[PATCH v3] linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

Michael Jeanson mjeanson@efficios.com
Wed Mar 13 20:51:15 GMT 2024


On 2024-03-13 15:28, Florian Weimer wrote:
> Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
> in commit 2c6b4b272e6b4d07303af25709051c3e96288f2d ("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: 2c6b4b272e6b4d07303af25709051c3e96288f2d
> Fixes: 1d350aa06091211863e41169729cee1bca39f72f

Would adding back the '#include <sys/rseq.h>' in nptl/descr.h have any adverse 
consequences?

It would restore the original behavior without any effect on architectures 
which do not define RSEQ_SIG.


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