[Ocfs2-devel] [PATCH] ocfs2/dlm: remove potential deadlock

[Wengang Wang]

Why atomic operations on dlm_refs need spinlock's protect?

        /* NOTE: Next three are protected by dlm_domain_lock */
        struct kref dlm_refs;
        enum dlm_ctxt_state dlm_state;
        unsigned int num_joins;

regards,
wengang.

On 10-06-16 14:52, Wengang Wang wrote:
> When we need to take both dlm_domain_lock and dlm->spinlock, we should take
> them in order of:
> dlm_domain_lock then dlm->spinlock.
> 
> There is pathes disobey this order. That is calling dlm_lockres_put with dlm->
> spinlock held. dlm_lockres_put() finally calls dlm_put() which take
> dlm_domain_lock.
> 
> The fix is moving the locking on dlm_domain_lock to dlm_ctxt_release() from
> dlm_put(). dlm_ctxt_release() is only called on the release of the last
> reference. Any path should not be holding dlm->spinlock when dropping the "last"
> reference.
> 
> Signed-off-by: Wengang Wang <wen.gang.wang at oracle.com>
> ---
>  fs/ocfs2/dlm/dlmdomain.c |   12 ++++--------
>  1 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ocfs2/dlm/dlmdomain.c b/fs/ocfs2/dlm/dlmdomain.c
> index ab82add..754baf2 100644
> --- a/fs/ocfs2/dlm/dlmdomain.c
> +++ b/fs/ocfs2/dlm/dlmdomain.c
> @@ -321,28 +321,24 @@ static void dlm_ctxt_release(struct kref *kref)
>  
>  	dlm = container_of(kref, struct dlm_ctxt, dlm_refs);
>  
> +	if (spin_is_locked(&dlm->spinlock))
> +		BUG();
>  	BUG_ON(dlm->num_joins);
>  	BUG_ON(dlm->dlm_state == DLM_CTXT_JOINED);
>  
> +	spin_lock(&dlm_domain_lock);
>  	/* we may still be in the list if we hit an error during join. */
>  	list_del_init(&dlm->list);
> -
>  	spin_unlock(&dlm_domain_lock);
>  
> -	mlog(0, "freeing memory from domain %s\n", dlm->name);
> -
>  	wake_up(&dlm_domain_events);
> -
> +	mlog(0, "freeing memory from domain %s\n", dlm->name);
>  	dlm_free_ctxt_mem(dlm);
> -
> -	spin_lock(&dlm_domain_lock);
>  }
>  
>  void dlm_put(struct dlm_ctxt *dlm)
>  {
> -	spin_lock(&dlm_domain_lock);
>  	kref_put(&dlm->dlm_refs, dlm_ctxt_release);
> -	spin_unlock(&dlm_domain_lock);
>  }
>  
>  static void __dlm_get(struct dlm_ctxt *dlm)
> -- 
> 1.6.6.1
> 
> 
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