On Thu, 20 Feb 2014, Mike Snitzer wrote: > On Thu, Feb 20 2014 at 6:01pm -0500, > Mikulas Patocka <mpatocka@xxxxxxxxxx> 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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
