Date: Fri, 18 Dec 2015 17:38:10 +0100 From: Mateusz Guzik <mjguzik@gmail.com> To: Don Lewis <truckman@FreeBSD.org> Cc: kostikbel@gmail.com, freebsd-current@freebsd.org Subject: Re: fork_findpid() - Fatal trap 12: page fault while in kernel mode Message-ID: <20151218163810.GB830@dft-labs.eu> In-Reply-To: <201512172233.tBHMXkNR096011@gw.catspoiler.org> References: <20151217195802.GA29200@dft-labs.eu> <201512172233.tBHMXkNR096011@gw.catspoiler.org>
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On Thu, Dec 17, 2015 at 02:33:46PM -0800, Don Lewis wrote: > 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. > Not sure I follow, can you rephrase/elaborate? > > 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. > Well I had such separate bitmaps in mind with addition of a combined "the id is in use bitmap". This would make the common case of finding a new pid reasonably fast. Access to all bitmaps would be protected with proctree lock, which matches current locking scheme anyway. -- Mateusz Guzik <mjguzik gmail.com>
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