linux-kernel - Re: [PATCH 2/3] userfaultfd: use fault

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Date:   Sun, 17 Dec 2017 09:10:10 +0200
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jason Baron <jbaron@...mai.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH 2/3] userfaultfd: use fault_wqh lock

Hi,

On Thu, Dec 14, 2017 at 04:23:43PM +0100, Christoph Hellwig wrote:
> From: Matthew Wilcox <mawilcox@...rosoft.com>
> 
> The epoll code currently uses the unlocked waitqueue helpers for managing

The userfaultfd code

> fault_wqh, but instead of holding the waitqueue lock for this waitqueue
> around these calls, it the waitqueue lock of fault_pending_wq, which is
> a different waitqueue instance.  Given that the waitqueue is not exposed
> to the rest of the kernel this actually works ok at the moment, but
> prevents the epoll locking rules from being enforced using lockdep.

ditto

> Switch to the internally locked waitqueue helpers instead.  This means
> that the lock inside fault_wqh now nests inside the fault_pending_wqh
> lock, but that's not a problem since it was entireyl unused before.

spelling: entirely

> Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
> [hch: slight changelog updates]
> Signed-off-by: Christoph Hellwig <hch@....de>

Reviewed-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>

> ---
>  fs/userfaultfd.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index ac9a4e65ca49..a39bc3237b68 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -879,7 +879,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
>  	 */
>  	spin_lock(&ctx->fault_pending_wqh.lock);
>  	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
> -	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
> +	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
>  	spin_unlock(&ctx->fault_pending_wqh.lock);
> 
>  	/* Flush pending events that may still wait on event_wqh */
> @@ -1045,7 +1045,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
>  			 * anyway.
>  			 */
>  			list_del(&uwq->wq.entry);
> -			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
> +			add_wait_queue(&ctx->fault_wqh, &uwq->wq);
> 
>  			write_seqcount_end(&ctx->refile_seq);
> 
> @@ -1194,7 +1194,7 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx,
>  		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
>  				     range);
>  	if (waitqueue_active(&ctx->fault_wqh))
> -		__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, range);
> +		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
>  	spin_unlock(&ctx->fault_pending_wqh.lock);
>  }
> 
> -- 
> 2.14.2
> 

-- 
Sincerely yours,
Mike.