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3e31009898
This commit defers reporting of RCU-preempt quiescent states at rcu_read_unlock_special() time when any of interrupts, softirq, or preemption are disabled. These deferred quiescent states are reported at a later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug offline operation. Of course, if another RCU read-side critical section has started in the meantime, the reporting of the quiescent state will be further deferred. This also means that disabling preemption, interrupts, and/or softirqs will act as an RCU-preempt read-side critical section. This is enforced by checking preempt_count() as needed. Some special cases must be handled on an ad-hoc basis, for example, context switch is a quiescent state even though both the scheduler and do_exit() disable preemption. In these cases, additional calls to rcu_preempt_deferred_qs() override the preemption disabling. Similar logic overrides disabled interrupts in rcu_preempt_check_callbacks() because in this case the quiescent state happened just before the corresponding scheduling-clock interrupt. In theory, this change lifts a long-standing restriction that required that if interrupts were disabled across a call to rcu_read_unlock() that the matching rcu_read_lock() also be contained within that interrupts-disabled region of code. Because the reporting of the corresponding RCU-preempt quiescent state is now deferred until after interrupts have been enabled, it is no longer possible for this situation to result in deadlocks involving the scheduler's runqueue and priority-inheritance locks. This may allow some code simplification that might reduce interrupt latency a bit. Unfortunately, in practice this would also defer deboosting a low-priority task that had been subjected to RCU priority boosting, so real-time-response considerations might well force this restriction to remain in place. Because RCU-preempt grace periods are now blocked not only by RCU read-side critical sections, but also by disabling of interrupts, preemption, and softirqs, it will be possible to eliminate RCU-bh and RCU-sched in favor of RCU-preempt in CONFIG_PREEMPT=y kernels. This may require some additional plumbing to provide the network denial-of-service guarantees that have been traditionally provided by RCU-bh. Once these are in place, CONFIG_PREEMPT=n kernels will be able to fold RCU-bh into RCU-sched. This would mean that all kernels would have but one flavor of RCU, which would open the door to significant code cleanup. Moving to a single flavor of RCU would also have the beneficial effect of reducing the NOCB kthreads by at least a factor of two. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> [ paulmck: Apply rcu_read_unlock_special() preempt_count() feedback from Joel Fernandes. ] [ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in response to bug reports from kbuild test robot. ] [ paulmck: Fix bug located by kbuild test robot involving recursion via rcu_preempt_deferred_qs(). ] |
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| ABI | ||
| accelerators | ||
| accounting | ||
| acpi | ||
| admin-guide | ||
| aoe | ||
| arm | ||
| arm64 | ||
| auxdisplay | ||
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| bus-devices | ||
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| cgroup-v1 | ||
| cma | ||
| connector | ||
| console | ||
| core-api | ||
| cpu-freq | ||
| cpuidle | ||
| crypto | ||
| dev-tools | ||
| device-mapper | ||
| devicetree | ||
| doc-guide | ||
| driver-api | ||
| driver-model | ||
| early-userspace | ||
| EDID | ||
| extcon | ||
| fault-injection | ||
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| ioctl | ||
| isdn | ||
| kbuild | ||
| kdump | ||
| kernel-hacking | ||
| laptops | ||
| leds | ||
| lightnvm | ||
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| locking | ||
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| rapidio | ||
| RCU | ||
| riscv | ||
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| scheduler | ||
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| sysctl | ||
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| x86 | ||
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| .gitignore | ||
| 00-INDEX | ||
| atomic_bitops.txt | ||
| atomic_t.txt | ||
| bt8xxgpio.txt | ||
| btmrvl.txt | ||
| bus-virt-phys-mapping.txt | ||
| Changes | ||
| clearing-warn-once.txt | ||
| CodingStyle | ||
| conf.py | ||
| cpu-load.txt | ||
| cputopology.txt | ||
| crc32.txt | ||
| dcdbas.txt | ||
| debugging-modules.txt | ||
| debugging-via-ohci1394.txt | ||
| dell_rbu.txt | ||
| digsig.txt | ||
| DMA-API-HOWTO.txt | ||
| DMA-API.txt | ||
| DMA-attributes.txt | ||
| DMA-ISA-LPC.txt | ||
| docutils.conf | ||
| dontdiff | ||
| efi-stub.txt | ||
| eisa.txt | ||
| flexible-arrays.txt | ||
| futex-requeue-pi.txt | ||
| gcc-plugins.txt | ||
| highuid.txt | ||
| hw_random.txt | ||
| hwspinlock.txt | ||
| index.rst | ||
| Intel-IOMMU.txt | ||
| intel_txt.txt | ||
| io-mapping.txt | ||
| io_ordering.txt | ||
| iostats.txt | ||
| IPMI.txt | ||
| IRQ-affinity.txt | ||
| IRQ-domain.txt | ||
| IRQ.txt | ||
| irqflags-tracing.txt | ||
| isa.txt | ||
| isapnp.txt | ||
| kernel-per-CPU-kthreads.txt | ||
| kobject.txt | ||
| kprobes.txt | ||
| kref.txt | ||
| ldm.txt | ||
| lockup-watchdogs.txt | ||
| logo.gif | ||
| logo.txt | ||
| lsm.txt | ||
| lzo.txt | ||
| mailbox.txt | ||
| Makefile | ||
| memory-barriers.txt | ||
| memory-hotplug.txt | ||
| men-chameleon-bus.txt | ||
| nommu-mmap.txt | ||
| ntb.txt | ||
| numastat.txt | ||
| padata.txt | ||
| parport-lowlevel.txt | ||
| percpu-rw-semaphore.txt | ||
| phy.txt | ||
| pi-futex.txt | ||
| pnp.txt | ||
| preempt-locking.txt | ||
| pwm.txt | ||
| rbtree.txt | ||
| remoteproc.txt | ||
| rfkill.txt | ||
| robust-futex-ABI.txt | ||
| robust-futexes.txt | ||
| rpmsg.txt | ||
| rtc.txt | ||
| SAK.txt | ||
| sgi-ioc4.txt | ||
| siphash.txt | ||
| SM501.txt | ||
| smsc_ece1099.txt | ||
| speculation.txt | ||
| static-keys.txt | ||
| SubmittingPatches | ||
| svga.txt | ||
| switchtec.txt | ||
| sync_file.txt | ||
| tee.txt | ||
| this_cpu_ops.txt | ||
| unaligned-memory-access.txt | ||
| vfio-mediated-device.txt | ||
| vfio.txt | ||
| video-output.txt | ||
| xillybus.txt | ||
| xz.txt | ||
| zorro.txt | ||
