From: Nathan Lynch nathanl@linux.ibm.com
[ Upstream commit fda0eb220021a97c1d656434b9340ebf3fc4704a ]
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.
Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") Signed-off-by: Nathan Lynch nathanl@linux.ibm.com Reviewed-by: Srikar Dronamraju srikar@linux.vnet.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210928214147.312412-3-nathanl@linux.ibm.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 bcb7b5f917be6..b325022ffa2b0 100644 --- a/arch/powerpc/include/asm/paravirt.h +++ b/arch/powerpc/include/asm/paravirt.h @@ -97,7 +97,23 @@ 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());
/* * Preemption can only happen at core granularity. This CPU