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

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (09/23/15 11:06), Vladimir Davydov wrote:
> 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?

the previous name was already null terminated, so it should be

	[name\0old_name\0]

	-ss

> 
> 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'

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