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tile: improve stack backtrace
This commit fixes a number of issues with the tile backtrace code.
- Don't try to identify userspace shared object or executable paths
if we are doing a backtrace from an interrupt; it's not legal,
and also unlikely to be interesting. Likewise, don't try to do
it for other address spaces, since d_path() assumes it is being
called in "current" context.
- Move "in_backtrace" from thread_struct to thread_info.
This way we can access it even if our stack thread_info has been
clobbered, which makes backtracing more robust.
- Avoid using "current" directly when testing for is_sigreturn().
Since "current" may be corrupt, we're better off using kbt->task
explicitly to look up the vdso_base for the current task.
Conveniently, this simplifies the internal APIs (we only need
one is_sigreturn() function now).
- Avoid bogus "Odd fault" warning when pc/sp/ex1 are all zero,
as is true for kernel threads above the last frame.
- Hook into Tejun Heo's dump_stack() framework in lib/dump_stack.c.
- Write last entry in save_stack_trace() as ULONG_MAX, not zero,
since ftrace (at least) relies on finding that marker.
- Implement save_stack_trace_regs() and save_strack_trace_user(),
and set CONFIG_USER_STACKTRACE_SUPPORT.
Signed-off-by: 
Chris Metcalf <cmetcalf@ezchip.com>