On Thu, Jun 06, 2013 at 09:49:11AM -0700, Alex Wang wrote:
> The 2/17. 3/17 look good to me.
Thanks.
> For this patch, want to ask few questions:
>
> 1. why does the previous implementation cannot guarantee thread safety (An
> example?)? Is this related to the sigchld_ related functions?
A single-threaded process can ensure that a signal handler doesn't
run during a section of code by blocking and unblocking the signal
around that section of code. A multithreaded process can't do that,
because the signal handler might get invoked from any thread.
> 2. For the "process_register()", the comment still talks about blocking
> SIGCHLD signal, which can be confusing
Good catch. I deleted that sentence from the comment, since it's no
longer correct. (It was also ungrammatical.)
> On Wed, Jun 5, 2013 at 1:05 PM, Ben Pfaff <b...@nicira.com> wrote:
>
> > Signed-off-by: Ben Pfaff <b...@nicira.com>
> > ---
> > lib/process.c | 103
> > ++++++++++++++++----------------------------------
> > lib/process.h | 1 +
> > ovsdb/ovsdb-server.c | 7 ++-
> > 3 files changed, 39 insertions(+), 72 deletions(-)
> >
> > diff --git a/lib/process.c b/lib/process.c
> > index 4102308..22dd266 100644
> > --- a/lib/process.c
> > +++ b/lib/process.c
> > @@ -44,9 +44,9 @@ struct process {
> > char *name;
> > pid_t pid;
> >
> > - /* Modified by signal handler. */
> > - volatile bool exited;
> > - volatile int status;
> > + /* State. */
> > + bool exited;
> > + int status;
> > };
> >
> > /* Pipe used to signal child termination. */
> > @@ -55,9 +55,6 @@ static int fds[2];
> > /* All processes. */
> > static struct list all_processes = LIST_INITIALIZER(&all_processes);
> >
> > -static bool sigchld_is_blocked(void);
> > -static void block_sigchld(sigset_t *);
> > -static void unblock_sigchld(const sigset_t *);
> > static void sigchld_handler(int signr OVS_UNUSED);
> >
> > /* Initializes the process subsystem (if it is not already initialized).
> > Calls
> > @@ -155,8 +152,6 @@ process_register(const char *name, pid_t pid)
> > struct process *p;
> > const char *slash;
> >
> > - ovs_assert(sigchld_is_blocked());
> > -
> > p = xzalloc(sizeof *p);
> > p->pid = pid;
> > slash = strrchr(name, '/');
> > @@ -181,7 +176,6 @@ process_register(const char *name, pid_t pid)
> > int
> > process_start(char **argv, struct process **pp)
> > {
> > - sigset_t oldsigs;
> > pid_t pid;
> > int error;
> >
> > @@ -192,16 +186,13 @@ process_start(char **argv, struct process **pp)
> > return error;
> > }
> >
> > - block_sigchld(&oldsigs);
> > pid = fork();
> > if (pid < 0) {
> > - unblock_sigchld(&oldsigs);
> > VLOG_WARN("fork failed: %s", strerror(errno));
> > return errno;
> > } else if (pid) {
> > /* Running in parent process. */
> > *pp = process_register(argv[0], pid);
> > - unblock_sigchld(&oldsigs);
> > return 0;
> > } else {
> > /* Running in child process. */
> > @@ -209,7 +200,6 @@ process_start(char **argv, struct process **pp)
> > int fd;
> >
> > fatal_signal_fork();
> > - unblock_sigchld(&oldsigs);
> > for (fd = 3; fd < fd_max; fd++) {
> > close(fd);
> > }
> > @@ -225,12 +215,7 @@ void
> > process_destroy(struct process *p)
> > {
> > if (p) {
> > - sigset_t oldsigs;
> > -
> > - block_sigchld(&oldsigs);
> > list_remove(&p->node);
> > - unblock_sigchld(&oldsigs);
> > -
> > free(p->name);
> > free(p);
> > }
> > @@ -265,13 +250,7 @@ process_name(const struct process *p)
> > bool
> > process_exited(struct process *p)
> > {
> > - if (p->exited) {
> > - return true;
> > - } else {
> > - char buf[_POSIX_PIPE_BUF];
> > - ignore(read(fds[0], buf, sizeof buf));
> > - return false;
> > - }
> > + return p->exited;
> > }
> >
> > /* Returns process 'p''s exit status, as reported by waitpid(2).
