linux-kernel - Re: [PATCH 1/3] virtio-crypto: fixup potential cpu stall when free unused bufs

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Message-ID: <2023060924-skinning-reset-e256@gregkh>
Date:   Fri, 9 Jun 2023 15:39:24 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Xianting Tian <xianting.tian@...ux.alibaba.com>
Cc:     arei.gonglei@...wei.com, mst@...hat.com, jasowang@...hat.com,
        xuanzhuo@...ux.alibaba.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, amit@...nel.org, arnd@...db.de,
        marcel@...tmann.org, johan.hedberg@...il.com, luiz.dentz@...il.com,
        linux-bluetooth@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xianting Tian <tianxianting.txt@...baba-inc.com>
Subject: Re: [PATCH 1/3] virtio-crypto: fixup potential cpu stall when free
 unused bufs

On Fri, Jun 09, 2023 at 09:18:15PM +0800, Xianting Tian wrote:
> From: Xianting Tian <tianxianting.txt@...baba-inc.com>
> 
> Cpu stall issue may happen if device is configured with multi queues
> and large queue depth, so fix it.
> 
> Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>
> ---
>  drivers/crypto/virtio/virtio_crypto_core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/crypto/virtio/virtio_crypto_core.c b/drivers/crypto/virtio/virtio_crypto_core.c
> index 1198bd306365..94849fa3bd74 100644
> --- a/drivers/crypto/virtio/virtio_crypto_core.c
> +++ b/drivers/crypto/virtio/virtio_crypto_core.c
> @@ -480,6 +480,7 @@ static void virtcrypto_free_unused_reqs(struct virtio_crypto *vcrypto)
>  			kfree(vc_req->req_data);
>  			kfree(vc_req->sgs);
>  		}
> +		cond_resched();

that's not "fixing a stall", it is "call the scheduler because we are
taking too long".  The CPU isn't stalled at all, just busy.

Are you sure this isn't just a bug in the code?  Why is this code taking
so long that you have to force the scheduler to run?  This is almost
always a sign that something else needs to be fixed instead.

thanks,

greg k-h