Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

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Would pairing one rseq_start with two rseq_finish do the trick
there ?

Yes, two rseq_finish works, as long as the extra rseq management overhead
is not substantial.

I've added a commit implementing rseq_finish2() in my rseq volatile
dev branch. You can fetch it at:

https://github.com/compudj/linux-percpu-dev/tree/rseq-fallback

I also have a separate test and benchmark tree in addition to the
kernel selftests here:

https://github.com/compudj/rseq-test

I named the first write a "speculative" write, and the second write
the "final" write.

Would you like to extend the test cases to cover your intended use-case ?

Hi Dave!

I just pushed a rseq_finish2() test in my rseq-fallback branch. It implements
a per-cpu buffer holding pointers, and pushes/pops items to/from it.

To use it:

cd tools/testing/selftests/rseq
./param_test -T b

(see -h for advanced usage)

Let me know if I got it right!

Hi Mathieu,

Thanks, you beat me to it.    I commented on the github, that's pretty much it.  

In the kernel, if rather than testing for:

if ((void __user *)instruction_pointer(regs) < post_commit_ip) {

we could test for both start_ip and post_commit_ip:

if ((void __user *)instruction_pointer(regs) < post_commit_ip
    && (void __user *)instruction_pointer(regs) >= start_ip) {

We could perform the failure path (storing NULL into the rseq_cs
field of struct rseq) in C rather than being required to do it in
assembly at addresses >= to post_commit_ip, all because the kernel
would test whether we are within the assembly block address range
using both the lower and upper bounds (start_ip and post_commit_ip).

Sounds reasonable to me.  I agree it would be best to move the failure path 
out of the asm if possible.--
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