Re: [patch] mm, oom: remove task_lock protecting comm printing

[Vladimir Davydov]

Hi,

On Tue, Sep 22, 2015 at 04:30:13PM -0700, David Rientjes wrote:
> The oom killer takes task_lock() in a couple of places solely to protect
> printing the task's comm.
> 
> A process's comm, including current's comm, may change due to
> /proc/pid/comm or PR_SET_NAME.
> 
> The comm will always be NULL-terminated, so the worst race scenario would
> only be during update.  We can tolerate a comm being printed that is in
> the middle of an update to avoid taking the lock.
> 
> Other locations in the kernel have already dropped task_lock() when
> printing comm, so this is consistent.

Without the protection, can't reading task->comm race with PR_SET_NAME
as described below?

Let T->comm[16] = "name\0rubbish1234"

CPU1                                    CPU2
----                                    ----
set_task_comm(T, "longname\0")
  T->comm[0] = 'l'
  T->comm[1] = 'o'
  T->comm[2] = 'n'
  T->comm[3] = 'g'
  T->comm[4] = 'n'
                                        printk("%s\n", T->comm)
                                          T->comm = "longnrubbish1234"
                                          OOPS: the string is not
                                                nil-terminated!
  T->comm[5] = 'a'
  T->comm[6] = 'm'
  T->comm[7] = 'e'
  T->comm[8] = '\0'

Thanks,
Vladimir
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