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Re: [dm-devel] dm-crypt: remove per-cpu structure




On Thu, 20 Feb 2014, Mike Snitzer wrote:

> 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?

Yes, it would complicate the code. The patch that removes percpu pointers 
shortens the file, using cpu hotplug hooks would make it bigger.

> > 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?

I will try it.

Mikulas


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