[PATCH] Fix pthreadpool fork behaviour
Volker Lendecke
Volker.Lendecke at SerNet.DE
Thu Aug 31 09:42:08 UTC 2017
On Thu, Aug 31, 2017 at 08:26:55AM +0200, Volker Lendecke via samba-technical wrote:
> > (note the change from away -> way in the second line of text).
>
> Sure, thanks! New patch attached.
Now really with patch. Same code, just the comment typo fixed.
Volker
--
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-------------- next part --------------
From b8317d11014d63e85349f32c761dbe9496cf1008 Mon Sep 17 00:00:00 2001
From: Volker Lendecke <vl at samba.org>
Date: Mon, 28 Aug 2017 16:38:19 +0200
Subject: [PATCH 1/2] pthreadpool: Fix fork behaviour
glibc's pthread_cond_wait(&c, &m) increments m.__data.__nusers, making
pthread_mutex_destroy return EBUSY. Thus we can't allow any thread waiting for
a job across a fork. Also, the state of the condvar itself is unclear across a
fork. Right now to me it looks like an initialized but unused condvar can be
used in the child. Busy worker threads don't cause any trouble here, they don't
hold mutexes or condvars. Also, they can't reach the condvar because _prepare
holds all mutexes.
Signed-off-by: Volker Lendecke <vl at samba.org>
---
lib/pthreadpool/pthreadpool.c | 67 +++++++++++++++++++++++++++++++++++++++++--
1 file changed, 65 insertions(+), 2 deletions(-)
diff --git a/lib/pthreadpool/pthreadpool.c b/lib/pthreadpool/pthreadpool.c
index f97cdcc6c15..23885aa6d11 100644
--- a/lib/pthreadpool/pthreadpool.c
+++ b/lib/pthreadpool/pthreadpool.c
@@ -89,6 +89,13 @@ struct pthreadpool {
* Number of idle threads
*/
int num_idle;
+
+ /*
+ * Condition variable indicating that we should quickly go
+ * away making way for fork() without anybody waiting on
+ * pool->condvar.
+ */
+ pthread_cond_t *prefork_cond;
};
static pthread_mutex_t pthreadpools_mutex = PTHREAD_MUTEX_INITIALIZER;
@@ -148,6 +155,7 @@ int pthreadpool_init(unsigned max_threads, struct pthreadpool **presult,
pool->num_threads = 0;
pool->max_threads = max_threads;
pool->num_idle = 0;
+ pool->prefork_cond = NULL;
ret = pthread_mutex_lock(&pthreadpools_mutex);
if (ret != 0) {
@@ -169,6 +177,47 @@ int pthreadpool_init(unsigned max_threads, struct pthreadpool **presult,
return 0;
}
+static void pthreadpool_prepare_pool(struct pthreadpool *pool)
+{
+ pthread_cond_t prefork_cond = PTHREAD_COND_INITIALIZER;
+ int ret;
+
+ ret = pthread_mutex_lock(&pool->mutex);
+ assert(ret == 0);
+
+ while (pool->num_idle != 0) {
+ /*
+ * Exit all idle threads, which are all blocked in
+ * pool->condvar. In the child we can destroy the
+ * pool, which would result in undefined behaviour in
+ * the pthread_cond_destroy(pool->condvar). glibc just
+ * blocks here.
+ */
+ pool->prefork_cond = &prefork_cond;
+
+ ret = pthread_cond_signal(&pool->condvar);
+ assert(ret == 0);
+
+ ret = pthread_cond_wait(&prefork_cond, &pool->mutex);
+ assert(ret == 0);
+
+ pool->prefork_cond = NULL;
+ }
+
+ ret = pthread_cond_destroy(&prefork_cond);
+ assert(ret == 0);
+
+ /*
+ * Probably it's well-defined somewhere: What happens to
+ * condvars after a fork? The rationale of pthread_atfork only
+ * writes about mutexes. So better be safe than sorry and
+ * destroy/reinit pool->condvar across a fork.
+ */
+
+ ret = pthread_cond_destroy(&pool->condvar);
+ assert(ret == 0);
+}
+
static void pthreadpool_prepare(void)
{
int ret;
@@ -180,8 +229,7 @@ static void pthreadpool_prepare(void)
pool = pthreadpools;
while (pool != NULL) {
- ret = pthread_mutex_lock(&pool->mutex);
- assert(ret == 0);
+ pthreadpool_prepare_pool(pool);
pool = pool->next;
}
}
@@ -194,6 +242,8 @@ static void pthreadpool_parent(void)
for (pool = DLIST_TAIL(pthreadpools);
pool != NULL;
pool = DLIST_PREV(pool)) {
+ ret = pthread_cond_init(&pool->condvar, NULL);
+ assert(ret == 0);
ret = pthread_mutex_unlock(&pool->mutex);
assert(ret == 0);
}
@@ -216,6 +266,8 @@ static void pthreadpool_child(void)
pool->head = 0;
pool->num_jobs = 0;
+ ret = pthread_cond_init(&pool->condvar, NULL);
+ assert(ret == 0);
ret = pthread_mutex_unlock(&pool->mutex);
assert(ret == 0);
}
@@ -406,6 +458,17 @@ static void *pthreadpool_server(void *arg)
&pool->condvar, &pool->mutex, &ts);
pool->num_idle -= 1;
+ if (pool->prefork_cond != NULL) {
+ /*
+ * Me must allow fork() to continue
+ * without anybody waiting on
+ * &pool->condvar.
