linux-kernel - Re: [PATCH 5/5] cachefiles: add missing lock protection when polling

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Date: Wed, 24 Apr 2024 13:46:47 +0800
From: Jia Zhu <zhujia.zj@...edance.com>
To: libaokun@...weicloud.com, netfs@...ts.linux.dev
Cc: dhowells@...hat.com, jlayton@...nel.org, jefflexu@...ux.alibaba.com,
 linux-cachefs@...hat.com, linux-erofs@...ts.ozlabs.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 Joseph Qi <joseph.qi@...ux.alibaba.com>,
 Gao Xiang <hsiangkao@...ux.alibaba.com>, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH 5/5] cachefiles: add missing lock protection when polling



在 2024/4/24 11:34, libaokun@...weicloud.com 写道:
> From: Jingbo Xu <jefflexu@...ux.alibaba.com>
> 
> Add missing lock protection in poll routine when iterating xarray,
> otherwise:
> 
> Even with RCU read lock held, only the slot of the radix tree is
> ensured to be pinned there, while the data structure (e.g. struct
> cachefiles_req) stored in the slot has no such guarantee.  The poll
> routine will iterate the radix tree and dereference cachefiles_req
> accordingly.  Thus RCU read lock is not adequate in this case and
> spinlock is needed here.
> 
> Fixes: b817e22b2e91 ("cachefiles: narrow the scope of triggering EPOLLIN events in ondemand mode")
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
> Reviewed-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
> Reviewed-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Thanks for catching this.

Reviewed-by: Jia Zhu <zhujia.zj@...edance.com>

> ---
>   fs/cachefiles/daemon.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/cachefiles/daemon.c b/fs/cachefiles/daemon.c
> index 6465e2574230..73ed2323282a 100644
> --- a/fs/cachefiles/daemon.c
> +++ b/fs/cachefiles/daemon.c
> @@ -365,14 +365,14 @@ static __poll_t cachefiles_daemon_poll(struct file *file,
>   
>   	if (cachefiles_in_ondemand_mode(cache)) {
>   		if (!xa_empty(&cache->reqs)) {
> -			rcu_read_lock();
> +			xas_lock(&xas);
>   			xas_for_each_marked(&xas, req, ULONG_MAX, CACHEFILES_REQ_NEW) {
>   				if (!cachefiles_ondemand_is_reopening_read(req)) {
>   					mask |= EPOLLIN;
>   					break;
>   				}
>   			}
> -			rcu_read_unlock();
> +			xas_unlock(&xas);
>   		}
>   	} else {
>   		if (test_bit(CACHEFILES_STATE_CHANGED, &cache->flags))