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Re: locking question: do_mmap(), do_munmap()

To: Manfred Spraul <manfreds@colorfullife.com>
Subject: Re: locking question: do_mmap(), do_munmap()
From: Ralf Baechle <ralf@oss.sgi.com>
Date: Fri, 15 Oct 1999 11:58:16 +0200
Cc: Kanoj Sarcar <kanoj@google.engr.sgi.com>, "Stephen C. Tweedie" <sct@redhat.com>, viro@math.psu.edu, andrea@suse.de, linux-kernel@vger.rutgers.edu, mingo@chiara.csoma.elte.hu, linux-mm@kvack.org, linux@engr.sgi.com, linux-mips@fnet.fr, linux-mips@vger.rutgers.edu
In-reply-to: <380435A6.93B4B75A@colorfullife.com>; from Manfred Spraul on Wed, Oct 13, 1999 at 09:32:54AM +0200
References: <199910130125.SAA66579@google.engr.sgi.com> <380435A6.93B4B75A@colorfullife.com>
On Wed, Oct 13, 1999 at 09:32:54AM +0200, Manfred Spraul wrote:

> Kanoj Sarcar wrote:
> > Here's a primitive patch showing the direction I am thinking of. I do not
> > have any problem with a spinning lock, but I coded this against 2.2.10,
> > where insert_vm_struct could go to sleep, hence I had to use sleeping
> > locks to protect the vma chain.
> 
> I found a few places where I don't know how to change them.
> 
> 1) arch/mips/mm/r4xx0.c:
> their flush_cache_range() function internally calls find_vma().
> flush_cache_range() is called by proc/mem.c, and it seems that this
> function cannot get the mmap semaphore.
> Currently, every caller of flush_cache_range() either owns the kernel
> lock or the mmap_sem.
> OTHO, this function contains a race anyway [src_vma can go away if
> handle_mm_fault() sleeps, src_vma is used at the end of the function.]

The sole reason for fiddling with the VMA is that we try to optimize
icache flushing for non-VM_EXEC vmas.  This optimization is broken
as the MIPS hardware doesn't make a difference between read and execute
in page permissions, so the icache might be dirty even though the vma
has no exec permission.  So I'll have to re-implement this whole things
anyway.  The other problem is an efficience problem.  A call like
flush_cache_range(some_mm_ptr, 0, TASK_SIZE) would take a minor eternity
and for MIPS64 a full eternity ...

  Ralf

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