Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
in commit 2c6b4b272e ("nptl:
Unconditionally use a 32-byte rseq area"). After that, it was
not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
compilation that provided a definition. This commit always checks
the rseq area for CPU number information before using the other
approaches.
This adds an unnecessary (but well-predictable) branch on
architectures which do not define RSEQ_SIG, but its cost is small
compared to the system call. Most architectures that have vDSO
acceleration for getcpu also have rseq support.
Fixes: 2c6b4b272e
Fixes: 1d350aa060
Reviewed-by: Arjun Shankar <arjun@redhat.com>
7a76f21867