Re: PATCH Multithreaded core dump support for the 2.5.14 (and 15) kernel.

[Andi Kleen <ak@suse.de>]

On Thu, May 16, 2002 at 10:13:40AM -0400, Mark Gross wrote:
> Also, does anyone know WHY the mmap_sem is needed in the elf_core_dump code, 
> and is this need still valid if I've suspended all the other processes that 
> could even touch that mm?  I.e. can I fix this by removing the down_write / 
> up_write in elf_core_dump?

The mmap_sem is needed to access current->mm (especially the vma list)
safely. Otherwise someone else sharing the mm_struct could modify it. 
If you make sure all others sharing the mm_struct are killed first 
(including now way for them to start new clones inbetween) then
the only loophole left would be remote access using /proc/pid/mem or ptrace. 
If you handle that too then it is probably safe to drop it. Unfortunately
I don't see a way to handle these remote users without at least 
taking it temporarily.

Of course there are other semaphores in involved in dumping too (e.g. the
VFS ->write code may take the i_sem or other private ones). I guess they 
won't be a big problem if you first kill and then dump later.

-Andi