[PATCH] lockdep: irqtrace subsystem, docs

IRQ-flags state tracing

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:

- 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

shouldnt be)