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Make default signal backend fully threadsafe

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Jason Toffaletti discovered with helgrind that our signal handler was
messing with evsig_base, which can be set from lots of places in the
code.  Ordinarly, we'd just stick a lock on it, except that it is
illegal (and genuinely error-prone) to call pthread_mutex_acquire()
from inside a signal handler.

The solution is to only store the fd we write to in a static variable,
write the signal number to the fd, and put evsig_cb in charge of
activating signal events.

I have no idea how we'll cope if we want to enable this to handle
siginfo (where available) in the future.
This commit is contained in:
Nick Mathewson 2010-09-15 01:40:02 -04:00
parent 720bd933c8
commit 95a7d418ab
2 changed files with 38 additions and 27 deletions
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1
evsignal-internal.h
View File

@ -47,7 +47,6 @@ struct evsig_info {
int ev_n_signals_added;
volatile sig_atomic_t evsig_caught;
sig_atomic_t evsigcaught[NSIG];
/* Array of previous signal handler objects before Libevent started
* messing with them. Used to restore old signal handlers. */

64
signal.c
View File

@ -108,6 +108,7 @@ static void *evsig_base_lock = NULL;
static struct event_base *evsig_base = NULL;
/* A copy of evsig_base->sigev_n_signals_added. */
static int evsig_base_n_signals_added = 0;
static evutil_socket_t evsig_base_fd = -1;
static void __cdecl evsig_handler(int sig);
@ -120,6 +121,7 @@ evsig_set_base(struct event_base *base)
EVSIGBASE_LOCK();
evsig_base = base;
evsig_base_n_signals_added = base->sig.ev_n_signals_added;
evsig_base_fd = base->sig.ev_signal_pair[0];
EVSIGBASE_UNLOCK();
}
@ -127,17 +129,40 @@ evsig_set_base(struct event_base *base)
static void
evsig_cb(evutil_socket_t fd, short what, void *arg)
{
static char signals[1];
static char signals[1024];
ev_ssize_t n;
int i;
int ncaught[NSIG];
struct event_base *base;
(void)arg; /* Suppress "unused variable" warning. */
base = arg;
n = recv(fd, signals, sizeof(signals), 0);
if (n == -1) {
int err = evutil_socket_geterror(fd);
if (! EVUTIL_ERR_RW_RETRIABLE(err))
event_sock_err(1, fd, "%s: recv", __func__);
memset(&ncaught, 0, sizeof(ncaught));
while (1) {
n = recv(fd, signals, sizeof(signals), 0);
if (n == -1) {
int err = evutil_socket_geterror(fd);
if (! EVUTIL_ERR_RW_RETRIABLE(err))
event_sock_err(1, fd, "%s: recv", __func__);
break;
} else if (n == 0) {
/* XXX warn? */
break;
}
for (i = 0; i < n; ++i) {
ev_uint8_t sig = signals[i];
if (sig < NSIG)
ncaught[sig]++;
}
}
EVBASE_ACQUIRE_LOCK(base, th_base_lock);
for (i = 0; i < NSIG; ++i) {
if (ncaught[i])
evmap_signal_active(base, i, ncaught[i]);
}
EVBASE_RELEASE_LOCK(base, th_base_lock);
}
int
@ -170,13 +195,12 @@ evsig_init(struct event_base *base)
base->sig.sh_old = NULL;
base->sig.sh_old_max = 0;
base->sig.evsig_caught = 0;
memset(&base->sig.evsigcaught, 0, sizeof(sig_atomic_t)*NSIG);
evutil_make_socket_nonblocking(base->sig.ev_signal_pair[0]);
evutil_make_socket_nonblocking(base->sig.ev_signal_pair[1]);
event_assign(&base->sig.ev_signal, base, base->sig.ev_signal_pair[1],
EV_READ | EV_PERSIST, evsig_cb, &base->sig.ev_signal);
EV_READ | EV_PERSIST, evsig_cb, base);
base->sig.ev_signal.ev_flags |= EVLIST_INTERNAL;
@ -275,6 +299,7 @@ evsig_add(struct event_base *base, int evsignal, short old, short events, void *
}
evsig_base = base;
evsig_base_n_signals_added = ++sig->ev_n_signals_added;
evsig_base_fd = base->sig.ev_signal_pair[0];
EVSIGBASE_UNLOCK();
event_debug(("%s: %d: changing signal handler", __func__, evsignal));
@ -352,6 +377,7 @@ evsig_handler(int sig)
#ifdef WIN32
int socket_errno = EVUTIL_SOCKET_ERROR();
#endif
ev_uint8_t msg;
if (evsig_base == NULL) {
event_warn(
@ -360,15 +386,13 @@ evsig_handler(int sig)
return;
}
evsig_base->sig.evsigcaught[sig]++;
evsig_base->sig.evsig_caught = 1;
#ifndef _EVENT_HAVE_SIGACTION
signal(sig, evsig_handler);
#endif
/* Wake up our notification mechanism */
send(evsig_base->sig.ev_signal_pair[0], "a", 1, 0);
msg = sig;
send(evsig_base_fd, &msg, 1, 0);
errno = save_errno;
#ifdef WIN32
EVUTIL_SET_SOCKET_ERROR(socket_errno);
@ -378,19 +402,6 @@ evsig_handler(int sig)
void
evsig_process(struct event_base *base)
{
struct evsig_info *sig = &base->sig;
sig_atomic_t ncalls;
int i;
base->sig.evsig_caught = 0;
for (i = 1; i < NSIG; ++i) {
ncalls = sig->evsigcaught[i];
if (ncalls == 0)
continue;
sig->evsigcaught[i] -= ncalls;
evmap_signal_active(base, i, ncalls);
}
}
void
@ -411,6 +422,7 @@ evsig_dealloc(struct event_base *base)
if (base == evsig_base) {
evsig_base = NULL;
evsig_base_n_signals_added = 0;
evsig_base_fd = -1;
}
EVSIGBASE_UNLOCK();