[Ocfs2-devel] [PATCH] ocfs2: Avoid livelock in ocfs2_readpage()

[Sunil Mushran <sunil.mushran@oracle.com>]

Looks reasonable. Was this something someone ran into or was
this reproduced only via a test workload?

On 06/23/2011 01:51 PM, Jan Kara wrote:
> When someone writes to an inode, readers accessing the same inode via
> ocfs2_readpage() just busyloop trying to get ip_alloc_sem because
> do_generic_file_read() looks up the page again and retries ->readpage()
> when previous attempt failed with AOP_TRUNCATED_PAGE. When there are enough
> readers, they can occupy all CPUs and in non-preempt kernel the system is
> deadlocked because writer holding ip_alloc_sem is never run to release the
> semaphore. Fix the problem by making reader block on ip_alloc_sem to break
> the busy loop.
>
> Signed-off-by: Jan Kara<jack@suse.cz>
> ---
>   fs/ocfs2/aops.c |    8 ++++++++
>   1 files changed, 8 insertions(+), 0 deletions(-)
>
> diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
> index ac97bca..0919e8f 100644
> --- a/fs/ocfs2/aops.c
> +++ b/fs/ocfs2/aops.c
> @@ -290,7 +290,15 @@ static int ocfs2_readpage(struct file *file, struct page *page)
>   	}
>
>   	if (down_read_trylock(&oi->ip_alloc_sem) == 0) {
> +		/*
> +		 * Unlock the page and cycle ip_alloc_sem so that we don't
> +		 * busyloop waiting for ip_alloc_sem to unlock
> +		 */
>   		ret = AOP_TRUNCATED_PAGE;
> +		unlock_page(page);
> +		unlock = 0;
> +		down_read(&oi->ip_alloc_sem);
> +		up_read(&oi->ip_alloc_sem);
>   		goto out_inode_unlock;
>   	}
>