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Possible License(s): GPL-2.0, LGPL-2.0, AGPL-1.0
 1IRQ-flags state tracing
 3started by Ingo Molnar <>
 5the "irq-flags tracing" feature "traces" hardirq and softirq state, in
 6that it gives interested subsystems an opportunity to be notified of
 7every hardirqs-off/hardirqs-on, softirqs-off/softirqs-on event that
 8happens in the kernel.
11and CONFIG_PROVE_RW_LOCKING to be offered by the generic lock debugging
12code. Otherwise only CONFIG_PROVE_MUTEX_LOCKING and
13CONFIG_PROVE_RWSEM_LOCKING will be offered on an architecture - these
14are locking APIs that are not used in IRQ context. (the one exception
15for rwsems is worked around)
17architecture support for this is certainly not in the "trivial"
18category, because lots of lowlevel assembly code deal with irq-flags
19state changes. But an architecture can be irq-flags-tracing enabled in a
20rather straightforward and risk-free manner.
22Architectures that want to support this need to do a couple of
23code-organizational changes first:
25- move their irq-flags manipulation code from their asm/system.h header
26  to asm/irqflags.h
28- rename local_irq_disable()/etc to raw_local_irq_disable()/etc. so that
29  the linux/irqflags.h code can inject callbacks and can construct the
30  real local_irq_disable()/etc APIs.
32- add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file
34and then a couple of functional changes are needed as well to implement
35irq-flags-tracing support:
37- in lowlevel entry code add (build-conditional) calls to the
38  trace_hardirqs_off()/trace_hardirqs_on() functions. The lock validator
39  closely guards whether the 'real' irq-flags matches the 'virtual'
40  irq-flags state, and complains loudly (and turns itself off) if the
41  two do not match. Usually most of the time for arch support for
42  irq-flags-tracing is spent in this state: look at the lockdep
43  complaint, try to figure out the assembly code we did not cover yet,
44  fix and repeat. Once the system has booted up and works without a
45  lockdep complaint in the irq-flags-tracing functions arch support is
46  complete.
47- if the architecture has non-maskable interrupts then those need to be
48  excluded from the irq-tracing [and lock validation] mechanism via
49  lockdep_off()/lockdep_on().
51in general there is no risk from having an incomplete irq-flags-tracing
52implementation in an architecture: lockdep will detect that and will
53turn itself off. I.e. the lock validator will still be reliable. There
54should be no crashes due to irq-tracing bugs. (except if the assembly
55changes break other code by modifying conditions or registers that
56shouldnt be)