linux-kernel - Re: [PATCH] Convert tlbstate_lock spin_lock to raw_spin

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Date:	Mon, 8 Sep 2008 11:04:31 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sujit Karataparambil <sjt.kar@...il.com>
cc:	linux-rt-users <linux-rt-users@...r.kernel.org>,
	xavier droubay <xavier.droubay@...il.com>,
	Chirag Jog <chirag@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Convert tlbstate_lock spin_lock to raw_spin_lock

[ Refrain from using "unlisted-recipients" please! ]

On Mon, 8 Sep 2008, Sujit Karataparambil wrote:

> Chirag,
> 
>   Kindly understand what xavier is trying to do. He is Trying to disable smp.
> The RT - Linux Code is dependent on the mutex lock to perform some
> soft/hard real time processing. This is an problem with the mutex lock.

It is not a problem with the mutex lock.

> 
> Kindly check with some one before signing off patches.

The patch is legit.

The problem is simply that functions like flush_tlb_current_task and 
flush_tlb_mm disable preemption and then call flush_tlb_others.
This function locks the tlbstate_lock which is currently a rtmutex. An 
rtmutex can not be called with preemption disabled. The tlbstate_lock is 
static, small and confined. I'm not sure we can change this code (flushing 
the TLB) in a way where we can allow preemption. This may just be a 
latency that we must hit.

-- Steve
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