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SubjectRe: [PATCH] lib/mpi: call cond_resched() from mpi_powm() loop
On Mon, Nov 06, 2017 at 10:19:51PM -0800, Eric Biggers wrote:
> From: Eric Biggers <>
> 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.
> Cc: # v4.12+
> Signed-off-by: Eric Biggers <>

Patch applied. Thanks.
Email: Herbert Xu <>
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