Re: ppoll() stuck on POLLIN while TCP peer is sending

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2013-01-07 at 12:25 +0000, Mel Gorman wrote:

> 
> > ===> 28014[28017]/stack <===
> > [<ffffffff8129fc1d>] release_sock+0xe5/0x11b
> > [<ffffffff812a642c>] sk_stream_wait_memory+0x1f7/0x1fc
> > [<ffffffff81040d5e>] autoremove_wake_function+0x0/0x2a
> > [<ffffffff812d8fc3>] tcp_sendmsg+0x710/0x86d
> > [<ffffffff8129a33e>] sock_sendmsg+0x7b/0x93
> > [<ffffffff8129a642>] sys_sendto+0xee/0x145
> > [<ffffffff8129a3bc>] sockfd_lookup_light+0x1a/0x50
> > [<ffffffff8129a668>] sys_sendto+0x114/0x145
> > [<ffffffff81000e34>] __switch_to+0x235/0x3c5
> > [<ffffffff81322769>] system_call_fastpath+0x16/0x1b
> > [<ffffffffffffffff>] 0xffffffffffffffff
> 
> This seems to be the guy that's stuck. It's waiting for more memory for
> the socket but who or what is allocating that memory? There are a few other
> bugs from over the weekend that I want to take a look at so I did not dig
> further or try to reproduce this bug yet. I'm adding Eric Dumazet back to
> the cc in case he has the quick answer.

Thanks Mel

It would not surprise me if sk_stream_wait_memory() have plain bug(s) or
race(s).

In 2010, in commit 482964e56e132 Nagendra Tomar fixed a pretty severe
long standing bug.

This path is not taken very often on most machines.

I would try the following patch :

diff --git a/net/core/stream.c b/net/core/stream.c
index f5df85d..6f09979 100644
--- a/net/core/stream.c
+++ b/net/core/stream.c
@@ -126,6 +126,7 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p)
 
 	while (1) {
 		set_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags);
+		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 
 		prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
 
@@ -139,7 +140,6 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p)
 		if (sk_stream_memory_free(sk) && !vm_wait)
 			break;
 
-		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 		sk->sk_write_pending++;
 		sk_wait_event(sk, &current_timeo, sk->sk_err ||
 						  (sk->sk_shutdown & SEND_SHUTDOWN) ||




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