* Nathan Lynch <nath...@linux.ibm.com> [2021-09-28 16:41:47]: > vcpu_is_preempted() can be used outside of preempt-disabled critical > sections, yielding warnings such as: > > BUG: using smp_processor_id() in preemptible [00000000] code: > systemd-udevd/185 > caller is rwsem_spin_on_owner+0x1cc/0x2d0 > CPU: 1 PID: 185 Comm: systemd-udevd Not tainted 5.15.0-rc2+ #33 > Call Trace: > [c000000012907ac0] [c000000000aa30a8] dump_stack_lvl+0xac/0x108 (unreliable) > [c000000012907b00] [c000000001371f70] check_preemption_disabled+0x150/0x160 > [c000000012907b90] [c0000000001e0e8c] rwsem_spin_on_owner+0x1cc/0x2d0 > [c000000012907be0] [c0000000001e1408] rwsem_down_write_slowpath+0x478/0x9a0 > [c000000012907ca0] [c000000000576cf4] filename_create+0x94/0x1e0 > [c000000012907d10] [c00000000057ac08] do_symlinkat+0x68/0x1a0 > [c000000012907d70] [c00000000057ae18] sys_symlink+0x58/0x70 > [c000000012907da0] [c00000000002e448] system_call_exception+0x198/0x3c0 > [c000000012907e10] [c00000000000c54c] system_call_common+0xec/0x250 > > The result of vcpu_is_preempted() is always used speculatively, and the > function does not access per-cpu resources in a (Linux) preempt-unsafe way. > Use raw_smp_processor_id() to avoid such warnings, adding explanatory > comments. > > Signed-off-by: Nathan Lynch <nath...@linux.ibm.com> > Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in > vcpu_is_preempted()")
Looks good to me. Reviewed-by: Srikar Dronamraju <sri...@linux.vnet.ibm.com> > --- > arch/powerpc/include/asm/paravirt.h | 18 +++++++++++++++++- > 1 file changed, 17 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/include/asm/paravirt.h > b/arch/powerpc/include/asm/paravirt.h > index 39f173961f6a..eb7df559ae74 100644 > --- a/arch/powerpc/include/asm/paravirt.h > +++ b/arch/powerpc/include/asm/paravirt.h > @@ -110,7 +110,23 @@ static inline bool vcpu_is_preempted(int cpu) > > #ifdef CONFIG_PPC_SPLPAR > if (!is_kvm_guest()) { > - int first_cpu = cpu_first_thread_sibling(smp_processor_id()); > + int first_cpu; > + > + /* > + * The result of vcpu_is_preempted() is used in a > + * speculative way, and is always subject to invalidation > + * by events internal and external to Linux. While we can > + * be called in preemptable context (in the Linux sense), > + * we're not accessing per-cpu resources in a way that can > + * race destructively with Linux scheduler preemption and > + * migration, and callers can tolerate the potential for > + * error introduced by sampling the CPU index without > + * pinning the task to it. So it is permissible to use > + * raw_smp_processor_id() here to defeat the preempt debug > + * warnings that can arise from using smp_processor_id() > + * in arbitrary contexts. > + */ > + first_cpu = cpu_first_thread_sibling(raw_smp_processor_id()); > > /* > * The PowerVM hypervisor dispatches VMs on a whole core > -- > 2.31.1 > -- Thanks and Regards Srikar Dronamraju