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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  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.

We could also modify the code to use per_bio_data to save one allocation.

Mikulas


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