Re: [PATCH] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted()
From: Srikar Dronamraju <hidden>
Date: 2021-09-22 07:58:14
Subsystem:
the rest · Maintainer:
Linus Torvalds
* Nathan Lynch [off-list ref] [2021-09-20 22:12:13]:
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 subject to invalidation by events inside and outside of Linux; it's just a best guess at a point in time. Use raw_smp_processor_id() to avoid such warnings.
Typically smp_processor_id() and raw_smp_processor_id() except for the CONFIG_DEBUG_PREEMPT. In the CONFIG_DEBUG_PREEMPT case, smp_processor_id() is actually debug_smp_processor_id(), which does all the checks. I believe these checks in debug_smp_processor_id() are only valid for x86 case (aka cases were they have __smp_processor_id() defined.) i.e x86 has a different implementation of _smp_processor_id() for stable and unstable
quoted hunk ↗ jump to hunk
Signed-off-by: Nathan Lynch <redacted> Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") --- arch/powerpc/include/asm/paravirt.h | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h index bcb7b5f917be..e429aca566de 100644 --- a/arch/powerpc/include/asm/paravirt.h +++ b/arch/powerpc/include/asm/paravirt.h@@ -97,7 +97,14 @@ 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; + + /* + * This is only a guess at best, and this function may be + * called with preemption enabled. Using raw_smp_processor_id() + * does not damage accuracy. + */ + first_cpu = cpu_first_thread_sibling(raw_smp_processor_id()); /* * Preemption can only happen at core granularity. This CPU-- 2.31.1
How about something like the below?
diff --git a/include/linux/smp.h b/include/linux/smp.h
index 510519e8a1eb..8c669e8ceb73 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h@@ -256,12 +256,14 @@ static inline int get_boot_cpu_id(void) */ #ifndef __smp_processor_id #define __smp_processor_id(x) raw_smp_processor_id(x) -#endif - +#else #ifdef CONFIG_DEBUG_PREEMPT extern unsigned int debug_smp_processor_id(void); # define smp_processor_id() debug_smp_processor_id() -#else +#endif +#endif + +#ifndef smp_processor_id # define smp_processor_id() __smp_processor_id() #endif
--
Thanks and Regards
Srikar Dronamraju