Re: [PATCH v2] ocfs2: ocfs2_downconvert_lock failure results in deadlock

[Joseph Qi]

On 8/30/21 12:46 PM, Gang He wrote:
> Usually, ocfs2_downconvert_lock() function always downconverts
> dlm lock to the expected level for satisfy dlm bast requests
> from the other nodes.
> But there is a rare situation. When dlm lock conversion is being
> canceled, ocfs2_downconvert_lock() function will return -EBUSY.
> You need to be aware that ocfs2_cancel_convert() function is
> asynchronous in fsdlm implementation.
> If we does not requeue this lockres entry, ocfs2 downconvert
> thread no longer handles this dlm lock bast request. Then, the
> other nodes will not get the dlm lock again, the current node's
> process will be blocked when acquire this dlm lock again.
> 
> Signed-off-by: Gang He <ghe@xxxxxxxx>

Reviewed-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
> ---
> v2: keep error print in ocfs2_downconvert_lock function, add more detailed
> comments in code, add msleep to avoid repeated attempts. 
> ---
>  fs/ocfs2/dlmglue.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 48fd369c29a4..f8f561850470 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -16,6 +16,7 @@
>  #include <linux/debugfs.h>
>  #include <linux/seq_file.h>
>  #include <linux/time.h>
> +#include <linux/delay.h>
>  #include <linux/quotaops.h>
>  #include <linux/sched/signal.h>
>  
> @@ -3912,6 +3913,17 @@ static int ocfs2_unblock_lock(struct ocfs2_super *osb,
>  	spin_unlock_irqrestore(&lockres->l_lock, flags);
>  	ret = ocfs2_downconvert_lock(osb, lockres, new_level, set_lvb,
>  				     gen);
> +	/* The dlm lock convert is being cancelled in background,
> +	 * ocfs2_cancel_convert() is asynchronous in fs/dlm,
> +	 * requeue it, try again later.
> +	 */
> +	if (ret == -EBUSY) {
> +		ctl->requeue = 1;
> +		mlog(ML_BASTS, "lockres %s, ReQ: Downconvert busy\n",
> +		     lockres->l_name);
> +		ret = 0;
> +		msleep(20);
> +	}
>  
>  leave:
>  	if (ret)
>