Re: [PATCH v2] net:qrtr: fix atomic idr allocation in qrtr_port_assign()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Adding the xarray maintainer...

On Sat, Mar 27, 2021 at 10:07:02PM +0800, Du Cheng wrote:
> add idr_preload() and idr_preload_end() around idr_alloc_u32(GFP_ATOMIC)
> due to internal use of per_cpu variables, which requires preemption
> disabling/enabling.
> 
> reported as "BUG: "using smp_processor_id() in preemptible" by syzkaller
> 
> Reported-by: syzbot+3eec59e770685e3dc879@syzkaller.appspotmail.com
> Signed-off-by: Du Cheng <ducheng2@gmail.com>
> ---
> changelog
> v1: change to GFP_KERNEL for idr_alloc_u32() but might sleep
> v2: revert to GFP_ATOMIC but add preemption disable/enable protection
> 
>  net/qrtr/qrtr.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/qrtr/qrtr.c b/net/qrtr/qrtr.c
> index edb6ac17ceca..6361f169490e 100644
> --- a/net/qrtr/qrtr.c
> +++ b/net/qrtr/qrtr.c
> @@ -722,17 +722,23 @@ static int qrtr_port_assign(struct qrtr_sock *ipc, int *port)
>  	mutex_lock(&qrtr_port_lock);
>  	if (!*port) {
>  		min_port = QRTR_MIN_EPH_SOCKET;
> +		idr_preload(GFP_ATOMIC);
>  		rc = idr_alloc_u32(&qrtr_ports, ipc, &min_port, QRTR_MAX_EPH_SOCKET, GFP_ATOMIC);
> +		idr_preload_end();

This seems "odd" to me.  We are asking idr_alloc_u32() to abide by
GFP_ATOMIC, so why do we need to "preload" it with the same type of
allocation?

Is there something in the idr/radix/xarray code that can't really handle
GFP_ATOMIC during a "normal" idr allocation that is causing this warning
to be hit?  Why is this change the "correct" one?

thanks,

greg k-h