Messages in this thread | | | Subject | Re: [Fastboot] [PATCH 1/3] stack overflow safe kdump (2.6.18-rc1-i386) - safe_smp_processor_id | From | Fernando Luis Vázquez Cao <> | Date | Tue, 11 Jul 2006 13:44:33 +0900 |
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On Tue, 2006-07-11 at 13:21 +0900, Fernando Luis Vázquez Cao wrote: > Hi Eric! > > On Mon, 2006-07-10 at 05:37 -0600, Eric W. Biederman wrote: > > Fernando Luis Vázquez Cao <fernando@oss.ntt.co.jp> writes: > > > > > Hi Keith, > > > > > > Thank you for the comments. > > > > > > On Mon, 2006-07-10 at 18:27 +1000, Keith Owens wrote: > > >> Fernando Luis Vazquez Cao (on Mon, 10 Jul 2006 16:50:52 +0900) wrote: > > >> >On the event of a stack overflow critical data that usually resides at > > >> >the bottom of the stack is likely to be stomped and, consequently, its > > >> >use should be avoided. > > >> > > > >> >In particular, in the i386 and IA64 architectures the macro > > >> >smp_processor_id ultimately makes use of the "cpu" member of struct > > >> >thread_info which resides at the bottom of the stack. x86_64, on the > > >> >other hand, is not affected by this problem because it benefits from > > >> >the use of the PDA infrastructure. > > >> > > > >> >To circumvent this problem I suggest implementing > > >> >"safe_smp_processor_id()" (it already exists in x86_64) for i386 and > > >> >IA64 and use it as a replacement for smp_processor_id in the reboot path > > >> >to the dump capture kernel. This is a possible implementation for i386. > > >> > > >> I agree with avoiding the use of thread_info when the stack might be > > >> corrupt. However your patch results in reading apic data and scanning > > >> NR_CPU sized tables for each IPI that is sent, which will slow down the > > >> sending of all IPIs, not just dump. > > > This patch only affects IPIs sent using send_IPI_allbutself which is > > > rarely called, so the impact in performance should be negligible. > > > > Well smp_call_function uses it so I don't know if rarely called applies. > > > > However when called with the NMI vector every instance of send_IPI_allbutself > > transforms this into send_IPI_mask. Which is why we need to know our current > > cpu in the first place. > > > > Therefore why don't we just do that explicitly in crash.c > > i.e. > > > > static void smp_send_nmi_allbutself(void) > > { > > cpumask_t mask = cpu_online_map; > > cpu_clear(safe_smp_processor_id(), mask); > > send_IPI_mask(mask, NMI_VECTOR); > > } > > > > That will guarantee that any effects this code paranoia may have > > are only seen in the crash dump path. > > That is a good idea, but I have on concern. In mach-default by default > we use __send_IPI_shortcut (no_broadcast==0) instead of send_IPI_mask. > Is it always safe to ignore the no_broadcast setting? In other words, > can __send_IPI_shortcut be replaced by send_IPI_mask safely? >From reading the code, it seems that send_IPI_mask is always safer (we avoid the risk of sending an IPI to an offline CPU) and with it we can certainly accomplish what we want. I will prepare new patches taking all your comments and advices.
Thank you,
Fernando
P.S.: Sorry for replying to myself...
> > The implementation of send_IPI_allbutself in the different architectures > follows: > > smp_send_nmi_allbutself > send_IPI_allbutself > > * mach-bigsmp > send_IPI_allbutself > cpu_clear(smp_processor_id(), mask) > send_IPI_mask > send_IPI_mask_sequence > apic_wait_icr_idle > > * mach-default > send_IPI_allbutself > __local_send_IPI_allbutself > if (no_broadcast) { > cpu_clear(smp_processor_id(), mask) > send_IPI_mask(mask, vector) > send_IPI_mask_bitmask > apic_wait_icr_idle > } else { > __send_IPI_shortcut(APIC_DEST_ALLBUT, vector) > apic_wait_icr_idle > } > > * mach-es7000 > send_IPI_allbutself > cpu_clear(smp_processor_id(), mask); > send_IPI_mask > send_IPI_mask_sequence > apic_wait_icr_idle > > * mach-numaq > send_IPI_allbutself > cpu_clear(smp_processor_id(), mask) > send_IPI_mask > send_IPI_mask_sequence > apic_wait_icr_idle > > * mach-summit > send_IPI_allbutself > cpu_clear(smp_processor_id(), mask) > send_IPI_mask > send_IPI_mask_sequence > apic_wait_icr_idle > > Regards, > > Fernando > > > > > > > >> It would be far cheaper to define > > >> a per-cpu variable containing the logical cpu number, set that variable > > >> once as each cpu is brought up and just read it in cases where you > > >> might not trust the integrity of struct thread_info. safe_smp_processor_id() > > >> resolves to just a read of the per cpu variable. > > > But to read a per-cpu variable you need to index the corresponding array > > > with processor id of the current CPU (see code below), but that is > > > precisely what we are trying to figure out. Anyway as > > > send_IPI_allbutself is not a fast path (correct if this assumption is > > > wrong) the current implementation of safe_smp_processor_id should be > > > fine. > > > > > > #define get_cpu_var(var) (*({ preempt_disable(); > > > &__get_cpu_var(var); })) > > > #define __get_cpu_var(var) per_cpu(var, smp_processor_id()) > > > > > > Am I missing something obvious? > > > > No. Except that other architectures have cheaper per pointers so they > > don't have that problem. > > > > Eric > > _______________________________________________ > fastboot mailing list > fastboot@lists.osdl.org > https://lists.osdl.org/mailman/listinfo/fastboot
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