Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.
1 IRQ-flags state tracing 2 3 started by Ingo Molnar <firstname.lastname@example.org> 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)