1IRQ-flags state tracing 2 3started by Ingo Molnar <mingo@redhat.com> 4 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. 9 10CONFIG_TRACE_IRQFLAGS_SUPPORT is needed for CONFIG_PROVE_SPIN_LOCKING 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) 16 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. 21 22Architectures that want to support this need to do a couple of 23code-organizational changes first: 24 25- add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file 26 27and then a couple of functional changes are needed as well to implement 28irq-flags-tracing support: 29 30- in lowlevel entry code add (build-conditional) calls to the 31 trace_hardirqs_off()/trace_hardirqs_on() functions. The lock validator 32 closely guards whether the 'real' irq-flags matches the 'virtual' 33 irq-flags state, and complains loudly (and turns itself off) if the 34 two do not match. Usually most of the time for arch support for 35 irq-flags-tracing is spent in this state: look at the lockdep 36 complaint, try to figure out the assembly code we did not cover yet, 37 fix and repeat. Once the system has booted up and works without a 38 lockdep complaint in the irq-flags-tracing functions arch support is 39 complete. 40- if the architecture has non-maskable interrupts then those need to be 41 excluded from the irq-tracing [and lock validation] mechanism via 42 lockdep_off()/lockdep_on(). 43 44in general there is no risk from having an incomplete irq-flags-tracing 45implementation in an architecture: lockdep will detect that and will 46turn itself off. I.e. the lock validator will still be reliable. There 47should be no crashes due to irq-tracing bugs. (except if the assembly 48changes break other code by modifying conditions or registers that 49shouldn't be) 50 51