path: root/Documentation/irqflags-tracing.txt
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-IRQ-flags state tracing
-started by Ingo Molnar <>
-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.
-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:
-- move their irq-flags manipulation code from their asm/system.h header
- to asm/irqflags.h
-- rename local_irq_disable()/etc to raw_local_irq_disable()/etc. so that
- the linux/irqflags.h code can inject callbacks and can construct the
- real local_irq_disable()/etc APIs.
-- 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)