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    SubjectRe: [PATCH 01/24] pidns: Remove races by stopping the caching of proc_mnt
    Louis Rilling <> writes:

    > On 09/07/10 8:58 -0700, Eric W. Biederman wrote:
    >> Having proc reference the pid_namespace and the pid_namespace
    >> reference proc is a serious reference counting problem, which has
    >> resulted in both leaks and use after free problems. Mount already
    >> knows how to go from a pid_namespace to a mount of proc, so we don't
    >> need to cache the proc mount.
    >> To do this I introduce get_proc_mnt and replace pid_ns->proc_mnt users
    >> with it. Additionally I remove pid_ns_(prepare|release)_proc as they
    >> are now unneeded.
    >> This is slightly less efficient but it is much easier to avoid the
    >> races. If efficiency winds up being a problem we can revisit our data
    >> structures.
    > IIUC, the difference between this solution and the first one I proposed is that
    > instead of pinning proc_mnt with mntget() at copy_process()-time, proc_mnt is
    > looked for and, if possible, mntget() at release_task()-time.
    > Could you elaborate on the trade-off, that is accessing proc_mnt at
    > copy_process()-time vs looking up proc_mnt at release_task()-time?

    A little code simplicity. But Serge was right there is cost a noticeable
    cost. About 5%-7% more on lat_proc from lmbench.

    The real benefit was simplicity.


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