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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 10 Sep 2019 12:33:12 +0100
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Subject: Re: Linux 5.3-rc8
To:     "Ahmed S. Darwish" <darwish.07@gmail.com>
Cc:     "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, zhangjs <zachary@baishancloud.com>,
        linux-ext4@vger.kernel.org,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
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On Tue, Sep 10, 2019 at 5:21 AM Ahmed S. Darwish <darwish.07@gmail.com> wrote:
>
> The commit b03755ad6f33 (ext4: make __ext4_get_inode_loc plug), [1]
> which was merged in v5.3-rc1, *always* leads to a blocked boot on my
> system due to low entropy.

Exactly what is it that blocks on entropy? Nobody should do that
during boot, because on some systems entropy is really really low
(think flash memory with polling IO etc).

That said, I would have expected that any PC gets plenty of entropy.
Are you sure it's entropy that is blocking, and not perhaps some odd
"forgot to unplug" situation?

> Can this even be considered a user-space breakage? I'm honestly not
> sure. On my modern RDRAND-capable x86, just running rng-tools rngd(8)
> early-on fixes the problem. I'm not sure about the status of older
> CPUs though.

It's definitely breakage, although rather odd. I would have expected
us to have other sources of entropy than just the disk. Did we stop
doing low bits of TSC from timer interrupts etc?

Ted, either way - ext4 IO patterns or random number entropy - this is
your code. Comments?

                 Linus