Subject: Re: [PATCH] atm: firestream: Replace GFP_ATOMIC with GFP_KERNEL in fs_send

From: Al Viro <[email-masked]>
Date: Fri, 26 Jan 2018 12:05:22 +0000

On Fri, Jan 26, 2018 at 04:00:27PM +0800, Jia-Ju Bai wrote:
> After checking all possible call chains to fs_send() here,
> my tool finds that fs_send() is never called in atomic context.
> And this function is assigned to a function pointer "dev->ops->send",
> which is only called by vcc_sendmsg() (net/atm/common.c)
> through vcc->dev->ops->send(), and vcc_sendmsg() calls schedule(),
> it indicates that fs_send() can call functions which may sleep.
> Thus GFP_ATOMIC is not necessary, and it can be replaced with GFP_KERNEL.
> 
> This is found by a static analysis tool named DCNS written by myself.

The trouble is, places like
net/atm/raw.c:65:       vcc->send = atm_send_aal0;
net/atm/raw.c:74:       vcc->send = vcc->dev->ops->send;
net/atm/raw.c:83:       vcc->send = vcc->dev->ops->send;
mean extra call chains.  It's *not* just vcc_sendmsg(), and e.g.
        ret = ATM_SKB(skb)->vcc->send(ATM_SKB(skb)->vcc, skb)
            ? DROP_PACKET : 1;
        bh_unlock_sock(sk_atm(vcc));
in pppoatm_send() definitely is called under a spinlock.

Looking through the driver (in advanced bitrot, as usual for drivers/atm),
I'd say that submit_queue() is fucked in head in the "queue full" case.
And judging by the history, had been thus since the original merge...