1======================= 2IRQ-flags state tracing 3======================= 4 5:Author: started by Ingo Molnar <mingo@redhat.com> 6 7The "irq-flags tracing" feature "traces" hardirq and softirq state, in 8that it gives interested subsystems an opportunity to be notified of 9every hardirqs-off/hardirqs-on, softirqs-off/softirqs-on event that 10happens in the kernel. 11 12CONFIG_TRACE_IRQFLAGS_SUPPORT is needed for CONFIG_PROVE_SPIN_LOCKING 13and CONFIG_PROVE_RW_LOCKING to be offered by the generic lock debugging 14code. Otherwise only CONFIG_PROVE_MUTEX_LOCKING and 15CONFIG_PROVE_RWSEM_LOCKING will be offered on an architecture - these 16are locking APIs that are not used in IRQ context. (the one exception 17for rwsems is worked around) 18 19Architecture support for this is certainly not in the "trivial" 20category, because lots of lowlevel assembly code deal with irq-flags 21state changes. But an architecture can be irq-flags-tracing enabled in a 22rather straightforward and risk-free manner. 23 24Architectures that want to support this need to do a couple of 25code-organizational changes first: 26 27- add and enable TRACE_IRQFLAGS_SUPPORT in their arch level Kconfig file 28 29and then a couple of functional changes are needed as well to implement 30irq-flags-tracing support: 31 32- in lowlevel entry code add (build-conditional) calls to the 33 trace_hardirqs_off()/trace_hardirqs_on() functions. The lock validator 34 closely guards whether the 'real' irq-flags matches the 'virtual' 35 irq-flags state, and complains loudly (and turns itself off) if the 36 two do not match. Usually most of the time for arch support for 37 irq-flags-tracing is spent in this state: look at the lockdep 38 complaint, try to figure out the assembly code we did not cover yet, 39 fix and repeat. Once the system has booted up and works without a 40 lockdep complaint in the irq-flags-tracing functions arch support is 41 complete. 42- if the architecture has non-maskable interrupts then those need to be 43 excluded from the irq-tracing [and lock validation] mechanism via 44 lockdep_off()/lockdep_on(). 45 46In general there is no risk from having an incomplete irq-flags-tracing 47implementation in an architecture: lockdep will detect that and will 48turn itself off. I.e. the lock validator will still be reliable. There 49should be no crashes due to irq-tracing bugs. (except if the assembly 50changes break other code by modifying conditions or registers that 51shouldn't be) 52 53