Re: what's the semaphore in requests for?

[Jens Axboe]

On Sun, Jul 29 2001, Peter T. Breuer wrote:
> "A month of sundays ago ptb wrote:"
> > What's the semaphore field in requests for?  Are driver writers supposed
> > to be using it?
> 
> It seems nobody knows.

Seems you don't get mail sent to you?! I answered this on the 24th

http://asimov.lib.uaa.alaska.edu/linux-kernel/archive/2001-Week-30/0165.html

> > The reason I ask is that I've been chasing an smp bug in a block driver
> > of mine for a week.  The bug only shows up in 2.4 kernels (not in same
> > code under 2.2.18) and only with smp ("nosmp" squashes it).  It only
> 
> I've made more progress in seeking this bug.  The test is
> just dd if=/dev/mine of=/dev/null bs=4k over 2GB of data.
> 
> 2 processors + 1 userspace helper daemon on device = no bug 
> 2 processors + 2 userspace helper daemon on device = bug  (lockup)
> 1 processors + 1 userspace helper daemon on device = no bug 
> 1 processors + 2 userspace helper daemon on device = no bug 
> 
> Seeing this, I added a semaphore that forces the helper daemons to
> exclude each other as they enter the kernel in their ioctl calls.
> Still the lockup occurred with two processors and two daemons.

And I'll restate here what I said then too -- SHOW THE CODE! Or send me
a crystal ball and I'll be happy to solve your races for you.

-- 
Jens Axboe