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SubjectRe: Patch 0727d35de ("Make initramfs honor CONFIG_DEVTMPFS_MOUNT") breaks boot
On Sun 2017-09-10 18:43:24, Rob Landley wrote:
> On 05/25/2017 01:13 AM, Michael Ellerman wrote:
> > Hi Rob,
> >
> > This is breaking a bunch of my powerpc boxes, for the exact same
> > reason, they use a config that has DEVTMPFS_MOUNT=y and that trips
> > up the initramfs.
>
> I've continued to use this locally but should probably make another
> stab at submitting upstream. The obvious workaround until debian fixes
> its 100% obvious bug seems to be:
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index f8893dc..f57d5df 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2417,7 +2417,17 @@ static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
> err = -EBUSY;
> if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
> path->mnt->mnt_root == path->dentry)
> + {
> + if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT) &&
> + !strcmp(path->mnt->mnt_sb->s_type->name, "devtmpfs"))
> + {
> + printk(KERN_WARNING "Debian bug workaround for devtmpfs overmount.");
> + printk(KERN_WARNING "This line doesn't output for some reason.");
> +
> + err = 0;
> + }
> goto unlock;
> + }
>
> err = -EINVAL;
> if (d_is_symlink(newmnt->mnt.mnt_root))
>
> Except for the second printk line: If you boot with rdinit=/bin/hush
> then the first time you mount -t devtmpfs /dev /dev after boot (with
> CONFIG_DEVTMPFS_MOUNT already having mounted it), you get the 0 return
> value but the last printk() doesn't output? The second and later times
> you repeat it, both printk() lines are output.
>
> What's up with printk?
>
> (I added the second printk because the _first_ one wasn't outputting
> that first time. Something is happening to flush the printk() queue
> instead of writing it out?

You need to add "\n" at the end of the line. Otherwise, it expects
that the message would continue and puts it into a cont buffer.
The buffer is flushed only when another non-continuous message
is added.

This problem is more visible since the commit 5c2992ee7fd8a29d0412
("printk: remove console flushing special cases for partial buffered
lines").

Hmm, Linus wanted to add a timer that would always flush
the cont buffer in a reasonable time frame. But there was a risk
of deadlocks because of circular timer<->printk dependency,
so it never happened.

Maybe, we could setup the timer via an irq_work. We already use
this trick for flushing deferred printk and waking klogd.
It is not nice but it would be easier than the previous mess.

Best Regards,
Petr

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