Re: [PATCH 2/4] exit: support non-blocking pidfds

On Tue, Sep 01, 2020 at 06:11:54PM +0200, Oleg Nesterov wrote:

On 08/31, Christian Brauner wrote:

quoted

--- a/kernel/exit.c
+++ b/kernel/exit.c

@@ -934,6 +934,7 @@ struct wait_opts {
 
 	wait_queue_entry_t		child_wait;
 	int			notask_error;
+	int			eagain_error;
 };
 
 static int eligible_pid(struct wait_opts *wo, struct task_struct *p)

@@ -1461,6 +1462,8 @@ static long do_wait(struct wait_opts *wo)
 
 notask:
 	retval = wo->notask_error;
+	if (!retval)
+		retval = wo->eagain_error;
 	if (!retval && !(wo->wo_flags & WNOHANG)) {
 		retval = -ERESTARTSYS;

I must have missed something but I don't understand why do we need
the new ->eagain_error and the change in do_wait().

quoted

@@ -1544,6 +1551,11 @@ static long kernel_waitid(int which, pid_t upid, struct waitid_info *infop,
 	wo.wo_flags	= options;
 	wo.wo_info	= infop;
 	wo.wo_rusage	= ru;
+	wo.eagain_error = 0;
+	if (f_flags & O_NONBLOCK) {
+		wo.wo_flags	|= WNOHANG;
+		wo.eagain_error	= -EAGAIN;
+	}
 	ret = do_wait(&wo);

Can't kernel_waitid() simply do

	if (f_flags & O_NONBLOCK)
		wo.wo_flags |= WNOHANG;
	ret = do_wait();
	if (!ret & (f_flags & O_NONBLOCK))
		ret = -EAGAIN;

?

Heh, indeed, that's even a smaller patch. Will change to that!

Thanks for the review, Oleg!
Christia