Srikar Dronamraju <sri...@linux.vnet.ibm.com> writes: > * Nathan Lynch <nath...@linux.ibm.com> [2021-09-22 11:01:12]: > >> Srikar Dronamraju <sri...@linux.vnet.ibm.com> writes: >> > * Nathan Lynch <nath...@linux.ibm.com> [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. >> >> Sorry, I don't follow... > > I meant, Unless CONFIG_DEBUG_PREEMPT, smp_processor_id() is defined as > raw_processor_id(). > >> >> > In the CONFIG_DEBUG_PREEMPT case, smp_processor_id() >> > is actually debug_smp_processor_id(), which does all the checks. >> >> Yes, OK. >> >> > I believe these checks in debug_smp_processor_id() are only valid for x86 >> > case (aka cases were they have __smp_processor_id() defined.) >> >> Hmm, I am under the impression that the checks in >> debug_smp_processor_id() are valid regardless of whether the arch >> overrides __smp_processor_id(). > > From include/linux/smp.h > > /* > * Allow the architecture to differentiate between a stable and unstable read. > * For example, x86 uses an IRQ-safe asm-volatile read for the unstable but a > * regular asm read for the stable. > */ > #ifndef __smp_processor_id > #define __smp_processor_id(x) raw_smp_processor_id(x) > #endif > > As far as I see, only x86 has a definition of __smp_processor_id. > So for archs like Powerpc, __smp_processor_id(), is always > defined as raw_smp_processor_id(). Right?
Sure, yes. > I would think debug_smp_processor_id() would be useful if __smp_processor_id() > is different from raw_smp_processor_id(). Do note debug_smp_processor_id() > calls raw_smp_processor_id(). I do not think the utility of debug_smp_processor_id() is related to whether the arch defines __smp_processor_id(). > Or can I understand how debug_smp_processor_id() is useful if > __smp_processor_id() is defined as raw_smp_processor_id()? So, for powerpc with DEBUG_PREEMPT unset, a call to smp_procesor_id() expands to __smp_processor_id() which expands to raw_smp_processor_id(), avoiding the preempt safety checks. This is working as intended. For powerpc with DEBUG_PREEMPT=y, a call to smp_processor_id() expands to the out of line call to debug_smp_processor_id(), which calls raw_smp_processor_id() and performs the checks, warning if called in an inappropriate context, as seen here. Also working as intended. AFAICT __smp_processor_id() is a performance/codegen-oriented hook, and not really related to the debug facility. Please see 9ed7d75b2f09d ("x86/percpu: Relax smp_processor_id()"). >> I think the stack trace here correctly identifies an incorrect use of >> smp_processor_id(), and the call site needs to be changed. Do you >> disagree? > > Yes the stack_trace shows that debug_smp_processor_id(). However what > I want to understand is why should we even call > debug_smp_processor_id(), when our __smp_processor_id() is defined as > raw_smp_processor_id(). smp_processor_id() should always expand to debug_smp_processor_id() when DEBUG_PREEMPT=y, regardless of whether the arch overrides __smp_processor_id(). That is how I understand the intent of the code as written.