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