[RFC PATCH for 5.8 0/4] rseq cpu_id ABI fix

Hi,

Recent integration of rseq into glibc unearthed an issue with inaccurate
cpu_id field for newly created tasks. This series includes a fix for the
underlying issue (meant to be backported to stable), as well as new rseq
flags to let user-space know that the kernel implements this fix, so
glibc and other rseq users can use this flag to know whether they can
safely use rseq without risk of corrupting their per-cpu data. This new
flag could either be added only to the master branch (no stable
backport) or backported to stable, depending on what seems the most
appropriate.

This is an RFC aiming for quick inclusion into the Linux kernel, unless
we prefer reverting the entire rseq glibc integration and try again in 6
months. Their upcoming release is on August 3rd, so we need to take a
decision on this matter quickly.

Thanks,

Mathieu

Mathieu Desnoyers (4):
  sched: Fix unreliable rseq cpu_id for new tasks
  rseq: Introduce RSEQ_FLAG_REGISTER
  rseq: Introduce RSEQ_FLAG_RELIABLE_CPU_ID
  rseq: selftests: Expect reliable cpu_id field

 include/uapi/linux/rseq.h           | 15 +++++-
 kernel/rseq.c                       | 81 ++++++++++++++++-------------
 kernel/sched/core.c                 |  2 +
 tools/testing/selftests/rseq/rseq.c | 10 +++-
 4 files changed, 71 insertions(+), 37 deletions(-)

-- 
2.17.1