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add pending state back to jl_thread_suspend_and_get_state-machine (Ju…
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…liaLang#55622)

Fixes an issue with JuliaLang#55500, where signals may abruptly abort the process
as they observe it is still processing the resume SIGUSR2 message and
are not able to wait for that processing to end before setting the new
message to exit.
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vtjnash authored Aug 29, 2024
1 parent e921dc8 commit da3468c
Showing 1 changed file with 55 additions and 10 deletions.
65 changes: 55 additions & 10 deletions src/signals-unix.c
Original file line number Diff line number Diff line change
Expand Up @@ -448,6 +448,7 @@ static int signal_caught_cond = -1;

int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
{
int err;
pthread_mutex_lock(&in_signal_lock);
jl_ptls_t ptls2 = jl_atomic_load_relaxed(&jl_all_tls_states)[tid];
jl_task_t *ct2 = ptls2 ? jl_atomic_load_relaxed(&ptls2->current_task) : NULL;
Expand All @@ -456,22 +457,51 @@ int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
pthread_mutex_unlock(&in_signal_lock);
return 0;
}
sig_atomic_t request = 0;
if (!jl_atomic_cmpswap(&ptls2->signal_request, &request, 1)) {
if (jl_atomic_load(&ptls2->signal_request) != 0) {
// something is wrong, or there is already a usr2 in flight elsewhere
// try to wait for it to finish or wait for timeout
struct pollfd event = {signal_caught_cond, POLLIN, 0};
do {
err = poll(&event, 1, timeout * 1000);
} while (err == -1 && errno == EINTR);
if (err == -1 || (event.revents & POLLIN) == 0) {
// not ready after timeout: cancel this request
pthread_mutex_unlock(&in_signal_lock);
return 0;
}
}
// check for any stale signal_caught_cond events
struct pollfd event = {signal_caught_cond, POLLIN, 0};
do {
err = poll(&event, 1, 0);
} while (err == -1 && errno == EINTR);
if (err == -1) {
pthread_mutex_unlock(&in_signal_lock);
return 0;
}
if ((event.revents & POLLIN) != 0) {
// consume it before continuing
eventfd_t got;
do {
err = read(signal_caught_cond, &got, sizeof(eventfd_t));
} while (err == -1 && errno == EINTR);
if (err != sizeof(eventfd_t)) abort();
assert(got == 1); (void) got;
}
sig_atomic_t request = jl_atomic_exchange(&ptls2->signal_request, 1);
assert(request == 0 || request == -1);
request = 1;
int err = pthread_kill(ptls2->system_id, SIGUSR2);
// wait for thread to acknowledge or timeout
struct pollfd event = {signal_caught_cond, POLLIN, 0};
err = pthread_kill(ptls2->system_id, SIGUSR2);
if (err == 0) {
// wait for thread to acknowledge or timeout
struct pollfd event = {signal_caught_cond, POLLIN, 0};
do {
err = poll(&event, 1, timeout * 1000);
} while (err == -1 && errno == EINTR);
if (err != 1 || (event.revents & POLLIN) == 0)
err = -1;
}
if ((event.revents & POLLIN) == 0) {
if (err == -1) {
// not ready after timeout: try to cancel this request
if (jl_atomic_cmpswap(&ptls2->signal_request, &request, 0)) {
pthread_mutex_unlock(&in_signal_lock);
Expand All @@ -487,7 +517,7 @@ int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
// Now the other thread is waiting on exit_signal_cond (verify that here by
// checking it is 0, and add an acquire barrier for good measure)
request = jl_atomic_load_acquire(&ptls2->signal_request);
assert(request == 0); (void) request;
assert(request == 0 || request == -1); (void) request;
jl_atomic_store_release(&ptls2->signal_request, 4); // prepare to resume normally, but later code may change this
*ctx = *signal_context;
return 1;
Expand Down Expand Up @@ -546,6 +576,7 @@ static void jl_exit_thread0(int signo, jl_bt_element_t *bt_data, size_t bt_size)
}

// request:
// -1: processing
// 0: nothing [not from here]
// 1: get state & wait for request
// 2: throw sigint if `!defer_signal && io_wait` or if force throw threshold
Expand All @@ -561,22 +592,36 @@ void usr2_handler(int sig, siginfo_t *info, void *ctx)
if (ptls == NULL)
return;
int errno_save = errno;
// acknowledge that we saw the signal_request
sig_atomic_t request = jl_atomic_exchange(&ptls->signal_request, 0);
sig_atomic_t request = jl_atomic_load(&ptls->signal_request);
if (request == 0)
return;
if (!jl_atomic_cmpswap(&ptls->signal_request, &request, -1))
return;
if (request == 1) {
signal_context = jl_to_bt_context(ctx);
// acknowledge that we saw the signal_request and set signal_context
int err;
eventfd_t got = 1;
err = write(signal_caught_cond, &got, sizeof(eventfd_t));
if (err != sizeof(eventfd_t)) abort();
sig_atomic_t processing = -1;
jl_atomic_cmpswap(&ptls->signal_request, &processing, 0);
// wait for exit signal
do {
err = read(exit_signal_cond, &got, sizeof(eventfd_t));
} while (err == -1 && errno == EINTR);
if (err != sizeof(eventfd_t)) abort();
assert(got == 1);
request = jl_atomic_exchange(&ptls->signal_request, 0);
request = jl_atomic_exchange(&ptls->signal_request, -1);
signal_context = NULL;
assert(request == 2 || request == 3 || request == 4);
}
int err;
eventfd_t got = 1;
err = write(signal_caught_cond, &got, sizeof(eventfd_t));
if (err != sizeof(eventfd_t)) abort();
sig_atomic_t processing = -1;
jl_atomic_cmpswap(&ptls->signal_request, &processing, 0);
if (request == 2) {
int force = jl_check_force_sigint();
if (force || (!ptls->defer_signal && ptls->io_wait)) {
Expand Down

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