linux-kernel - RE: [PATCH 4.19 56/81] kernel/sysctl.c: do not override max

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Date:   Thu, 17 Oct 2019 11:25:47 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Michal Hocko' <mhocko@...nel.org>, Pavel Machek <pavel@...x.de>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: RE: [PATCH 4.19 56/81] kernel/sysctl.c: do not override max_threads
 provided by userspace

From: Michal Hocko
> Sent: 17 October 2019 12:05
...
> > Plus, I don't see any locking here, should this be WRITE_ONCE() at
> > minimum?
> 
> Why would that matter? Do you expect several root processes race to set
> the value?

One of them wins. No one is going to notice is the value is set an extra time.

WRITE_ONCE() is rarely required.
Probably only if other code is going to update the value after seeing the first write.
(eg if you are unlocking a mutex - although they have to be more complex)

READ_ONCE() is a different matter.
IMHO the compiler shouldn't be allowed to do more reads than the source requests.

	David

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