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Date: Wed, 20 Mar 2002 13:37:09 -0500
From: Daniel Jacobowitz <dan@debian.org>
To: "Vamsi Krishna S ." <vamsi@in.ibm.com>
Cc: Mark Gross <mgross@unix-os.sc.intel.com>, Pavel Machek <pavel@suse.cz>,
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Subject: Re: [PATCH] multithreaded coredumps for elf exeecutables
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On Wed, Mar 20, 2002 at 11:36:30AM +0530, Vamsi Krishna S . wrote:
> There is serialization at higher level. We take a write lock
> on current->mm->mmap_sem at the beginning of elf_core_dump
> function which is released just before leaving the function.
> So, if one thread enters elf_core_dump and starts dumping core,
> no other thread (same mm) of the same process can start
> dumping.
> 
> static int elf_core_dump(long signr, struct pt_regs * regs, struct file * file)
> {
> 	...
> 	...
>         /* now stop all vm operations */
>         down_write(&current->mm->mmap_sem);
> 	...
> 	...
> 	...
>         up_write(&current->mm->mmap_sem);
>         return has_dumped;
> }

That's not a feature, it's a bug.  You can't take the mmap_sem before
collecting thread status; it will cause a deadlock on at least ia64,
where some registers are collected from user memory.

(Thanks to Manfred Spraul for explaining that to me.)

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer
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