netdev - Re: [PATCH net v3 2/7] net/rds: Get rid of "wait_clean_list

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Date:   Tue, 16 Jul 2019 17:26:35 -0700
From:   santosh.shilimkar@...cle.com
To:     Gerd Rausch <gerd.rausch@...cle.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, rds-devel@....oracle.com
Cc:     David Miller <davem@...emloft.net>
Subject: Re: [PATCH net v3 2/7] net/rds: Get rid of "wait_clean_list_grace"
 and add locking

On 7/16/19 3:28 PM, Gerd Rausch wrote:
> Waiting for activity on the "clean_list" to quiesce is no substitute
> for proper locking.
> 
> We can have multiple threads competing for "llist_del_first"
> via "rds_ib_reuse_mr", and a single thread competing
> for "llist_del_all" and "llist_del_first" via "rds_ib_flush_mr_pool".
> 
> Since "llist_del_first" depends on "list->first->next" not to change
> in the midst of the operation, simply waiting for all current calls
> to "rds_ib_reuse_mr" to quiesce across all CPUs is woefully inadequate:
> 
> By the time "wait_clean_list_grace" is done iterating over all CPUs to see
> that there is no concurrent caller to "rds_ib_reuse_mr", a new caller may
> have just shown up on the first CPU.
> 
> Furthermore, <linux/llist.h> explicitly calls out the need for locking:
>   * Cases where locking is needed:
>   * If we have multiple consumers with llist_del_first used in one consumer,
>   * and llist_del_first or llist_del_all used in other consumers,
>   * then a lock is needed.
> 
> Also, while at it, drop the unused "pool" parameter
> from "list_to_llist_nodes".
> 
> Signed-off-by: Gerd Rausch <gerd.rausch@...cle.com>
> ---
Acked-by: Santosh Shilimkar <santosh.shilimkar@...cle.com>