[PATCH 01/24] pidns: Remove races by stopping the caching of proc_mnt

[Eric W. Biederman]

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
>> 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.

Eric