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linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

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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>
This commit is contained in:
Florian Weimer
2024-03-15 19:08:24 +01:00
parent 73c26018ed
commit 7a76f21867
1 changed files with 0 additions and 8 deletions
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8
sysdeps/unix/sysv/linux/sched_getcpu.c
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@@ -33,17 +33,9 @@ vsyscall_sched_getcpu (void)
return r == -1 ? r : cpu; return r == -1 ? r : cpu;
} }
#ifdef RSEQ_SIG
int int
sched_getcpu (void) sched_getcpu (void)
{ {
int cpu_id = THREAD_GETMEM_VOLATILE (THREAD_SELF, rseq_area.cpu_id); int cpu_id = THREAD_GETMEM_VOLATILE (THREAD_SELF, rseq_area.cpu_id);
return __glibc_likely (cpu_id >= 0) ? cpu_id : vsyscall_sched_getcpu (); return __glibc_likely (cpu_id >= 0) ? cpu_id : vsyscall_sched_getcpu ();
} }
#else /* RSEQ_SIG */
int
sched_getcpu (void)
{
return vsyscall_sched_getcpu ();
}
#endif /* RSEQ_SIG */