netdev - Re: [PATCH 2.6.35.7] net: Fix the condition passed to sk_wait

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Message-ID: <667106.4951.qm@web53706.mail.re2.yahoo.com>
Date:	Sat, 2 Oct 2010 01:22:16 -0700 (PDT)
From:	Nagendra Tomar <tomer_iisc@...oo.com>
To:	netdev@...r.kernel.org
Cc:	davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.35.7] net: Fix the condition passed to sk_wait_event()

Resending ...

The condition (3rd arg) passed to sk_wait_event() in sk_stream_wait_memory() and sk_stream_wait_connect() are incorrect.
The incorrect check in sk_stream_wait_memory() causes the following soft lockup in tcp_sendmsg() when the global tcp memory pool has exhausted. The check in sk_stream_wait_connect() was found by code audit.

>>> snip <<<

localhost kernel: BUG: soft lockup - CPU#3 stuck for 11s! [sshd:6429]
localhost kernel: CPU 3:
localhost kernel: RIP: 0010:[sk_stream_wait_memory+0xcd/0x200]  [sk_stream_wait_memory+0xcd/0x200] sk_stream_wait_memory+0xcd/0x200
localhost kernel: Call Trace:
localhost kernel:  [sk_stream_wait_memory+0x1b1/0x200] sk_stream_wait_memory+0x1b1/0x200
localhost kernel:  [<ffffffff802557c0>] autoremove_wake_function+0x0/0x40
localhost kernel:  [ipv6:tcp_sendmsg+0x6e6/0xe90] tcp_sendmsg+0x6e6/0xce0
localhost kernel:  [sock_aio_write+0x126/0x140] sock_aio_write+0x126/0x140
localhost kernel:  [xfs:do_sync_write+0xf1/0x130] do_sync_write+0xf1/0x130
localhost kernel:  [<ffffffff802557c0>] autoremove_wake_function+0x0/0x40
localhost kernel:  [hrtimer_start+0xe3/0x170] hrtimer_start+0xe3/0x170
localhost kernel:  [vfs_write+0x185/0x190] vfs_write+0x185/0x190
localhost kernel:  [sys_write+0x50/0x90] sys_write+0x50/0x90
localhost kernel:  [system_call+0x7e/0x83] system_call+0x7e/0x83

>>> snip <<<

What is happening is, that the sk_wait_event() condition passed from
sk_stream_wait_memory() evaluates to true for the case of tcp global memory
exhaustion. This is because both sk_stream_memory_free() and vm_wait are true which causes sk_wait_event() to *not* call schedule_timeout(). 
Hence sk_stream_wait_memory() returns immediately to the caller w/o sleeping.
This causes the caller to again try allocation, which again fails and again
calls sk_stream_wait_memory(), and so on.

Signed-off-by: Nagendra Singh Tomar <tomer_iisc@...oo.com>
---
--- linux-2.6.35.7/net/core/stream.c.orig	2010-03-23 23:46:45.000000000 +0530
+++ linux-2.6.35.7/net/core/stream.c	2010-03-24 00:21:09.000000000 +0530
@@ -73,9 +73,8 @@ int sk_stream_wait_connect(struct sock *
 		prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
 		sk->sk_write_pending++;
 		done = sk_wait_event(sk, timeo_p,
-				     !sk->sk_err &&
-				     !((1 << sk->sk_state) &
-				       ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)));
+				     ((1 << sk->sk_state) &
+				       (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)));
 		finish_wait(sk_sleep(sk), &wait);
 		sk->sk_write_pending--;
 	} while (!done);
@@ -144,10 +143,9 @@ int sk_stream_wait_memory(struct sock *s

 		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) &&
-						  sk_stream_memory_free(sk) &&
-						  vm_wait);
+		sk_wait_event(sk, &current_timeo, sk->sk_err ||
+						  (sk->sk_shutdown & SEND_SHUTDOWN) ||
+						  (sk_stream_memory_free(sk) && !vm_wait));
 		sk->sk_write_pending--;

 		if (vm_wait) {

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