+ */
+ pthread_cond_signal(pool->prefork_cond);
+ pthreadpool_server_exit(pool);
+ return NULL;
+ }
+
if (res == ETIMEDOUT) {
if (pool->num_jobs == 0) {
--
2.11.0
From 27972e86255890f03fcda7e0bf48af28c71f0f43 Mon Sep 17 00:00:00 2001
From: Volker Lendecke <vl at samba.org>
Date: Tue, 29 Aug 2017 21:57:54 +0200
Subject: [PATCH 2/2] pthreadpool: Test fork with an active thread
Signed-off-by: Volker Lendecke <vl at samba.org>
---
lib/pthreadpool/tests.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 114 insertions(+)
diff --git a/lib/pthreadpool/tests.c b/lib/pthreadpool/tests.c
index c4d2e6a1382..999118286eb 100644
--- a/lib/pthreadpool/tests.c
+++ b/lib/pthreadpool/tests.c
@@ -7,6 +7,7 @@
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
+#include <signal.h>
#include "pthreadpool_pipe.h"
#include "pthreadpool_tevent.h"
@@ -192,6 +193,113 @@ static int test_fork(void)
return 0;
}
+static void busyfork_job(void *private_data)
+{
+ return;
+}
+
+static int test_busyfork(void)
+{
+ struct pthreadpool_pipe *p;
+ int fds[2];
+ struct pollfd pfd;
+ pid_t child, waitret;
+ int ret, jobnum, wstatus;
+
+ ret = pipe(fds);
+ if (ret == -1) {
+ perror("pipe failed");
+ return -1;
+ }
+
+ ret = pthreadpool_pipe_init(1, &p);
+ if (ret != 0) {
+ fprintf(stderr, "pthreadpool_pipe_init failed: %s\n",
+ strerror(ret));
+ return -1;
+ }
+
+ ret = pthreadpool_pipe_add_job(p, 1, busyfork_job, NULL);
+ if (ret != 0) {
+ fprintf(stderr, "pthreadpool_add_job failed: %s\n",
+ strerror(ret));
+ return -1;
+ }
+
+ ret = pthreadpool_pipe_finished_jobs(p, &jobnum, 1);
+ if (ret != 1) {
+ fprintf(stderr, "pthreadpool_pipe_finished_jobs failed\n");
+ return -1;
+ }
+
+ poll(NULL, 0, 200);
+
+ child = fork();
+ if (child < 0) {
+ perror("fork failed");
+ return -1;
+ }
+
+ if (child == 0) {
+ ret = pthreadpool_pipe_destroy(p);
+ if (ret != 0) {
+ fprintf(stderr, "pthreadpool_pipe_destroy failed: "
+ "%s\n", strerror(ret));
+ exit(1);
+ }
+ exit(0);
+ }
+
+ ret = close(fds[1]);
+ if (ret == -1) {
+ perror("close failed");
+ return -1;
+ }
+
+ pfd = (struct pollfd) { .fd = fds[0], .events = POLLIN };
+
+ ret = poll(&pfd, 1, 5000);
+ if (ret == -1) {
+ perror("poll failed");
+ return -1;
+ }
+ if (ret == 0) {
+ fprintf(stderr, "Child did not exit for 5 seconds\n");
+ /*
+ * The child might hang forever in
+ * pthread_cond_destroy for example. Be kind to the
+ * system and kill it.
+ */
+ kill(child, SIGTERM);
+ return -1;
+ }
+ if (ret != 1) {
+ fprintf(stderr, "poll returned %d -- huh??\n", ret);
+ return -1;
+ }
+
+ poll(NULL, 0, 200);
+
+ waitret = waitpid(child, &wstatus, WNOHANG);
+ if (waitret != child) {
+ fprintf(stderr, "waitpid returned %d\n", (int)waitret);
+ return -1;
+ }
+
+ if (!WIFEXITED(wstatus)) {
+ fprintf(stderr, "child did not properly exit\n");
+ return -1;
+ }
+
+ ret = WEXITSTATUS(wstatus);
+ if (ret != 0) {
+ fprintf(stderr, "child returned %d\n", ret);
+ return -1;
+ }
+
+ return 0;
+}
+
static void test_tevent_wait(void *private_data)
{
int *timeout = private_data;
@@ -301,6 +409,12 @@ int main(void)
return 1;
}
+ ret = test_busyfork();
+ if (ret != 0) {
+ fprintf(stderr, "test_busyfork failed\n");
+ return 1;
+ }
+
printf("success\n");
return 0;
}
--
2.11.0
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