linux-kernel - Re: [PATCH] ipc/sem.c: Convert kfree_rcu() to call

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Date:   Wed, 30 Dec 2020 08:19:41 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     qiang.zhang@...driver.com
Cc:     akpm@...ux-foundation.org, manfred@...orfullife.com,
        gustavoars@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipc/sem.c: Convert kfree_rcu() to call_rcu() in freeary
 function

On Wed, Dec 30, 2020 at 08:00:38PM +0800, qiang.zhang@...driver.com wrote:
> From: Zqiang <qiang.zhang@...driver.com>
> 
> Due to freeary function is called with spinlock be held,
> the synchronize_rcu function may be called in kfree_rcu
> function, the schedule may be happen in spinlock critical
> region, need to replace kfree_rcu() with call_rcu().

Except that the call to kfree_rcu() below has two arguments, and thus
provides a link for queuing the callback.  It will never directly invoke
synchronize_rcu().  It is only the single-argument variant of kfree_rcu()
that might invoke synchronize_rcu().

Or are you seeing lockdep or might-sleep failures with the current code?
If so, please post the relevant portions of the console output.

							Thanx, Paul

> Fixes: 693a8b6eecce ("ipc,rcu: Convert call_rcu(free_un) to kfree_rcu()")
> Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> ---
>  ipc/sem.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/ipc/sem.c b/ipc/sem.c
> index f6c30a85dadf..12c3184347d9 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1132,6 +1132,13 @@ static int count_semcnt(struct sem_array *sma, ushort semnum,
>  	return semcnt;
>  }
>  
> +static void free_un(struct rcu_head *head)
> +{
> +	struct sem_undo *un = container_of(head, struct sem_undo, rcu);
> +
> +	kfree(un);
> +}
> +
>  /* Free a semaphore set. freeary() is called with sem_ids.rwsem locked
>   * as a writer and the spinlock for this semaphore set hold. sem_ids.rwsem
>   * remains locked on exit.
> @@ -1152,7 +1159,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
>  		un->semid = -1;
>  		list_del_rcu(&un->list_proc);
>  		spin_unlock(&un->ulp->lock);
> -		kfree_rcu(un, rcu);
> +		call_rcu(&un->rcu, free_un);
>  	}
>  
>  	/* Wake up all pending processes and let them fail with EIDRM. */
> -- 
> 2.17.1
>