On Tue, Aug 11, 2020 at 11:12:36AM -0700, Josh Triplett wrote:
> As far as I can tell, O_NONBLOCK has no effect on a pidfd. When calling
> waitid on a pidfd for a running process, it always blocks unless you
> provide WNOHANG.
>
> I don't think anything depends on that behavior. Would it be possible to
> make O_NONBLOCK on a pidfd cause waitid on a running process to return
> EWOULDBLOCK?
>
> This would make it easier to use pidfd in some non-blocking event loops.
Hey Josh,
Just to see I did a _horrible_ draft (cf. [1]) and it seems doable to me
and if you can provide a good rationale and a use-case then I think that
would be ok.
[1]:
diff --git a/kernel/exit.c b/kernel/exit.c
index 727150f28103..b43a0e126cee 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -934,6 +934,7 @@ struct wait_opts {
wait_queue_entry_t child_wait;
int notask_error;
+ bool wo_pidfd_nonblock;
};
static int eligible_pid(struct wait_opts *wo, struct task_struct *p)
@@ -1462,6 +1463,11 @@ static long do_wait(struct wait_opts *wo)
notask:
retval = wo->notask_error;
if (!retval && !(wo->wo_flags & WNOHANG)) {
+ if (wo->wo_pidfd_nonblock) {
+ retval = -EWOULDBLOCK;
+ goto end;
+ }
+
retval = -ERESTARTSYS;
if (!signal_pending(current)) {
schedule();
@@ -1495,9 +1501,10 @@ static long kernel_waitid(int which, pid_t upid, struct
waitid_info *infop,
int options, struct rusage *ru)
{
struct wait_opts wo;
+ struct fd f;
struct pid *pid = NULL;
enum pid_type type;
- long ret;
+ long ret = 0;
if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED|
__WNOTHREAD|__WCLONE|__WALL))
@@ -1505,6 +1512,7 @@ static long kernel_waitid(int which, pid_t upid, struct
waitid_info *infop,
if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
return -EINVAL;
+ wo.wo_pidfd_nonblock = false;
switch (which) {
case P_ALL:
type = PIDTYPE_MAX;
@@ -1531,9 +1539,22 @@ static long kernel_waitid(int which, pid_t upid, struct
waitid_info *infop,
if (upid < 0)
return -EINVAL;
- pid = pidfd_get_pid(upid);
+ f = fdget(upid);
+ if (!f.file)
+ return ERR_PTR(-EBADF);
+
+ pid = pidfd_pid(f.file);
+
+ if (f.file->f_flags & O_NONBLOCK)
+ wo.wo_pidfd_nonblock = true;
+
if (IS_ERR(pid))
- return PTR_ERR(pid);
+ ret = PTR_ERR(pid);
+ else
+ get_pid(pid);
+ fdput(f);
+ if (ret)
+ return ret;
break;
default:
return -EINVAL;
