Re: [PATCH] mm: rcu-protected get_mm_exe_file()

[Konstantin Khlebnikov]

On 16.03.2015 17:07, Oleg Nesterov wrote:
> On 03/16, Konstantin Khlebnikov wrote:
> > +/**
> > + * set_mm_exe_file - change a reference to the mm's executable file
> > + *
> > + * This changes mm's executale file (shown as symlink /proc/[pid]/exe).
> > + *
> > + * Main users are mmput(), sys_execve() and sys_prctl(PR_SET_MM_MAP/EXE_FILE).
> > + * Callers prevent concurrent invocations: in mmput() nobody alive left,
> > + * in execve task is single-threaded, prctl holds mmap_sem exclusively.
> > + */
> >   void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
> >   {
> > +	struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
> > +			!atomic_read(&mm->mm_users) || current->in_execve ||
> > +			lock_is_held(&mm->mmap_sem));
> > +
> >   	if (new_exe_file)
> >   		get_file(new_exe_file);
> > -	if (mm->exe_file)
> > -		fput(mm->exe_file);
> > -	mm->exe_file = new_exe_file;
> > +	rcu_assign_pointer(mm->exe_file, new_exe_file);
> > +	if (old_exe_file)
> > +		fput(old_exe_file);
> >   }
>
> Yes, I think this is correct, __fput() does call_rcu(file_free_rcu). And
> much better than the new lock ;)
>
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
>
>
>
> So I think the patch is fine, but personally I dislike the "prctl holds
> mmap_sem exclusively" and rcu_dereference_protected().
>
> I mean, I think we can do another cleanup on top of this change.
>
> 	1. set_mm_exe_file() should be called by exit/exec only, so
> 	   it should use
>
> 		rcu_dereference_protected(mm->exe_file,
> 					atomic_read(&mm->mm_users) <= 1);
>
> 	2. prctl() should not use it, it can do
>
> 	   get_file(new_exe);
> 	   old_exe = xchg(&mm->exe_file);
> 	   if (old_exe)
> 	   	fput(old_exe);

I think smp_mb() is required before xchg() or
probably this stuff should be hidden inside yet another magic RCU macro
( with two screens of comments =)

> 	3. and we can remove down_write(mmap_sem) from prctl paths.
>
> 	   Actually we can do this even without xchg() above, but we might
> 	   want to kill MMF_EXE_FILE_CHANGED and test_and_set_bit() check.
>
> What do you think?
>
> Oleg.


--
Konstantin
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