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IRQ-flags state tracing
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: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)