Re: NBD Cleanup patch and bugfix in ll_rw_blk.c

[Jens Axboe]

On Wed, Feb 28 2001, Linus Torvalds wrote:
> As far as I can tell, the patch will trigger only for a not-empty request
> list, where the elevator decides to put the new request at the head of the
> queue.
> 
> Which is probably unlikely (and with the current elevator it might even be
> impossible). But it does not look impossible in theory. And I'd really
> prefer _not_ to have something that
>  - looks completely bogus from a design standpoint
>  - might be buggy under some rather unlikely circumstances
> 
> Reading them together they become "might cause disk corruption in some
> really hard-to-trigger circumstances". No, thank you.
> 
> Note that I suspect that all current drivers (or at least the common ones)
> have protection against being called multiple times, simply because 2.2.x
> used to do it. Which again means that you'd probably never see problems
> in practice. But that doesn't make it _right_. 

I think most of the "we want to disable plugging" behaviour stems
from the way task queues behave. Once somebody starts a tq_disk
run, the list is fried and walked one by one. Both old loop
and nbd drop the io_request_lock and block, possibly waiting
for I/O to be done (at least the loop case, don't know about
ndb). But this I/O won't be done just because the target plug every
now and then just happens to be queued behind the nbd/loop one and a new
tq_disk run won't start it.

-- 
Jens Axboe