Re: [RFC PATCH v8 1/9] Restartable sequences system call

On Sat, Aug 27, 2016 at 5:21 AM, Pavel Machek [off-list ref] wrote:

Hi!

quoted

Expose a new system call allowing each thread to register one userspace
memory area to be used as an ABI between kernel and user-space for two
purposes: user-space restartable sequences and quick access to read the
current CPU number value from user-space.

* Restartable sequences (per-cpu atomics)

Restartables sequences allow user-space to perform update operations on
per-cpu data without requiring heavy-weight atomic operations.

The restartable critical sections (percpu atomics) work has been started
by Paul Turner and Andrew Hunter. It lets the kernel handle restart of
critical sections. [1] [2] The re-implementation proposed here brings a
few simplifications to the ABI which facilitates porting to other
architectures and speeds up the user-space fast path. A locking-based
fall-back, purely implemented in user-space, is proposed here to deal
with debugger single-stepping. This fallback interacts with rseq_start()
and rseq_finish(), which force retries in response to concurrent
lock-based activity.

Hmm. Purely software fallback needed for singlestepping... Looks like this is malware
writer's dream come true...

Also if you ever get bug in the restartable code, debugger will be useless to debug it...
unless new abilities are added to debuggers to manually schedule threads on CPUs.

Is this good idea?

We've had some off-list discussion.

I have a revised version which incoprorates some of Mattheiu's
improvements, but avoids this requirement nearly ready for posting.

- Paul