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Documentation / irqflags-tracing.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	IRQ-flags state tracing
2	
3	started by Ingo Molnar <mingo@redhat.com>
4	
5	the "irq-flags tracing" feature "traces" hardirq and softirq state, in
6	that it gives interested subsystems an opportunity to be notified of
7	every hardirqs-off/hardirqs-on, softirqs-off/softirqs-on event that
8	happens in the kernel.
9	
10	CONFIG_TRACE_IRQFLAGS_SUPPORT is needed for CONFIG_PROVE_SPIN_LOCKING
11	and CONFIG_PROVE_RW_LOCKING to be offered by the generic lock debugging
12	code. Otherwise only CONFIG_PROVE_MUTEX_LOCKING and
13	CONFIG_PROVE_RWSEM_LOCKING will be offered on an architecture - these
14	are locking APIs that are not used in IRQ context. (the one exception
15	for rwsems is worked around)
16	
17	architecture support for this is certainly not in the "trivial"
18	category, because lots of lowlevel assembly code deal with irq-flags
19	state changes. But an architecture can be irq-flags-tracing enabled in a
20	rather straightforward and risk-free manner.
21	
22	Architectures that want to support this need to do a couple of
23	code-organizational changes first:
24	
25	- move their irq-flags manipulation code from their asm/system.h header
26	  to asm/irqflags.h
27	
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.
31	
32	- add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file
33	
34	and then a couple of functional changes are needed as well to implement
35	irq-flags-tracing support:
36	
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().
50	
51	in general there is no risk from having an incomplete irq-flags-tracing
52	implementation in an architecture: lockdep will detect that and will
53	turn itself off. I.e. the lock validator will still be reliable. There
54	should be no crashes due to irq-tracing bugs. (except if the assembly
55	changes break other code by modifying conditions or registers that
56	shouldn't be)
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