2011/12/2 Andriy Gapon <a...@freebsd.org>: > on 02/12/2011 20:40 John Baldwin said the following: >> On 12/2/11 12:18 PM, Attilio Rao wrote: >>> 2011/12/2 John Baldwin<j...@freebsd.org>: >>>> On 12/2/11 5:05 AM, Andriy Gapon wrote: >>>>> >>>>> on 02/12/2011 06:36 John Baldwin said the following: >>>>>> >>>>>> Ah, ok (I had thought SCHEDULER_STOPPED was going to always be true when >>>>>> kdb was >>>>>> active). But I think these two changes should cover critical_exit() ok. >>>>>> >>>>> >>>>> I attempted to start a discussion about this a few times already :-) >>>>> Should we treat kdb context the same as SCHEDULER_STOPPED context (in the >>>>> current definition) ? That is, skip all locks in the same fashion? >>>>> There are pros and contras. >>>> >>>> >>>> kdb should not block on locks, no. Most debugger commands should not go >>>> near locks anyway unless they are intended to carefully modify the existing >>>> system in a safe manner (such as the 'kill' command which should only be >>>> using try locks and fail if it cannot safely post the signal). >>> >>> The biggest problem to KDB as the same as panic is that doing proper >>> 'continue' is impossible. >>> One of the features of the 'skip-locking' path is that it doesn't take >>> into account fast locking paths, where sometimes the lock can succeed >>> and other fails and you don't know about them. Also the restarted CPUs >>> can find corrupted datas (as they can be arbitrarely updated), I'm >>> sure it is too much panic prone. >> >> Yes, my thought is that kdb commands, etc. should be using dedicated routines >> that do not use locks whenever possible. The problem of a user >> calling an arbitrary routine is not solvable (so I don't think we should try >> to >> solve that, you use 'call' at your own risk), but built-in commands should >> explicitly either 1) not use locking, or 2) only use try locks and fail out >> cleanly (including dropping any try locks acquired) if a try fails. Now, >> that's >> an ideal view, I don't know how close we are to that in practice or if it is >> a >> realistically attainable goal. >> > > > I agree with what Attilio and you say. Initially it was tempting for me to > apply the same SCHEDULER_STOPPED stopped medicine to the kdb_active context, > but > after trying to deal with kdb_active x SCHEDULER_STOPPED x ukbd situation I > really changed my mind. > > > I would classify the code that can be called in kdb_active context as follows: > o debugger code proper (kdb, ddb, gdb stub, etc) - this obviously must not > (doesn't have to) use any locking > > o code that can be invoked via 'call' command - this is essentially any code > and > I don't think that it can/should do anything special for the kdb_active > context [*] > > o debugger helper routines - those that do something trivial should not > acquire > any locks; those that access shared resources should try the relevant locks > and > bail out if a resource can be in inconsistent state, or should be equipped to > deal correctly with such a state; this is the same as what you say above > > o common code that the debuggers have to use - most obviously this is console > code and drivers that serve a particular console; on one hand those drivers > can > have a non-trivial state that must be lock protected during normal operation, > on > the other hand the debugger must disregard those locks and grab its console; > this is the most complex case in my opinion.
Thanks for summarizing this up. However, please note that code in 2 and 4 entries may have the same issues or being the same thing, in practice. Anyway, I'm thinking now that if we really want to stop CPUs when entering KDB (and I think we do) functions at 2 and 4 should basically just be totally lockless or in general being totally re-entrant because when we restart CPUs we don't really want them finding datas to be corrupted. Also, skipping locking, is totally broken for this very specific reason. Functions at point 2 and 4 should be totally lockless then and possibly just work on read mode. For point 2, specifically, I think we need an explicit KPI to define functions within the subsystem themselves (something like DB_SHOW_COMMAND()) which marks undoublty functions to be called within ddb (the only KDB backend we implement right now) and likely for functions at point 4 we need to find a way to stress their belonging to the KDB area. Attilio -- Peace can only be achieved by understanding - A. Einstein _______________________________________________ firstname.lastname@example.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"