[virtual memory] page_table_lock & mmap_sem
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
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.
participants (3)
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Rik van Riel -
sk.syed2 -
Venkatram Tummala