* 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? 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(). Or can I understand how debug_smp_processor_id() is useful if __smp_processor_id() is defined as raw_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(). -- Thanks and Regards Srikar Dronamraju