> > @@ -313,6 +292,35 @@ process_status_msg(int status)
> > return ds_cstr(&ds);
> > }
> >
> > +/* Executes periodic maintenance activities required by the process
> > module. */
> > +void
> > +process_run(void)
> > +{
> > + char buf[_POSIX_PIPE_BUF];
> > +
> > + if (!list_is_empty(&all_processes) && read(fds[0], buf, sizeof buf) >
> > 0) {
> > + struct process *p;
> > +
> > + LIST_FOR_EACH (p, node, &all_processes) {
> > + if (!p->exited) {
> > + int retval, status;
> > + do {
> > + retval = waitpid(p->pid, &status, WNOHANG);
> > + } while (retval == -1 && errno == EINTR);
> > + if (retval == p->pid) {
> > + p->exited = true;
> > + p->status = status;
> > + } else if (retval < 0) {
> > + VLOG_WARN("waitpid: %s", strerror(errno));
> > + p->exited = true;
> > + p->status = -1;
> > + }
> > + }
> > + }
> > + }
> > +}
> > +
> > +
> > /* Causes the next call to poll_block() to wake up when process 'p' has
> > * exited. */
> > void
> > @@ -353,50 +361,5 @@ process_search_path(const char *name)
> > static void
> > sigchld_handler(int signr OVS_UNUSED)
> > {
> > - struct process *p;
> > -
> > - COVERAGE_INC(process_sigchld);
> > - LIST_FOR_EACH (p, node, &all_processes) {
> > - if (!p->exited) {
> > - int retval, status;
> > - do {
> > - retval = waitpid(p->pid, &status, WNOHANG);
> > - } while (retval == -1 && errno == EINTR);
> > - if (retval == p->pid) {
> > - p->exited = true;
> > - p->status = status;
> > - } else if (retval < 0) {
> > - /* XXX We want to log something but we're in a signal
> > - * handler. */
> > - p->exited = true;
> > - p->status = -1;
> > - }
> > - }
> > - }
> > ignore(write(fds[1], "", 1));
> > }
> > -
> > -static bool
> > -sigchld_is_blocked(void)
> > -{
> > - sigset_t sigs;
> > -
> > - xpthread_sigmask(SIG_SETMASK, NULL, &sigs);
> > - return sigismember(&sigs, SIGCHLD);
> > -}
> > -
> > -static void
> > -block_sigchld(sigset_t *oldsigs)
> > -{
> > - sigset_t sigchld;
> > -
> > - sigemptyset(&sigchld);
> > - sigaddset(&sigchld, SIGCHLD);
> > - xpthread_sigmask(SIG_BLOCK, &sigchld, oldsigs);
> > -}
> > -
> > -static void
> > -unblock_sigchld(const sigset_t *oldsigs)
> > -{
> > - xpthread_sigmask(SIG_SETMASK, oldsigs, NULL);
> > -}
> > diff --git a/lib/process.h b/lib/process.h
> > index 6d1410b..d17737d 100644
> > --- a/lib/process.h
> > +++ b/lib/process.h
> > @@ -33,6 +33,7 @@ bool process_exited(struct process *);
> > int process_status(const struct process *);
> > char *process_status_msg(int);
> >
> > +void process_run(void);
> > void process_wait(struct process *);
> >
> > char *process_search_path(const char *);
> > diff --git a/ovsdb/ovsdb-server.c b/ovsdb/ovsdb-server.c
> > index b9afd8c..20e1964 100644
> > --- a/ovsdb/ovsdb-server.c
> > +++ b/ovsdb/ovsdb-server.c
> > @@ -230,8 +230,11 @@ main(int argc, char *argv[])
> > for (i = 0; i < n_dbs; i++) {
> > ovsdb_trigger_run(dbs[i].db, time_msec());
> > }
> > - if (run_process && process_exited(run_process)) {
> > - exiting = true;
> > + if (run_process) {
> > + process_run();
> > + if (process_exited(run_process)) {
> > + exiting = true;
> > + }
> > }
> >
> > /* update Manager status(es) every 5 seconds */
> > --
> > 1.7.2.5
> >
> > _______________________________________________
> > dev mailing list
> > dev@openvswitch.org
> > http://openvswitch.org/mailman/listinfo/dev
> >
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