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    SubjectRe: [RFC][PATCH] ps command race fix take2 [1/4] list token
    On Tue, 22 Aug 2006 10:56:08 -0600 (Eric W. Biederman) wrote:

    > KAMEZAWA Hiroyuki <> writes:
    > > This is ps command race fix take2. Unfortunately, against 2.6.18-rc4.
    > > I'll rebase this to appropriate kernel if O.K. (I think this is RFC)
    > >
    > > This patch implements Paul Jackson's idea, 'inserting false link in task list'.
    > Currently the tasklist_lock is one of the more highly contended locks in
    > the kernel. Adding an extra place it is taken is undesirable.
    yes. taking lock is a probem.
    I know current readdir() uses 8192 bytes buffer for getdents64(). Then,
    maybe write-lock will be acquired all-tgids/400+ times for inserting token
    (in 32bit system).

    > If could see a better algorithm for sending a signal to all processes
    > in a process groups we could remove the tasklist_lock entirely.
    Sorry, could you explain more ?

    > In addition you only solves half the readdir problems. You don't solve
    > the seek problem which is returning to an offset you had been to
    > before. A relatively rare case but...
    Ah, I should add lseek handler for proc root. Okay.

    > > Good point of this approach is cost of searching task is O(N) (N=num of tgids).
    > > Bad point is lock and kmalloc/kfree.
    > > I didin't modified thread_list and cpuset's proc list, maybe future work.
    > >
    > > If searching pid bitmap is better, please take Erics.
    > My patch at least needs a good changelog but I believe it will work
    > better and can be further improved with a better pid data structure
    > if there is actually a problem there. Given that I don't take
    > any locks it should be much friendlier at scale, and the code
    > was simpler.
    yes. it has several good points and simple.
    My patch's point is just using task_list if we can, because it exists for keeping
    all tasks(tgids).

    > However I will miss a few newly forked processes and I don't think your
    > technique will miss any. Still neither will miss a process that
    > existed the entire time.
    > If nothing else I think it was worth posting so we could contrast the two.
    please post again. I think comparing the two is good.
    I will post take3 with improved comments and lseek handler, and so on.


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