linux-kernel - Re: [RFC PATCH] drivers/char/random.c: Is reducing locking range like this safe?

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Date:	9 Jun 2014 05:23:17 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, tytso@....edu
Cc:	hpa@...ux.intel.com, linux-kernel@...r.kernel.org,
	mingo@...nel.org, price@....edu
Subject: Re: [RFC PATCH] drivers/char/random.c: Is reducing locking range like this safe?

> Sigh, adventures in "unable to mount root filesystem" currently underway.

PEBKAC resolved.  (I had CONFIG_ATA=m for some testing, but forgot
to change it back to y when compiling a kernel I expected to boot.)

32 MiB write:

	    Without patch		With patch
0 readers: 0.495523 0.494998		0.515995 0.516026
1 reader:  0.842245 0.845308		0.704283 0.704318
2 readers: 1.18658  1.18762		0.904635 0.904844
3 readers: 1.49615  1.49616		1.14311  1.16836

I haven't rebooted back to the without-patch kernel to see if the 20 ms
loss is consistent, but obviously that needs fixing.  (There's more than
just that patch I posted in there.)

Anyway, it's noticeable, but maybe not worth fixing.

The interesting question is whether we should get rid of out[].
That's obscurity that contributes no security, and thus a Bad
Thing by definition.
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