Messages in this thread | | | Date | Tue, 7 Nov 2017 10:38:30 -0800 (PST) | From | Mat Martineau <> | Subject | Re: [PATCH] lib/mpi: call cond_resched() from mpi_powm() loop |
| |
Eric,
On Mon, 6 Nov 2017, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com> > > 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?
Thanks for the fix.
Mat
> > Cc: stable@vger.kernel.org # v4.12+ > Signed-off-by: Eric Biggers <ebiggers@google.com> > --- > lib/mpi/mpi-pow.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c > index e24388a863a7..f089a52dbbdb 100644 > --- a/lib/mpi/mpi-pow.c > +++ b/lib/mpi/mpi-pow.c > @@ -26,6 +26,7 @@ > * however I decided to publish this code under the plain GPL. > */ > > +#include <linux/sched.h> > #include <linux/string.h> > #include "mpi-internal.h" > #include "longlong.h" > @@ -263,6 +264,8 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) > break; > e = ep[i]; > c = BITS_PER_MPI_LIMB; > + > + cond_resched(); > } > > /* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT > -- > 2.15.0 > >
-- Mat Martineau Intel OTC
| |