linux-kernel - Re: [RFC] [PATCH] Pre-emption control for userspace

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Message-ID: <20140304222356.41c55bbc@alan.etchedpixels.co.uk>
Date:	Tue, 4 Mar 2014 22:23:56 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Khalid Aziz <khalid.aziz@...cle.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	Ingo Molnar <mingo@...nel.org>, peterz@...radead.org,
	akpm@...ux-foundation.org, andi.kleen@...el.com, rob@...dley.net,
	viro@...iv.linux.org.uk, oleg@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace

Obvious bug

| Usage model is a thread mmaps this file during initialization. It then
| writes a 1 to the mmap'd file after it grabs the lock in its critical
| section where it wants immunity from pre-emption.

You need to write it first or you can be pre-empted taking the lock
before asking for immunity.

Presumably you could equally use something to group tasks (say a control
group of some form) and implement voluntary pre-emption within the group
only when in user space. Ie they only pre-empt each other by yielding but
they can be pre-empted by other tasks outside the group ?

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