronize_rcu();

	printf("closing fds\n");
	fflush(stdout);

	/* Clean up the stale data */
	close(_p[0]);
	close(_p[1]);
	close(_epfd);
	
	printf("closed fds (%d, %d, %d)\n", _p[0], _p[1], _epfd);
	fflush(stdout);

	/*
	 * Test is done. Join all the threads so that we give time for
	 * races to show up.
	 */
	niterations = 0;
	for (; nthreads > 0; nthreads--) {
		unsigned long long thread_iterations;

		rc = pthread_join(threads[nthreads - 1],
				  (void *)&thread_iterations);
		niterations += thread_iterations;
	}

	after.tv_sec -= before.tv_sec;
	after.tv_nsec -= before.tv_nsec;
	if (after.tv_nsec < 0) {
		--after.tv_sec;
		after.tv_nsec += 1000000000;
	}
	printf("%f iterations per second\n", (double)niterations/((double)after.tv_sec + (double)after.tv_nsec/1000000000.0));
	exit(EXIT_SUCCESS);
error:
	/* This is trashy testcase code -- it doesn't do full cleanup! */
	for (; nthreads > 0; nthreads--)
		rc = pthread_cancel(threads[nthreads - 1]);
	exit(EXIT_FAILURE);
}

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