Re: [PATCH bpf v2 1/3] bpf: fix a rcu_sched stall issue with bpf task/task_file iterator

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Aug 18, 2020 at 05:30:37PM -0700, Yonghong Song wrote:
> 
> 
> On 8/18/20 5:05 PM, Alexei Starovoitov wrote:
> > On Tue, Aug 18, 2020 at 03:23:09PM -0700, Yonghong Song wrote:
> > > 
> > > We did not use cond_resched() since for some iterators, e.g.,
> > > netlink iterator, where rcu read_lock critical section spans between
> > > consecutive seq_ops->next(), which makes impossible to do cond_resched()
> > > in the key while loop of function bpf_seq_read().
> > 
> > but after this patch we can, right?
> 
> We can do cond_resched() after seq->op->stop(). See more below.
> 
> > 
> > > +/* maximum visited objects before bailing out */
> > > +#define MAX_ITER_OBJECTS	1000000
> > > +
> > >   /* bpf_seq_read, a customized and simpler version for bpf iterator.
> > >    * no_llseek is assumed for this file.
> > >    * The following are differences from seq_read():
> > > @@ -79,7 +82,7 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
> > >   {
> > >   	struct seq_file *seq = file->private_data;
> > >   	size_t n, offs, copied = 0;
> > > -	int err = 0;
> > > +	int err = 0, num_objs = 0;
> > >   	void *p;
> > >   	mutex_lock(&seq->lock);
> > > @@ -135,6 +138,7 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
> > >   	while (1) {
> > >   		loff_t pos = seq->index;
> > > +		num_objs++;
> > >   		offs = seq->count;
> > >   		p = seq->op->next(seq, p, &seq->index);
> > >   		if (pos == seq->index) {
> > > @@ -153,6 +157,15 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
> > >   		if (seq->count >= size)
> > >   			break;
> > > +		if (num_objs >= MAX_ITER_OBJECTS) {
> > > +			if (offs == 0) {
> > > +				err = -EAGAIN;
> > > +				seq->op->stop(seq, p);
> > > +				goto done;
> > > +			}
> > > +			break;
> > > +		}
> > > +
> > 
> > should this block be after op->show() and error processing?
> > Otherwise bpf_iter_inc_seq_num() will be incorrectly incremented?
> 
> The purpose of op->next() is to calculate the "next" object position,
> stored in the seq private data. So for next read() syscall, start()
> will try to fetch the data based on the info in seq private data.
> 
> This is true for conditions "if (seq->count >= size) break"
> in the above so next op->start() can try to locate the correct
> object. The same is for this -EAGAIN thing.
> 
> > 
> > >   		err = seq->op->show(seq, p);
> > >   		if (err > 0) {
> > >   			bpf_iter_dec_seq_num(seq);
> > 
> > After op->stop() we can do cond_resched() in all cases,
> > since rhashtable walk does rcu_unlock in stop() callback, right?
> 
> Yes, we can. I am thinking since we return to user space,
> cond_resched() might not be needed since returning to user space
> will trigger some kind of scheduling. This patch fixed
> the rcu stall issue. But if my understanding is incorrect,
> I am happy to add cond_reched().

ahh. you're correct on both counts. Applied all three patches to bpf tree. thanks!