On Tue, Nov 07, 2017 at 10:38:30AM -0800, Mat Martineau wrote: > > Eric, > > On Mon, 6 Nov 2017, Eric Biggers wrote: > > >From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > >On a non-preemptible kernel, if KEYCTL_DH_COMPUTE is called with the > >largest permitted inputs (16384 bits), the kernel spends 10+ seconds > >doing modular exponentiation in mpi_powm() without rescheduling. If all > >threads do it, it locks up the system. Moreover, it can cause > >rcu_sched-stall warnings. > > > >Notwithstanding the insanity of doing this calculation in kernel mode > >rather than in userspace, fix it by calling cond_resched() as each bit > >from the exponent is processed. It's still noninterruptible, but at > >least it's preemptible now. > > cond_resched() is in the outer loop and gets called every > BITS_PER_LONG bits. That seems to be often enough for the system > that was taking 10+ seconds, and might be ok for slower processors. > > Was your intent to call cond_resched() for every bit as you > described in the commit message? > You're right, the cond_resched() is actually once per "limb", not once per bit. With the largest permitted inputs (16384 bits), each limb of the exponent takes about 38 milliseconds on an x86_64 CPU. Therefore on some other CPUs it will probably take 100+ milliseconds, which is much too long. So I guess it should do cond_resched() for each bit. I'll send a revised patch... Eric
