Re: [PATCH -mm -next] ipc,sem: untangle RCU locking withfind_alloc_undo

[Mike Galbraith]

On Tue, 2013-03-26 at 16:00 -0400, Rik van Riel wrote: 
> On Tue, 26 Mar 2013 14:07:14 -0400
> Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> 
> > > Not necessarily, we do release everything at the end of the function: 
> > >     out_unlock_free:
> > > 	sem_unlock(sma, locknum);
> > 
> > Ow, there's a rcu_read_unlock() in sem_unlock()? This complicates things even
> > more I suspect. If un is non-NULL we'll be unlocking rcu lock twice?
> 
> Sasha, this patch should resolve the RCU tangle, by making sure
> we only ever take the rcu_read_lock once in semtimedop.
> 
> ---8<---
> 
> The ipc semaphore code has a nasty RCU locking tangle, with both
> find_alloc_undo and semtimedop taking the rcu_read_lock(). The
> code can be cleaned up somewhat by only taking the rcu_read_lock
> once.
> 
> There are no other callers to find_alloc_undo.
> 
> This should also solve the trinity issue reported by Sasha Levin.

I take it this is on top of the patchlet Sasha submitted?

(I hit rcu stall banging on patch set in rt with 60 synchronized core
executive model if I let it run long enough, fwtw)

> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  ipc/sem.c |   31 +++++++++----------------------
>  1 files changed, 9 insertions(+), 22 deletions(-)
> 
> diff --git a/ipc/sem.c b/ipc/sem.c
> index f46441a..2ec2945 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -1646,22 +1646,23 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>  			alter = 1;
>  	}
>  
> +	INIT_LIST_HEAD(&tasks);
> +
>  	if (undos) {
> +		/* On success, find_alloc_undo takes the rcu_read_lock */
>  		un = find_alloc_undo(ns, semid);
>  		if (IS_ERR(un)) {
>  			error = PTR_ERR(un);
>  			goto out_free;
>  		}
> -	} else
> +	} else {
>  		un = NULL;
> +		rcu_read_lock();
> +	}
>  
> -	INIT_LIST_HEAD(&tasks);
> -
> -	rcu_read_lock();
>  	sma = sem_obtain_object_check(ns, semid);
>  	if (IS_ERR(sma)) {
> -		if (un)
> -			rcu_read_unlock();
> +		rcu_read_unlock();
>  		error = PTR_ERR(sma);
>  		goto out_free;
>  	}
> @@ -1693,22 +1694,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>  	 */
>  	error = -EIDRM;
>  	locknum = sem_lock(sma, sops, nsops);
> -	if (un) {
> -		if (un->semid == -1) {
> -			rcu_read_unlock();
> -			goto out_unlock_free;
> -		} else {
> -			/*
> -			 * rcu lock can be released, "un" cannot disappear:
> -			 * - sem_lock is acquired, thus IPC_RMID is
> -			 *   impossible.
> -			 * - exit_sem is impossible, it always operates on
> -			 *   current (or a dead task).
> -			 */
> -
> -			rcu_read_unlock();
> -		}
> -	}
> +	if (un && un->semid == -1)
> +		goto out_unlock_free;
>  
>  	error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current));
>  	if (error <= 0) {
> --
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