Subject: Re: PR/34293 CVS commit: src/sys/dev
To: Michael van Elst <email@example.com>
From: Manuel Bouyer <firstname.lastname@example.org>
Date: 09/06/2006 23:23:02
On Mon, Sep 04, 2006 at 11:23:37PM +0200, Michael van Elst wrote:
> On Mon, Sep 04, 2006 at 08:55:06PM +0000, Manuel Bouyer wrote:
> > > vndstrategy isn't called from interrupt context.
> > It is, if called from another driver (e.g. Xen block device).
> I believe that this is not allowed. The strategy routine is
> part of the top half of the driver, it operates in process
There was a discussion on tech-kern when I introduced the Xen
backend driver, and the conclusion was that it's valid to call
a strategy routine from interrupt context, and drivers should deal
> Saying that, I scanned other drivers and found several that may
> sleep in their strategy() routine. Most promiment scsistrategy()
> does this (by virtue of calling scsipi_command which waits for
> a free scsi transfer context in scsipi_get_xs).
This is a pseudo-strategy, part of the SCSI-specific ioctls implementation.
It's internal details, it's not exported to other drivers.
> > This patch broke vnd for all Xen users, where it worked fine before.
> I agree. The patch probably leaves a lot space for improvement. But
> the real bug is calling the strategy() routine from interrupt context.
You're redefining the strategy() interface here.
> > I understand all of this. But 1) the patch is broken (calling tsleep in
> > a strategy routine) 2) whenever it's not a problem for other drivers is
> > a matter of opinion. If has probably performances inpacts.
> Calling tsleep() in strategy is allowed.
Where did you get this ?
> > > performances on systems with multiple drives.
> > No, I meant if all buffers end up in a single device queue.
> That's what happening without the patch.
And that's what happens in other driver's queue too.
Manuel Bouyer <email@example.com>
NetBSD: 26 ans d'experience feront toujours la difference