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SubjectRe: [RFC PATCH v1 11/31] ARC: Low level IRQ/Trap/Exception(non-MMU) Handling
On Friday 16 November 2012 10:28 AM, Al Viro wrote:
>> + ; --------- check for signals/restore-sigmask ------------
>> + bbit0 r9, TIF_SIGPENDING, chk_next_work
>> +
>> + ; save CALLEE Regs.
>> + ; (i) If this signal causes coredump - full regfile needed
>> + ; (ii) If signal is SIGTRAP/SIGSTOP, task is being traced thus
>> + ; tracer might call PEEKUSR for a CALLEE reg
>> + ;
>> + ; NOTE: SP will grow up by size of CALLEE Reg-File
>> + SAVE_CALLEE_SAVED_USER ; clobbers r12
>> +
>> + ; save location of saved Callee Regs @ thread_struct->callee
>> + GET_CURR_TASK_FIELD_PTR TASK_THREAD, r10
>> + st sp, [r10, THREAD_CALLEE_REG]
>> +
>> + bl @do_signal
>> +
>> + ; unwind SP for cheap discard of Callee saved Regs
>> + DISCARD_CALLEE_SAVED_USER
> Uh-oh... And what if tracer wanted to modify callee-saved regs?

So the solution would be to either unconditionally restore all the 13 callee regs
- or add additional state (struct thread) where ptrace makes a note that it wrote
to a callee reg which is used here to conditional-ize the restore. Former is
simpler to do - although it might ill-affect micro-benchmarks such as LMBench
lat_sig. Anyhow correctness comes before optimization.

>
>> + b resume_user_mode_begin ; loop back to start of U mode ret
>> +
>> + ; --- notify_resume ---
>> +chk_next_work:
>> + btst r9, TIF_NOTIFY_RESUME
>> + blnz @do_notify_resume
>> +
>> + ;--------- All things done, go back to Userland ------
>> +
>> + b restore_regs
> No. After NOTIFY_RESUME stuff you need to recheck SIGPENDING. This should
> go to resume_user_mode_begin, not restore regs.

Thanks for pointing this - fixed in v2 series.

> Another problem here is
> IRQ handling - you hit do_signal()/do_notify_resume() with IRQs disabled,
> which is broken - you need to re-enable it before going into either.

Thanks - fixed that as well. I presume it is broken for "interrupt latency
reasons" and not because IRQs could remain disabled all the way into usermode -
since given the way RTIE insn works - that won't happen. Although independently
this was indeed cause of a WARN_ON_ONCE when bh were enabled with IRQs disabled.



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