On Wed 2016-09-28 10:18:45, Sergey Senozhatsky wrote: > On (09/27/16 18:02), Petr Mladek wrote: > > The main trick is that we replace the per-CPU function pointer > > by a preempt_count-like variable that could track the printk context. > > > > I know that Sergey has another ideas in this area. But I wanted to see > > how this approach would look like. > > well, yes. I was looking at WARN_*_DEFERRED [1] for some time, and, I > think, the maintenance cost of that solution is just too high: > > a) every existing WARN_* in sched/timekeeping/who knows where else > must be evaluated to ensure that in can't be called from printk() > path. if `false' - then the corresponding macro must be replaced > with _DEFERRED flavor. > > b) any patch that adds new WARN_* usages must be additionally checked > to ensure that each of new WARN_* macros cannot be called from printk > path. if `false' -- the corresponding macro must be replaced with > _DEFERRED flavor. > > c) any patch that refactors the code or moves some function calls around > etc. must be additionally checked for any accidental WARN_* from printk > path. even though if none of the patches added any new WARN_* to the code. > > b) apart from WARN_* there can be `accidental' pr_err/pr_debug/etc. not > necessarily newly added (see 'c'). > > > that's too much. > > it takes a lot of additional effort, because both reviewer and contributor > must consider printk() internals. and, what's worse, if something goes > unnoticed we end up having a printk() deadlock again. > > so I decided to address some of printk() issues in printk.c, not in > kernel/time/timekeeping.c or kernel/sched/core.c or anywhere else.
I see the point. Your approach (alt buffer) adds some complexity to the printk code but it allows to remove printk_deferred()/WARN_DEFERRED() and all the risk of it. I am going to look closely on it. Best Regards, Petr
