Subject: Re: Does BUFQ_GET/PUT need a lock?
To: None <email@example.com>
From: Juergen Hannken-Illjes <firstname.lastname@example.org>
Date: 07/21/2004 15:16:42
On Tue, Jul 20, 2004 at 09:54:58PM -0700, Gordon Waidhofer wrote:
> Defined in sys/buf.h and kern/subr_disk.c, the
> BUFQ_PUT() and BUFQ_GET() mechanisms manipulate
> disk I/O request queues. A device driver's strategy()
> routine uses BUFQ_PUT() to put a new request in
> an advantageous place in the queue. The device
> driver's start routine uses BUFQ_GET() to obtain the
> next request to issue to the device.
> The strategy() routine can be entered a variety
> of ways. The most likely path is through VOP_STRATEGY().
> The strategy() routine merges requests for
> multiple partitions into one queue. The vnode
> locks may provide locking for individual partitions
> but not for the overall device. So I don't
> see any way for the VOPs to provide exclusion.
> I expected to find a spinlock on the bufq_state
> structure, but did not.
> So I think there is a race condition or I'm
> missing something.
> Multiple CPUs can enter strategy(), perhaps
> with different partitions each, and try to
> BUFQ_PUT(). There is no lock to keep things
> sane. Similarly, the device start() routine
> could conflict with threads running in strategy().
> I have a pressing concern that multiple
> threads be able to instigate disk I/O.
> Any help appreciated.
The disk driver is responsible for locking.
As the bufq usually is part of a drivers control struct this
struct should get the spinlock.
Jürgen Hannken-Illjes - email@example.com