From 9c335c0b8daf56b9f73479d00b1dd726e1fcca09 Mon Sep 17 00:00:00 2001
From: "J. Bruce Fields" <bfields@redhat.com>
Date: Tue, 26 Oct 2010 11:32:03 -0400
Subject: [PATCH] svcrpc: fix wspace-checking race

We call svc_xprt_enqueue() after something happens which we think may
require handling from a server thread.  To avoid such events being lost,
svc_xprt_enqueue() must guarantee that there will be a svc_serv() call
from a server thread following any such event.  It does that by either
waking up a server thread itself, or checking that XPT_BUSY is set (in
which case somebody else is doing it).

But the check of XPT_BUSY could occur just as someone finishes
processing some other event, and just before they clear XPT_BUSY.

Therefore it's important not to clear XPT_BUSY without subsequently
doing another svc_export_enqueue() to check whether the xprt should be
requeued.

The xpo_wspace() check in svc_xprt_enqueue() breaks this rule, allowing
an event to be missed in situations like:

	data arrives
	call svc_tcp_data_ready():
	call svc_xprt_enqueue():
	set BUSY
	find no write space
				svc_reserve():
				free up write space
				call svc_enqueue():
				test BUSY
	clear BUSY

So, instead, check wspace in the same places that the state flags are
checked: before taking BUSY, and in svc_receive().

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
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