random...@fastmail.us:
> On Wed, Jun 3, 2015, at 10:43, Marko Rauhamaa wrote:
>> However, the child process needs to be prepared for os.close() to
>> block indefinitely because of an NFS problem or because SO_LINGER has
>> been specified by the parent, for example. Setting the close-on-exec
>> flag doesn't help there.
>
> Out of curiosity, does exec block in this situation?
I didn't try it, but it is apparent in the source code:
========================================================================
void do_close_on_exec(struct files_struct *files)
{
unsigned i;
struct fdtable *fdt;
/* exec unshares first */
spin_lock(&files->file_lock);
for (i = 0; ; i++) {
unsigned long set;
unsigned fd = i * BITS_PER_LONG;
fdt = files_fdtable(files);
if (fd >= fdt->max_fds)
break;
set = fdt->close_on_exec[i];
if (!set)
continue;
fdt->close_on_exec[i] = 0;
for ( ; set ; fd++, set >>= 1) {
struct file *file;
if (!(set & 1))
continue;
file = fdt->fd[fd];
if (!file)
continue;
rcu_assign_pointer(fdt->fd[fd], NULL);
__put_unused_fd(files, fd);
spin_unlock(&files->file_lock);
filp_close(file, files);
cond_resched();
spin_lock(&files->file_lock);
}
}
spin_unlock(&files->file_lock);
}
int filp_close(struct file *filp, fl_owner_t id)
{
int retval = 0;
if (!file_count(filp)) {
printk(KERN_ERR "VFS: Close: file count is 0\n");
return 0;
}
if (filp->f_op->flush)
retval = filp->f_op->flush(filp, id);
if (likely(!(filp->f_mode & FMODE_PATH))) {
dnotify_flush(filp, id);
locks_remove_posix(filp, id);
}
fput(filp);
return retval;
}
========================================================================
Now, the kernel NFS code specifies a flush() method, which can block and
even fail.
Sockets don't have a flush() method. So closing a socket cannot fail.
However, fput(), which decrements the reference count, may block if
lingering has been specified for the socket.
So I wasn't all that wrong earlier after all: whoever closes a socket
last will linger. Thus, the parent (who wanted to linger) might zip
through closing a socket while the unwitting child process will suffer
the lingering delay before it gets to exec.
However, there's this comment in inet_release():
/* [...]
* If the close is due to the process exiting, we never
* linger..
*/
Marko
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