[Devel] Re: [PATCH 01/24] pidns: Remove races by stopping the
caching of proc_mnt
Eric W. Biederman
ebiederm at xmission.com
Sun Jul 11 10:25:18 EDT 2010
Louis Rilling <Louis.Rilling at kerlabs.com> 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
> 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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