On Fri 25-11-16 16:48:40, Ted Tso wrote: > On Fri, Nov 25, 2016 at 11:51:26AM -0800, Linus Torvalds wrote: > > We do have filesystem code that is just disgusting. As an example: > > fs/afs/ tends to have these crazy "_enter()/_exit()" macros in every > > single function. If you want that, use the function tracer. That seems > > to be just debugging code that has been left around for others to > > stumble over. I do *not* believe that we should encourage that kind of > > "machine gun spray" use of tracepoints. > > There is a reason why people want to be able to do that, and that's > because kprobes doesn't give you access to the arguments and return > codes to the functions. Maybe there could be a way to do this more > easily using DWARF information and EBPF magic, perhaps? It won't help > for inlined functions, of course, but most of the functions where > people want to do this aren't generally functions which are going to > be inlined, but rather things like write_begin, writepages, which are > called via a struct ops table and so will never be inlined to begin > with. Actually, you can print register & stack contents from a kprobe and you can get a function return value from a kretprobe (see Documentation/trace/kprobetrace.txt). Since calling convention is fixed (arg 1 in RDI, arg 2 in RSI...) you can relatively easily dump function arguments on entry and dump return value on return for arbitrary function of your choice. I was already debugging issues like that several times (in VFS actually because of missing trace points ;)). You can even create a kprobe to dump register contents in the middle of the function (although there it takes more effort reading the dissasembly to see what you are interested in). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
