On 17 Dec, Mateusz Guzik wrote: > On Thu, Dec 17, 2015 at 11:48:08AM -0800, Don Lewis wrote: >> On 17 Dec, Konstantin Belousov wrote: >> > On Wed, Dec 16, 2015 at 11:08:02AM -0800, Don Lewis wrote: >> >> I used to have a patch the deferred linking the new process into >> >> proctree/allproc until it was fully formed. The motivation was to get >> >> rid of all of the PRS_NEW stuff scattered around the source. >> >> Unfortunately the patch bit-rotted and I'm pretty sure that I lost it. >> > >> > I had similar tought for a second as one of the possibilities to fix the >> > issue, but rejected it outright due to the way the pid allocator works. >> > The loop which faulted is the allocator, it depends on the new pid being >> > linked early to detect the duplicated alloc. >> > >> > What you wrote could be done, but this restructuring requires the separate >> > pid allocator, and probably it must repeat all quirks and subtle behaviour >> > of the current algorithm. But I do not object, PRS_NEW is a trouble >> > on its own. >> >> I don't think it requires any changes to the allocater. It should only >> be necessary to delay the call to fork_findpid() until we are ready to >> link the new proc into allproc. Basically, drop the locks at the >> beginning of do_fork(), then grab them again somewhere near the end >> (probably where we are currently mark the process as PRS_NORMAL) and >> move the call to fork_findpid(), the p2->p_pid assignment, and the list >> manipulation code to a location after that. >> >> It's probably not quite that simple though ... > > That would mean you would need to be able to deconstruct the process > because you cannot guarantee there are any pids left, which may or may > not be easily doable.
It doesn't look like we handle that properly in the current code. I think fork_findpid() will loop forever. It shouldn't be possible if maxproc < pid_max / 3, or maybe pid_max / 2. It might be a good idea to enforce this. > The current method is going to bite us performance-wise anyway and an > allocater which does not require a walk over the tree is necessary in > the long run. Seems like a bitmap (or a bunch of bitmaps) is the way to > go here. I think that separate bitmaps for process, process group, and session ids would be needed. It would waste some space, but it's probably more efficent to use a byte array and store all the bits for the pid together. > Meanwhile one can add a special process permanently in PRS_NEW state and > poisoned pointers in debug kernels to help ensuring that all loops > handle the case. > > Not signing up for any of this work though. _______________________________________________ freebsd-current@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
