Re: [PATCH] Re: [dm-devel] kmalloc after down_write?

[Ming Zhang <mingz ele uri edu>]

the chance to block another snapshot creation is tiny. but block the
map() is possible.

i do not see anything that makes this patch not as good as the original
one.

ming

On Fri, 2004-11-19 at 15:51, Kevin Corry wrote:
> On Friday 19 November 2004 2:27 pm, Alasdair G Kergon wrote:
> > On Fri, Nov 19, 2004 at 02:16:52PM -0600, Kevin Corry wrote:
> > > In register_snapshot(), move the kmalloc() outside the _origins_lock.
> >
> > Maybe - but snapshots and memory allocations don't mix particularly well,
> > and these are complex trade-offs, so for now I'd rather leave this code
> > as it is (i.e. not introduce new allocations), until there's strong
> > evidence one way or the other e.g. from testing or more detailed code
> > analysis.
> 
> I wasn't implying you needed to (or even should) accept that patch. :) Just 
> put it out there as an example of how to reduce the time spent holding that 
> lock without having to drop and reaquire it. I guess I should have marked my 
> response as [RFC|PATCH].
> 
> Personally, I think this is one area where we really don't have to worry about 
> lock contention. It's very unlikely in real-world scenarios that two 
> different processes are going to be creating snapshots at the same time. 
> Thinking about EVMS, I know there's no way to activate two snapshots 
> simultaneously. Thinking about LVM2, you'd have to run two copies of lvcreate 
> at the same point in time, which also seems kind of hard to imagine in 
> practice.
> 
> Thus, I'd say the patch is mostly unnecessary.