unw_init_frame_info() and activation records
Quoted 3 lines · Piet/Pete Delaney
The kdb code at least seems to support the
NEW_UNWINDcode by usingunw_init_frame_info(). You code mentions that it should be using activation records and I was wondering what that's all about.
kdb for i386 uses an internal structure that holds the activation information about each frame. Some of the stack data is arch independent, some is arch dependent, I would like to use a common internal representation for kdb stack information across all architectures so the common code is easier to maintain.
The activation record was my first attempt at abstracting stack info, but it is too i386 specific. One day I will do a better abstraction method, for now kdb for each architecture handles its own stack decoding.
Quoted 2 lines
Perhaps you could take a few minutes and provide a few pointers on how to save time in adding unwind support.
For IA64, use the kernel unwind routines. The main problem on ia64 was to distinguish between functions that were running and functions that were sleeping, they have different starting points for unwinding. Detecting current is not enough, because you need to access current tasks on other cpus.
Running tasks do not have a switch_stack structure, this structure is required to prime the unwind code. To ensure that all tasks have a switch_stack, arch/ia64/kdb/kdbasupport.c::kdba_main_loop() is invoked for each process that enters kdb, kdba_main_loop() calls unw_init_running() to build switch_stack before continuing with the rest of the kdb processing.
The primary kdb cpu tries to drive all the other cpus into kdb, which will invoke kdba_main_loop() for the running process on each cpu and set up a switch_stack. This can fail if the other cpus are ignoring interrupts. Trying to backtrace a process in a disabled spin loop on another cpu will not work with this method.
This is just one symptom of the larger problem that kdb cannot break into disabled spin loops on ia64, even NMI is ignored. IMNSHO the real fix is to change the cli/sti code on ia64 so it does not mask NMI. David Mosberger has rejected this for the standard kernel, but I intend to add it as a debugging option for kdb on ia64. I don't like changing such low level routines, it makes a debugging kernel different from a normal kernel. But sometimes it is the only way to debug disabled spin lock problems, so it will be an additional kdb option.