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Diffstat (limited to 'Documentation/core-api/irq/irqflags-tracing.rst')
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diff --git a/Documentation/core-api/irq/irqflags-tracing.rst b/Documentation/core-api/irq/irqflags-tracing.rst new file mode 100644 index 000000000000..bdd208259fb3 --- /dev/null +++ b/Documentation/core-api/irq/irqflags-tracing.rst @@ -0,0 +1,52 @@ +======================= +IRQ-flags state tracing +======================= + +:Author: started by Ingo Molnar <mingo@redhat.com> + +The "irq-flags tracing" feature "traces" hardirq and softirq state, in +that it gives interested subsystems an opportunity to be notified of +every hardirqs-off/hardirqs-on, softirqs-off/softirqs-on event that +happens in the kernel. + +CONFIG_TRACE_IRQFLAGS_SUPPORT is needed for CONFIG_PROVE_SPIN_LOCKING +and CONFIG_PROVE_RW_LOCKING to be offered by the generic lock debugging +code. Otherwise only CONFIG_PROVE_MUTEX_LOCKING and +CONFIG_PROVE_RWSEM_LOCKING will be offered on an architecture - these +are locking APIs that are not used in IRQ context. (the one exception +for rwsems is worked around) + +Architecture support for this is certainly not in the "trivial" +category, because lots of lowlevel assembly code deal with irq-flags +state changes. But an architecture can be irq-flags-tracing enabled in a +rather straightforward and risk-free manner. + +Architectures that want to support this need to do a couple of +code-organizational changes first: + +- add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file + +and then a couple of functional changes are needed as well to implement +irq-flags-tracing support: + +- in lowlevel entry code add (build-conditional) calls to the + trace_hardirqs_off()/trace_hardirqs_on() functions. The lock validator + closely guards whether the 'real' irq-flags matches the 'virtual' + irq-flags state, and complains loudly (and turns itself off) if the + two do not match. Usually most of the time for arch support for + irq-flags-tracing is spent in this state: look at the lockdep + complaint, try to figure out the assembly code we did not cover yet, + fix and repeat. Once the system has booted up and works without a + lockdep complaint in the irq-flags-tracing functions arch support is + complete. +- if the architecture has non-maskable interrupts then those need to be + excluded from the irq-tracing [and lock validation] mechanism via + lockdep_off()/lockdep_on(). + +In general there is no risk from having an incomplete irq-flags-tracing +implementation in an architecture: lockdep will detect that and will +turn itself off. I.e. the lock validator will still be reliable. There +should be no crashes due to irq-tracing bugs. (except if the assembly +changes break other code by modifying conditions or registers that +shouldn't be) + |