Re: [PATCH] fix sd open/remove race

[James Bottomley <James.Bottomley@steeleye.com>]

On Fri, 2004-04-09 at 10:55, Alan Stern wrote:
> Hmm.  That wouldn't matter if you cleared the indicator in sd_remove.  
> Then the race would only be sd_remove against sd_open.  Also, you do have
> to admit, your way of doing it is more obscure.  You should at least add
> comments explaining this 3-way race and the fact that the semaphore is
> already acquired during scsi_disk_release.

But, as I pointed out before, you can't clear the indicator in sd_remove
because, fundamentally, that's not where the race is.  The fundamental
race is the get/put one making sure no-one can get a reference to a
freed object.

sd_remove is just the disconnection indication.  It triggers a put of
the object, causing garbage collection if it's the last put, but that's
all it does.  To mediate the race, the synchronisation has to be around
object acquisition/deallocation.

I can add comments, but the placement of the semaphores is natural to
mediate the object acquisition/deallocation race.  The three thread
illustration was just the first thing that came into my head for
explaining what problems could be caused by not doing natural semaphore
placement.

James