linux-ext4 - Re: Linux 5.3-rc8

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Message-ID: <20190916014050.GA7002@darwi-home-pc>
Date:   Mon, 16 Sep 2019 03:40:50 +0200
From:   "Ahmed S. Darwish" <darwish.07@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Lennart Poettering <mzxreary@...inter.de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        zhangjs <zachary@...shancloud.com>, linux-ext4@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.3-rc8

On Sun, Sep 15, 2019 at 09:29:55AM -0700, Linus Torvalds wrote:
> On Sat, Sep 14, 2019 at 11:51 PM Lennart Poettering
> <mzxreary@...inter.de> wrote:
> >
> > Oh man. Just spend 5min to understand the situation, before claiming
> > this was garbage or that was garbage. The code above does not block
> > boot.
> 
> Yes it does. You clearly didn't read the thread.
> 
> > It blocks startup of services that explicit order themselves
> > after the code above. There's only a few services that should do that,
> > and the main system boots up just fine without waiting for this.
> 
> That's a nice theory, but it doesn't actually match reality.
> 
> There are clearly broken setups that use this for things that it
> really shouldn't be used for. Asking for true randomness at boot
> before there is any indication that randomness exists, and then just
> blocking with no further action that could actually _generate_ said
> randomness.
> 
> If your description was true that the system would come up and be
> usable while the blocked thread is waiting for that to happen, things
> would be fine.
>

A small note here, especially after I've just read the commit log of
72dbcf721566 ('Revert ext4: "make __ext4_get_inode_loc plug"'), which
unfairly blames systemd there.

Yes, the systemd-random-seed(8) process blocks, but this is an
isolated process, and it's only there as a synchronization point and
to load/restore random seeds from disk across reboots.

The wisdom of having a sysnchronization service ("before/after urandom
CRNG is inited") can be debated. That service though, and systemd in
general, did _not_ block the overall system boot.

What blocked the system boot was GDM/gnome-session implicitly calling
getrandom() for the Xorg MIT cookie. This was shown in the strace log
below:

   https://lkml.kernel.org/r/20190910173243.GA3992@darwi-home-pc

thanks,

-- 
darwi
http://darwish.chasingpointers.com