[RFC PATCH] relay: avoid relay_open_buf inproperly fails in buffer-only mode

[Philo Lu <lulie@linux.alibaba.com>]

In buffer-only mode, relay_open(NULL, NULL, ...) is used to create the
buffer first, where chan->has_base_filename is not set. Though we still
need to call chan->cb->create_buf_file in relay_open_buf() to retrieve
global info for global buffer, the create_buf_file callback should
return NULL. With IS_ERR_OR_NULL() check, relay_open fails because of
the returned NULL dentry, so this patch reverts back to the WARN_ON()
version and add a comment for this behavior.

Here is an example after fix:
```
struct dentry *my_create_buf_file(const char *filename,
            struct dentry *parent, umode_t mode,
            struct rchan_buf *buf, int *is_global)
{
    if (!filename)
        return NULL;

    return debugfs_create_file(filename, mode, parent, buf,
                &relay_file_operations);
}

relay_cb.create_buf_file = my_create_buf_file
relay_chan = relay_open(NULL, NULL,
                    subbuf_size, subbuf_num,
                    &relay_cb, NULL);
relay_late_setup_files(relay_chan, filename, parent);
```

But before fix, the create_buf_file callback must be something like:
```
struct dentry *my_create_buf_file(const char *filename,
            struct dentry *parent, umode_t mode,
            struct rchan_buf *buf, int *is_global)
{
    if (!filename)
        return ERR_PTR(1); // a valid ptr is necessary for relay_open

    return debugfs_create_file(filename, mode, parent, buf,
                &relay_file_operations);
}
```

I'm not sure if this revertion proper because it may break existing use
cases. I think we can also remove the WARN_ON check instead.

Fixes: 2c1cf00eeacb ("relay: check return of create_buf_file() properly")
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
---
 kernel/relay.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index 83fe0325cde1..0700745447c1 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -395,7 +395,8 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
 		dentry = chan->cb->create_buf_file(NULL, NULL,
 						   S_IRUSR, buf,
 						   &chan->is_global);
-		if (IS_ERR_OR_NULL(dentry))
+		/* has_base_filename not set, so dentry should be NULL */
+		if (WARN_ON(dentry))
 			goto free_buf;
 	}

--
2.32.0.3.g01195cf9f