linux-kernel - RE: [PATCH rev2 1/4] dmaengine: enable mutliple clients and operations

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Message-ID: <BD524EA7912ED5469DFD0BAEF6BC752F296540@orsmsx411.amr.corp.intel.com>
Date:	Tue, 1 Aug 2006 11:03:28 -0700
From:	"Leech, Christopher" <christopher.leech@...el.com>
To:	"David Miller" <davem@...emloft.net>,
	"Williams, Dan J" <dan.j.williams@...el.com>
Cc:	<linux-kernel@...r.kernel.org>, <neilb@...e.de>,
	<galak@...nel.crashing.org>, <alan@...rguk.ukuu.org.uk>
Subject: RE: [PATCH rev2 1/4] dmaengine: enable mutliple clients and operations

Sorry Dave,

The delay on that issue is my fault.  While Dan is an Intel employee,
he's in a totally separate part of the company (and a different state)
from me and the other networking folks.

I was trying to determine if reordering so that the spin_lock was inside
the lock_cpu_hotplug would work, or if something else was needed, Dan
suggested making the net_dma_event_lock a mutex, but then as Andrew
pointed out as long as a lock is being held preemption is disabled and
we're safe from cpu hotplug.

Simple patch to remove the lock_cpu_hotplug/unlock_cpu_hotplug calls as
Andrew suggested coming up.

- Chris

> -----Original Message-----
> From: David Miller [mailto:davem@...emloft.net] 
> Sent: Monday, July 31, 2006 11:12 PM
> To: Williams, Dan J
> Cc: linux-kernel@...r.kernel.org; neilb@...e.de; 
> galak@...nel.crashing.org; Leech, Christopher; 
> alan@...rguk.ukuu.org.uk
> Subject: Re: [PATCH rev2 1/4] dmaengine: enable mutliple 
> clients and operations
> 
> 
> 
> Can I ask that the known bugs in the I/O AT DMA code be fixed
> before we start adding new features to it?
> 
> Specifically, the lock_cpu_hotplug() call in net_dma_rebalance()
> is still there and being invoked with a spinlock held.  The
> spinlock is grabbed by the caller, netdev_dma_event() which
> grabs the net_dma_event_lock spinlock.
> 
> You cannot invoke lock_cpu_hotplug() while holding a spinlock
> because lock_cpu_hotplug(), as seen in kernel/cpu.c, takes
> a semaphore which can sleep.  Sleeping while holding a spinlock
> is not allowed.
> 
> This is the second time I have tried to make the Intel developers
> aware of this bug.  So please fix this problem.
> 
> Thanks a lot.
> 
-
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