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SubjectRe: [PATCH 1/4] comm: Introduce comm_lock spinlock to protect task->comm access
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On Wed, 2011-05-18 at 11:01 +0900, KOSAKI Motohiro wrote:
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 5e62d26..34fa611 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -998,17 +998,28 @@ static void flush_old_files(struct files_struct * files)
> >
> > char *get_task_comm(char *buf, struct task_struct *tsk)
> > {
> > - /* buf must be at least sizeof(tsk->comm) in size */
> > - task_lock(tsk);
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&tsk->comm_lock, flags);
> > strncpy(buf, tsk->comm, sizeof(tsk->comm));
> > - task_unlock(tsk);
> > + spin_unlock_irqrestore(&tsk->comm_lock, flags);
> > return buf;
> > }
> >
> > void set_task_comm(struct task_struct *tsk, char *buf)
> > {
> > + unsigned long flags;
> > +
> > + /*
> > + * XXX - Even though comm is protected by comm_lock,
> > + * we take the task_lock here to serialize against
> > + * current users that directly access comm.
> > + * Once those users are removed, we can drop the
> > + * task locking& memsetting.
> > + */
>
> If we provide __get_task_comm(), we can't remove memset() forever.

True enough. I'll fix that comment up then.

>
> > task_lock(tsk);
> > + spin_lock_irqsave(&tsk->comm_lock, flags);
>
> This is strange order. task_lock() doesn't disable interrupt.

Strange order? Can you explain why you think that is? Having comm_lock
as an inner-most lock seems quite reasonable, given the limited nature
of what it protects.

> And, can you please document why we need interrupt disabling?

Since we might access current->comm from irq context. Where would you
like this documented? Just there in the code?

thanks
-john




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