linux-kernel - Re: [PATCH] ocfs2: ocfs2_downconvert

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Date:   Thu, 26 Aug 2021 16:23:13 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Gang He <ghe@...e.com>, mark@...heh.com, jlbec@...lplan.org
Cc:     linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] ocfs2: ocfs2_downconvert_lock failure results in deadlock



On 8/26/21 2:10 PM, Gang He wrote:
> Usually, ocfs2_downconvert_lock() function always downconverts
> dlm lock to the expected level for satisfy dlm bast requests

s/for/to

> 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@...e.com>
> ---
>  fs/ocfs2/dlmglue.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 48fd369c29a4..c454c218fbfe 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -3671,13 +3671,11 @@ static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
>  			     OCFS2_LOCK_ID_MAX_LEN - 1);
>  	lockres_clear_pending(lockres, generation, osb);
>  	if (ret) {
> -		ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);
> +		if (ret != -EBUSY)
> +			ocfs2_log_dlm_error("ocfs2_dlm_lock", ret, lockres);

Do we have to treat EBUSY as a normal case?

>  		ocfs2_recover_from_dlm_error(lockres, 1);
> -		goto bail;
>  	}
>  
> -	ret = 0;
> -bail:
>  	return ret;
>  }
>  
> @@ -3912,6 +3910,13 @@ 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);
> +	/* ocfs2_cancel_convert() is in progress, try again later */
> +	if (ret == -EBUSY) {
> +		ctl->requeue = 1;
> +		mlog(ML_BASTS, "lockres %s, ReQ: Downconvert busy\n",
> +		     lockres->l_name);
> +		ret = 0;

Ditto. If EBUSY is not a normal case, I'd like just requeue it but not
change it to normal return code.
You know ML_BASTS is always switched off in production environment.

Thanks,
Joseph

> +	}
>  
>  leave:
>  	if (ret)
>