On Wed, May 6, 2020 at 2:13 PM Sreyan Chakravarty <sreyan32@xxxxxxxxx> wrote: > > I am reading Robert Love's Book Linux Kernel Development(https://rads.stackoverflow.com/amzn/click/com/B003V4ATI0). > > It uses the 2.6.33 kernel for demonstration. > > I have been going through certain parts of the source and can't find out where is the initial definition of many things. A lot of things are just used, like "magic" without me finding the definition. > > One example: > > static struct sched_entity *__pick_next_entity(struct cfs_rq *cfs_rq) > { > struct rb_node *left = cfs_rq->rb_leftmost; > > if (!left) > return NULL; > > return rb_entry(left, struct sched_entity, run_node); > } > > static struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq) > { > struct rb_node *last = rb_last(&cfs_rq->tasks_timeline); > > if (!last) > return NULL; > > return rb_entry(last, struct sched_entity, run_node); > } > > > This in kernel/sched_fair.c lines 384 and 394 in 2.6.33 kernel. > > Where is the run_node coming from ? > > I have grep ed(https://livegrep.com/search/linuxq=run_node&fold_case=auto®ex=false&context=true) the entire source base over here, and I have not found any definition of run_node that would allow it to be used like this. > > There is a declaration(https://github.com/torvalds/linux/blob/219d54332a09e8d8741c1e1982f5eae56099de85/include/linux/sched.h#L448) in the sched_entity structure, but nothing outside of it that would allow it to be used like this. > > I cannot understand how things are organized, its really confusing. > > What is going on ? I believe you are assuming "rb_entry" is a C function and run_node is a variable, but it is not. Instead rb_entry is a macro that takes the argument as "plain source code strings" and creates an C-code expression out of that. If you look at the macro definitions of rb_entry and follow that to the truth, you will how everything magically resolves to C code that a C compiler could actually parse and make sense of. I hope that helps. Lukas _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
