linux-kernel - Re: [PATCH 2/5] sched: Fix cfs_bandwidth misuse of hrtimer_expires

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Message-ID: <xm2661sxyqq3.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Wed, 16 Oct 2013 12:12:52 -0700
From:	bsegall@...gle.com
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, pjt@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] sched: Fix cfs_bandwidth misuse of hrtimer_expires_remaining

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Oct 16, 2013 at 11:16:17AM -0700, Ben Segall wrote:
>> + * Requires cfs_b->lock for hrtimer_expires_remaining to be safe against the
>> + * hrtimer base being cleared by __hrtimer_start_range_ns. This is possibly
>> + * incorrect if we ever built with CPU_HOTPLUG.
>
> Do clarify; most people build with hotplug enabled; due to wanting
> suspend/resume stuff etc..

Whoops, that comment turned out to be incorrect anyway - I was worried
about migrate_timers, but it is safe as it is. The following should be
accurate:

+ * Requires cfs_b->lock for hrtimer_expires_remaining to be safe against the
+ * hrtimer base being cleared by __hrtimer_start_range_ns. In the case of
+ * migrate_hrtimers, base is never cleared, so we are fine.
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