Re: [PATCH] acquire mmap semaphore in pagemap_read.

[Alexey Dobriyan]

On Thu, Mar 12, 2009 at 11:33:08AM +0100, Martin Schwidefsky wrote:
> --- linux-2.6/fs/proc/task_mmu.c
> +++ linux-2.6-patched/fs/proc/task_mmu.c
> @@ -716,7 +716,9 @@ static ssize_t pagemap_read(struct file 
>  	 * user buffer is tracked in "pm", and the walk
>  	 * will stop when we hit the end of the buffer.
>  	 */
> +	down_read(&mm->mmap_sem);
>  	ret = walk_page_range(start_vaddr, end_vaddr, &pagemap_walk);
> +	up_read(&mm->mmap_sem);

This will introduce "put_user under mmap_sem" which is deadlockable.
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