Messages in this thread | | | Date | Mon, 14 Jan 2013 14:12:33 -0800 | From | David Daney <> | Subject | Re: [PATCH] mips: function tracer: Fix broken function tracing |
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On 01/14/2013 01:10 PM, Alan Cooper wrote: > I already tried using "adddiu sp, sp, 8" and it caused the kernel to > randomly crash. After many hours of debugging the reason occurred to > me while in bed in the middle of the night. The problem is that if we > get an interrupt between the add 8 and the add -8 instructions, we > trash the existing stack. > > The problem with the 2 nop approach is that there are a series of > subroutines used to write each nop and these nested subroutines are > traceable.
This seems like a bug. The low-level code used to do code patching probably should be CFLAGS_REMOVE_file.o = -pg
> This means on the second call to these subroutines they > execute with only one nop and crash. I could write some new code > that wrote the 2 nops at once, but (now that I understand > "stop_machine") with the branch likely solution we should be able to > stop using "stop_machine" when we write nops to the 20-30 thousand > Linux functions. It looks like other platforms have stopped using > stop_machine.
I don't particularly object to the 'branch likely solution', but I think the failures of the other approaches indicates underlying bugs in the tracing code. Those bugs should probably be fixed.
David Daney
> > Al > > On Fri, Jan 11, 2013 at 12:01 PM, David Daney <ddaney.cavm@gmail.com> wrote: >> On 01/11/2013 06:33 AM, Al Cooper wrote: >>> >>> Function tracing is currently broken for all 32 bit MIPS platforms. >>> When tracing is enabled, the kernel immediately hangs on boot. >>> This is a result of commit b732d439cb43336cd6d7e804ecb2c81193ef63b0 >>> that changes the kernel/trace/Kconfig file so that is no longer >>> forces FRAME_POINTER when FUNCTION_TRACING is enabled. >>> >>> MIPS frame pointers are generally considered to be useless because >>> they cannot be used to unwind the stack. Unfortunately the MIPS >>> function tracing code has bugs that are masked by the use of frame >>> pointers. This commit fixes the bugs so that MIPS frame pointers do >>> not need to be enabled. >>> >>> The bugs are a result of the odd calling sequence used to call the trace >>> routine. This calling sequence is inserted into every tracable function >>> when the tracing CONFIG option is enabled. This sequence is generated >>> for 32bit MIPS platforms by the compiler via the "-pg" flag. >>> Part of the sequence is "addiu sp,sp,-8" in the delay slot after every >>> call to the trace routine "_mcount" (some legacy thing where 2 arguments >>> used to be pushed on the stack). The _mcount routine is expected to >>> adjust the sp by +8 before returning. >>> >>> One of the bugs is that when tracing is disabled for a function, the >>> "jalr _mcount" instruction is replaced with a nop, but the >>> "addiu sp,sp,-8" is still executed and the stack pointer is left >>> trashed. When frame pointers are enabled the problem is masked >>> because any access to the stack is done through the frame >>> pointer and the stack pointer is restored from the frame pointer when >>> the function returns. This patch uses a branch likely instruction >>> "bltzl zero, f1" instead of "nop" to disable the call because this >>> instruction will not execute the "addiu sp,sp,-8" instruction in >>> the delay slot. The other possible solution would be to nop out both >>> the jalr and the "addiu sp,sp,-8", but this would need to be interrupt >>> and SMP safe and would be much more intrusive. >> >> >> I thought all CPUs were in stop_machine() when the modifications were done, >> so that there is no issue with multi-word instruction patching. >> >> Am I wrong about this? >> >> So really I think you can do two NOP just as easily. >> >> The only reason I bring this up is that I am not sure all 32-bit CPUs >> implement the 'Likely' branch variants. Also there may be an affect on the >> branch predictor. >> >> A third possibility would be to replace the JALR with 'ADDIU SP,SP,8' That >> way the following adjustment would be canceled out. This would work well on >> single-issue CPUs, but the instructions may not be able to dual-issue on a >> multi issue machine due to data dependencies. >> >> David Daney >> >> >>> >>> A few other bugs were fixed where the _mcount routine itself did not >>> always fix the sp on return. >>> >> > >
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