----- On Oct 23, 2017, at 7:30 PM, Ben Maurer bmaurer@xxxxxx wrote: >> if (!((long)ip - (long)start_ip <= (long)post_commit_offset)) >> return 1; > >> This introduces an issue here: if "ip" is lower than "start_ip", we >> can incorrectly think we are in a critical section, when we are in >> fact not. > > This shouldn't be an issue if we used unsigned numbers. Eg if start_ip is X and > post_commit_offset is L, then (ip - X <= L) means that if ip is less than X ip > - X will be signed, which will become a large unsigned value. > >> or to the kernel to set it back to NULL if it finds out that it is >> preempting/delivering a signal over an instruction pointer outside >> of the current rseq_cs start_ip/post_commit_ip range (lazy clear). > > I see, lazy clear makes sense. Still, if during most execution periods the user > code enters some rseq section (likely if rseq is used for something like > malloc) on every context switch this code will have to be run. > >> Moreover, this modification would add a subtraction on the common case >> (ip - start_ip), and makes the ABI slightly uglier. > > We could benchmark it but the subtraction should be similar in cost to the extra > comparison but reducing the number of branches seems like it will help as well. > FWIW GCC attempts to translate this kind of sequence to a subtract and compare: > https://godbolt.org/g/5DGLvo. > > I agree the ABI is uglier, but since we're mucking with every context switch I > thought I'd point it out. Thanks for following up on this. I did not initially realize the importance of doing the unsigned comparison. I've pushed a commit in my private dev branch implementing your suggestion. https://github.com/compudj/linux-percpu-dev/commit/4cf8e9104636b51741c0118f2c88519e3acab7aa Thanks! Mathieu > >> If I understand well, you are proposing to speed up .so load time by >> means of removing relocations of pointers within rseq_cs, done by >> making those relative to the rseq_cs address. > > Yeah, I think this may be overkill as optimization. -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
