Commits
Al Cooper committed 58b69401c79
MIPS: Function tracer: Fix broken function tracing 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 don't 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 traceable 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. So when not disabled, the original jalr and addiu will be there, so _mcount has to adjust sp. The problem 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 writes two nops starting at the address of the "jalr _mcount" instruction whenever tracing is disabled. This means that the "addiu sp,sp.-8" will be converted to a nop along with the "jalr". When disabled, there will be two nops. This is SMP safe because the first time this happens is during ftrace_init() which is before any other processor has been started. Subsequent calls to enable/disable tracing when other CPUs ARE running will still be safe because the enable will only change the first nop to a "jalr" and the disable, while writing 2 nops, will only be changing the "jalr". This patch also stops using stop_machine() to call the tracer enable/disable routines and calls them directly because the routines are SMP safe. When the kernel first boots we have to be able to handle the gcc generated jalr, addui sequence until ftrace_init gets a chance to run and change the sequence. At this point mcount just adjusts the stack and returns. When ftrace_init runs, we convert the jalr/addui to nops. Then whenever tracing is enabled we convert the first nop to a "jalr mcount+8". The mcount+8 entry point skips the stack adjust. [ralf@linux-mips.org: Folded in Steven Rostedt's build fix.] Signed-off-by: Al Cooper <alcooperx@gmail.com> Cc: rostedt@goodmis.org Cc: ddaney.cavm@gmail.com Cc: linux-mips@linux-mips.org Cc: linux-kernel@vger.kernel.org Patchwork: https://patchwork.linux-mips.org/patch/4806/ Patchwork: https://patchwork.linux-mips.org/patch/4841/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>