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Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: There isn't much to be done here: just mark the document title as such and add a :Author:. While here, use upper case at the beginning of a few paragraphs that start with lower case. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
53 lines
2.3 KiB
Plaintext
53 lines
2.3 KiB
Plaintext
=======================
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IRQ-flags state tracing
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=======================
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:Author: 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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- 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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shouldn't be)
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