* [virtual memory] page_table_lock & mmap_sem
@ 2011-03-29 22:30 Venkatram Tummala
2011-03-30 3:58 ` sk.syed2
2011-03-31 15:14 ` Rik van Riel
0 siblings, 2 replies; 3+ messages in thread
From: Venkatram Tummala @ 2011-03-29 22:30 UTC (permalink / raw)
To: kernelnewbies
Hi,
Why do we need both page_table_lock & mmap_sem in the kernel. Why isn't
mmap_sem good enough for serialization. As far as i see it, all attempts to
change the page table hierarchy in the kernel is done after holding the
write lock on mmap_sem. As we are already holding the write lock, we do we
need an extra page_table_lock (spinlock) ? Are there any code paths in the
kernel which attempt to modify the page table hierarchy without holding the
write lock on mmap_sem ?
page_table_lock is a finer grained lock than mmap_sem but page_table_lock is
only taken after holding a write lock on mmap_sem (as far as i can tell).
What is the reasoning behind having an extra spinlock?
Thanks.
Venkatram Tummala
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* [virtual memory] page_table_lock & mmap_sem
2011-03-29 22:30 [virtual memory] page_table_lock & mmap_sem Venkatram Tummala
@ 2011-03-30 3:58 ` sk.syed2
2011-03-31 15:14 ` Rik van Riel
1 sibling, 0 replies; 3+ messages in thread
From: sk.syed2 @ 2011-03-30 3:58 UTC (permalink / raw)
To: kernelnewbies
On Tue, Mar 29, 2011 at 5:30 PM, Venkatram Tummala
<venkatram867@gmail.com> wrote:
> Hi,
> Why do we need both page_table_lock & mmap_sem in the kernel. Why isn't
See linux/Documentation/vm/locking
-syed
^ permalink raw reply [flat|nested] 3+ messages in thread
* [virtual memory] page_table_lock & mmap_sem
2011-03-29 22:30 [virtual memory] page_table_lock & mmap_sem Venkatram Tummala
2011-03-30 3:58 ` sk.syed2
@ 2011-03-31 15:14 ` Rik van Riel
1 sibling, 0 replies; 3+ messages in thread
From: Rik van Riel @ 2011-03-31 15:14 UTC (permalink / raw)
To: kernelnewbies
On 03/29/2011 06:30 PM, Venkatram Tummala wrote:
> Hi,
>
> Why do we need both page_table_lock & mmap_sem in the kernel. Why isn't
> mmap_sem good enough for serialization. As far as i see it, all attempts
> to change the page table hierarchy in the kernel is done after holding
> the write lock on mmap_sem. As we are already holding the write lock, we
> do we need an extra page_table_lock (spinlock) ? Are there any code
> paths in the kernel which attempt to modify the page table hierarchy
> without holding the write lock on mmap_sem ?
Yes, there are.
Look at the page fault code, which takes a read lock
on mmap_sem.
This allows multiple threads in a multi-threaded process
to handle page faults simultaneously.
--
All rights reversed.
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2011-03-29 22:30 [virtual memory] page_table_lock & mmap_sem Venkatram Tummala
2011-03-30 3:58 ` sk.syed2
2011-03-31 15:14 ` Rik van Riel
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