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U2FsdGVkX19uezF/7fntY+v/gz7rKvh92IJ+BF8pv6YoRk9lw+mhh3eM6sYqwEsCb06tQE4lQv6vOQ2O06zjROJgO2yCBssx821r1Ce+6aXVT2bazFMeNVYA+qT5Olqwa4zvRwILlomaUT0cC5QQjyPCt6NfIf6cGxnBcOtTT5SOxUAoZMSetoiKek2bd4DczldxlVr4ZmitR+ojXljsSTOX23YwmSNRgCKrttAls4FfvxM2lfGqwrysg79EamnPq2r8Cjccg7le85admLtEiNKnFeuBV7P7UqGuYJdAzRIQXKv9uGnhKUdcRTkHHWx0HNnYhdhsFPo75R17KrCFm9wcEB83iZhaQfhqxW+xlk8mmRuQYXaYaAutMGm4z6BRLLvOsrp76JEfbVMi5Q56CQkmEimOTDS+wqfR72aCWfa1+BS5snaPSamJXe+5xB0KZL5Jr8GxBhn/9qjewICGutSuweUEdogQ7gP4uEd0osl6VDjuL3WBLKrbHmheNKtI9aQYd+wpU8Oa31/Qfka28Nv3Fdvo/d9Ody/yzaklohfD4ysWnf/My6Q3lp9q98/Qz8L8iheiIaZiHDMmJx5WthTYtFVaCWR8GcTyeSziCjFG0xJZlVhvuPSKOz7G2abBfMu5vmpcQxa1XvxDGux4Y7JTetZzIAHrQhPqvr3Bfe6DQXF9lQxG2t7Pxvq664/ocYbEGIP3WP07KfN0E/2he47EdzIhhiSeCnTN5X7GZvKFtatq/2Ikon2z4zz3WbSyWHoklnRcyNKvXidSpoFQ2o3Nmzvyk1PJqaAmOvcSPQP0MaOiFpH+Epq44mSTZoEldCu6X1DmfEBHUEDu9FT4kIPKFvVEvD43qRGu+mHbg0bZ+kOLn99A5w2uc2ZaUtIScFrafPpyundHZSLT9JJ26JpkA9UX6TYdxrX15/oo4rWu3XFAq0ad8ThdC9uvNLSxSTs0cT0By7Ym0ZqVjWu l7vxM/Ci 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Steven Rostedt writes: > On Tue, 7 Nov 2023 13:57:31 -0800 > Ankur Arora wrote: > >> Note: this commit is badly broken. Only here for discussion. >> >> Configurations with ARCH_NO_PREEMPT support preempt_count, but might >> not be tested well enough under PREEMPTION to support it might not >> be demarcating the necessary non-preemptible sections. >> >> One way to handle this is by limiting them to PREEMPT_NONE mode, not >> doing any tick enforcement and limiting preemption to happen only at >> user boundary. >> >> Unfortunately, this is only a partial solution because eager >> rescheduling could still happen (say, due to RCU wanting an >> expedited quiescent period.) And, because we do not trust the >> preempt_count accounting, this would mean preemption inside an >> unmarked critical section. > > Is preempt_count accounting really not trust worthy? I think the concern was that we might not be marking all the sections that might be non-preemptible. Plus, given that these archs have always been !preemption, it is unlikely that they would work without changes. I think basically we don't have a clue if they work or not since haven't ever been tested. > That is, if we preempt at preempt_count() going to zero but nowhere else, > would that work? At least it would create some places that can be resched. I'm not sure we can be sure. I can imagine places where it should be preempt_disable() ; spin_lock() ; ... ; spin_unlock(); preempt_enable() but the preempt_disable/_enable() are MIA. I think even so it is a pretty good idea. We could decompose ARCH_NO_PREEMPT into ARCH_NO_PREEMPT_COUNT and ARCH_NO_PREEMPT_IRQ. And, as you imply, PREEMPTION (or rather PREEMPT_COUNT) only depends on ARCH_NO_PREEMPT_COUNT, not the ARCH_NO_PREEMPT_IRQ. And this change might make the task of fixing this simpler since you would only have to worry about neighborhood and paths leading to preempt_enable(). void irqentry_exit_cond_resched(void) { - if (!preempt_count()) { + if (IS_DISABLED(CONFIG_ARCH_NO_PREEMPT_IRQ) && !preempt_count()) { /* Sanity check RCU and thread stack */ rcu_irq_exit_check_preempt(); Geert, if you think it might help I can send out a patch. > What's the broken part of these archs? The assembly? Not sure anyone knows. But, assuming m68k is representative of the other three ARCH_NO_PREEMPT ones (might be better placed, because COLDFIRE m68k already supports preemption) the patches Geert had sent out add: - preempt_enable/_disable() pairs to the cache/tlb flush logic - a need-resched check and call to preempt_schedule_irq() in the exception return path. m68k support: https://lore.kernel.org/all/7858a184cda66e0991fd295c711dfed7e4d1248c.1696603287.git.geert@linux-m68k.org/#t (The series itself ran into an mmput() bug which might or might not have anything to do with preemption.) > If that's the case, as > long as the generic code has the preempt_count() I would think that would > be trust worthy. I'm also guessing that in_irq() and friends are still > reliable. True. -- ankur