Re: dm-crypt: remove per-cpu structure

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Feb 20 2014 at  7:10pm -0500,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 20 Feb 2014, Mike Snitzer wrote:
> 
> > On Thu, Feb 20 2014 at  6:01pm -0500,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > Dm-crypt used per-cpu structures to hold pointers to ablkcipher_request.
> > > The code assumed that the work item keeps executing on a single CPU, so it
> > > used no synchronization when accessing this structure.
> > > 
> > > When we disable a CPU by writing zero to
> > > /sys/devices/system/cpu/cpu*/online, the work item could be moved to
> > > another CPU. This causes crashes in dm-crypt because the code starts using
> > > a wrong ablkcipher_request.
> > > 
> > > This patch fixes this bug by removing the percpu definition. The structure
> > > ablkcipher_request is accessed via a pointer from convert_context.
> > > Consequently, if the work item is rescheduled to a different CPU, the
> > > thread still uses the same ablkcipher_request.
> > 
> > Hi Mikulas,
> > 
> > Obviously avoiding crashes is more important than performance.
> > 
> > But are we losing performance by switching away from using percpu?  Do
> > we care?  I'd like to see the header to speak to the potential for
> > slowdown (if there is any).
> 
> There is one more allocation per request than before. I don't know how 
> much does it cost.

OK, any reason you didn't fix this up by using cpu hotplug hooks like
Tejun suggested?  Too complicated?
 
> We could also modify the code to use per_bio_data to save one allocation.

OK, sounds like a good win.  Can you write a separate followup patch
that makes use of per_bio_data?