Re: [Patch net-next] atm: remove an unnecessary loop

[David Miller]

From: Chas Williams <3ch...@gmail.com>
Date: Sat, 11 Mar 2017 19:41:36 -0500

> From: Francois Romieu 
> 
> Andrey reported this kernel warning:
> 
> WARNING: CPU: 0 PID: 4114 at kernel/sched/core.c:7737 
> __might_sleep+0x149/0x1a0
> do not call blocking ops when !TASK_RUNNING; state=1 set at
> [] prepare_to_wait+0x182/0x530
> 
> The deeply nested alloc_skb is a problem.
> 
> Diagnosis: nesting is wrong. It makes zero sense. Fix it and the
> implicit task state change problem automagically goes away.
> 
> alloc_skb() does not need to be in the "while" loop.
> 
> alloc_skb() does not need to be in the {prepare_to_wait/add_wait_queue ...
> finish_wait/remove_wait_queue} block.
> 
> I claim that:
> - alloc_tx() should only perform the "wait_for_decent_tx_drain" part
> - alloc_skb() ought to be done directly in vcc_sendmsg
> - alloc_skb() failure can be handled gracefully in vcc_sendmsg
> - alloc_skb() may use a (m->msg_flags & MSG_DONTWAIT) dependent
>   GFP_{KERNEL / ATOMIC} flag
> 
> Reported-by: Andrey Konovalov 
> Reviewed-and-Tested-by: Chas Williams <3ch...@gmail.com>
> Signed-off-by: Chas Williams <3ch...@gmail.com>

Applied, thanks a lot Chas.

[Patch net-next] atm: remove an unnecessary loop

[Chas Williams]

From: Francois Romieu 

Andrey reported this kernel warning:

WARNING: CPU: 0 PID: 4114 at kernel/sched/core.c:7737 __might_sleep+0x149/0x1a0
do not call blocking ops when !TASK_RUNNING; state=1 set at
[] prepare_to_wait+0x182/0x530

The deeply nested alloc_skb is a problem.

Diagnosis: nesting is wrong. It makes zero sense. Fix it and the
implicit task state change problem automagically goes away.

alloc_skb() does not need to be in the "while" loop.

alloc_skb() does not need to be in the {prepare_to_wait/add_wait_queue ...
finish_wait/remove_wait_queue} block.

I claim that:
- alloc_tx() should only perform the "wait_for_decent_tx_drain" part
- alloc_skb() ought to be done directly in vcc_sendmsg
- alloc_skb() failure can be handled gracefully in vcc_sendmsg
- alloc_skb() may use a (m->msg_flags & MSG_DONTWAIT) dependent
  GFP_{KERNEL / ATOMIC} flag

Reported-by: Andrey Konovalov 
Reviewed-and-Tested-by: Chas Williams <3ch...@gmail.com>
Signed-off-by: Chas Williams <3ch...@gmail.com>
---
 net/atm/common.c | 22 +-
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/net/atm/common.c b/net/atm/common.c
index 9613381..f06422f 100644
--- a/net/atm/common.c
+++ b/net/atm/common.c
@@ -62,21 +62,16 @@ static void vcc_remove_socket(struct sock *sk)
write_unlock_irq(_sklist_lock);
 }
 
-static struct sk_buff *alloc_tx(struct atm_vcc *vcc, unsigned int size)
+static bool vcc_tx_ready(struct atm_vcc *vcc, unsigned int size)
 {
-   struct sk_buff *skb;
struct sock *sk = sk_atm(vcc);
 
if (sk_wmem_alloc_get(sk) && !atm_may_send(vcc, size)) {
pr_debug("Sorry: wmem_alloc = %d, size = %d, sndbuf = %d\n",
 sk_wmem_alloc_get(sk), size, sk->sk_sndbuf);
-   return NULL;
+   return false;
}
-   while (!(skb = alloc_skb(size, GFP_KERNEL)))
-   schedule();
-   pr_debug("%d += %d\n", sk_wmem_alloc_get(sk), skb->truesize);
-   atomic_add(skb->truesize, >sk_wmem_alloc);
-   return skb;
+   return true;
 }
 
 static void vcc_sock_destruct(struct sock *sk)
@@ -606,7 +601,7 @@ int vcc_sendmsg(struct socket *sock, struct msghdr *m, 
size_t size)
eff = (size+3) & ~3; /* align to word boundary */
prepare_to_wait(sk_sleep(sk), , TASK_INTERRUPTIBLE);
error = 0;
-   while (!(skb = alloc_tx(vcc, eff))) {
+   while (!vcc_tx_ready(vcc, eff)) {
if (m->msg_flags & MSG_DONTWAIT) {
error = -EAGAIN;
break;
@@ -628,6 +623,15 @@ int vcc_sendmsg(struct socket *sock, struct msghdr *m, 
size_t size)
finish_wait(sk_sleep(sk), );
if (error)
goto out;
+
+   skb = alloc_skb(eff, GFP_KERNEL);
+   if (!skb) {
+   error = -ENOMEM;
+   goto out;
+   }
+   pr_debug("%d += %d\n", sk_wmem_alloc_get(sk), skb->truesize);
+   atomic_add(skb->truesize, >sk_wmem_alloc);
+
skb->dev = NULL; /* for paths shared with net_device interfaces */
ATM_SKB(skb)->atm_options = vcc->atm_options;
if (!copy_from_iter_full(skb_put(skb, size), size, >msg_iter)) {
-- 
2.7.4