[question] shm_nattch in sys_shmat?

[question] shm_nattch in sys_shmat?

[Matthew Dobson]

Hello all!
	In case it wasn't obvious from the subject, I've got a question about a 
piece of code in ipc/shm.c:sys_shmat(), more specifically about the use 
of the shm_nattch counter.  This is supposed to be used to count the 
number of times the shared memory segment has been attatched to a 
processes adress space.  For example, shm_open & shm_mmap both increment 
shm_nattch, and shm_close decrements shm_nattch.  I would be inclined to 
think that sys_shmat should increment this counter, to keep track of a 
new attatchment of the shared segment to a processes adress space. 
sys_shmat, does in fact increment shm_nattch, but only to decrement it 
again a few lines later, as seen in this code snippet.  Can anyone 
please explain why this is?

 >	file = shp->shm_file;
 >	size = file->f_dentry->d_inode->i_size;
 >>>	shp->shm_nattch++;
 >	shm_unlock(shp);
 >
 >	down_write(&current->mm->mmap_sem);
 >	if (addr && !(shmflg & SHM_REMAP)) {
 >		user_addr = ERR_PTR(-EINVAL);
 >		if (find_vma_intersection(current->mm, addr, addr + size))
 >			goto invalid;
 >		/*
 >		 * If shm segment goes below stack, make sure there is some
 >		 * space left for the stack to grow (at least 4 pages).
 >		 */
 >		if (addr < current->mm->start_stack &&
 >		    addr > current->mm->start_stack - size - PAGE_SIZE * 5)
 >			goto invalid;
 >	}
 >
 >	user_addr = (void*) do_mmap (file, addr, size, prot, flags, 0);
 >
 >invalid:
 >	up_write(&current->mm->mmap_sem);
 >
 >	down (&shm_ids.sem);
 >	if(!(shp = shm_lock(shmid)))
 >		BUG();
 >>>	shp->shm_nattch--;
 >	if(shp->shm_nattch == 0 &&
 >	   shp->shm_flags & SHM_DEST)
 >		shm_destroy (shp);
 >	else
 >		shm_unlock(shp);
 >	up (&shm_ids.sem);

Thanks!

-Matt

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Re: [question] shm_nattch in sys_shmat?

[Christoph Rohland]

Hi Matthew,

On Fri, 31 Jan 2003, Matthew Dobson wrote:
> 	sys_shmat, does in fact increment shm_nattch, but only to
> 	decrement it again a few lines later, as seen in this code
> 	snippet.  Can anyone please explain why this is?

sys_shmat temporarily increases shm_nattch to make sure it's never zero:

>  >>>	shp->shm_nattch++;

Make sure shm_nattch is greater than zero.

>  >	user_addr = (void*) do_mmap (file, addr, size, prot,

map the segment which increments shm_nattch in shm_mmap accounting for
the actual mapping

>  >>>	shp->shm_nattch--;

Correct it again.

Greetings
		Christoph


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Re: [question] shm_nattch in sys_shmat?

[Matthew Dobson]

Christoph Rohland wrote:
> Hi Matthew,
> 
> On Fri, 31 Jan 2003, Matthew Dobson wrote:
> 
>>	sys_shmat, does in fact increment shm_nattch, but only to
>>	decrement it again a few lines later, as seen in this code
>>	snippet.  Can anyone please explain why this is?
> 
> 
> sys_shmat temporarily increases shm_nattch to make sure it's never zero:
> 
> 
>> >>>	shp->shm_nattch++;
> 
> 
> Make sure shm_nattch is greater than zero.
> 
> 
>> >	user_addr = (void*) do_mmap (file, addr, size, prot,
> 
> 
> map the segment which increments shm_nattch in shm_mmap accounting for
> the actual mapping
> 
> 
>> >>>	shp->shm_nattch--;
> 
> 
> Correct it again.
> 
> Greetings
> 		Christoph

Ah ha...   I hadn't followed the do_mmap call chain deep enough to 
notice that it would call the shm_mmap call through the f_op function 
pointer.  Thanks for pointing that out.  It makes much more sense now. 
A small comment in there would make it *much* more obvious what is going on.

Cheers!

-Matt

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