linux-kernel - Re: Using random in interrupts for RT

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Message-ID: <1345572733.5069.9.camel@gandalf.local.home>
Date:	Tue, 21 Aug 2012 14:12:13 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	RT <linux-rt-users@...r.kernel.org>,
	Theodore Tso <tedtso@...il.com>
Subject: Re: Using random in interrupts for RT

On Tue, 2012-08-14 at 10:30 -0400, Steven Rostedt wrote:
> Thomas,
> 
> Ben Hutchings asked me if we still need "genirq: Disable random call on
> preempt-rt" for -rt? With commit 902c098a366 "random: use lockless
> techniques in the interrupt path" there is no more locks used. But does
> it still produce high latencies?

Ben,

Looks that the #ifndef can't be removed just yet. The code is not
totally lockless...


add_interrupt_randomness ->
	credit_entropy_bits ->
		kill_fasync ->
			kill_fasync_rcu ->
				spin_lock_irqsave(&fa->fa_lock, flags)

to make things worse, that spinlock turns into a mutex on -rt, so it may
crash the box if triggered.

-- Steve


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