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svcrpc: fix wspace-checking race

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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>
This commit is contained in:
J. Bruce Fields 2010-10-26 11:32:03 -04:00
parent b176331627
commit 9c335c0b8d
1 changed files with 11 additions and 22 deletions
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33
net/sunrpc/svc_xprt.c
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@ -302,6 +302,15 @@ static void svc_thread_dequeue(struct svc_pool *pool, struct svc_rqst *rqstp)
list_del(&rqstp->rq_list);
}
static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
{
if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE)))
return true;
if (xprt->xpt_flags & ((1<<XPT_DATA)|(1<<XPT_DEFERRED)))
return xprt->xpt_ops->xpo_has_wspace(xprt);
return false;
}
/*
* Queue up a transport with data pending. If there are idle nfsd
* processes, wake 'em up.
@ -314,8 +323,7 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
struct svc_rqst *rqstp;
int cpu;
if (!(xprt->xpt_flags &
((1<<XPT_CONN)|(1<<XPT_DATA)|(1<<XPT_CLOSE)|(1<<XPT_DEFERRED))))
if (!svc_xprt_has_something_to_do(xprt))
return;
cpu = get_cpu();
@ -345,25 +353,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
BUG_ON(xprt->xpt_pool != NULL);
xprt->xpt_pool = pool;
/* Handle pending connection */
if (test_bit(XPT_CONN, &xprt->xpt_flags))
goto process;
/* Handle close in-progress */
if (test_bit(XPT_CLOSE, &xprt->xpt_flags))
goto process;
/* Check if we have space to reply to a request */
if (!xprt->xpt_ops->xpo_has_wspace(xprt)) {
/* Don't enqueue while not enough space for reply */
dprintk("svc: no write space, transport %p not enqueued\n",
xprt);
xprt->xpt_pool = NULL;
clear_bit(XPT_BUSY, &xprt->xpt_flags);
goto out_unlock;
}
process:
if (!list_empty(&pool->sp_threads)) {
rqstp = list_entry(pool->sp_threads.next,
struct svc_rqst,
@ -744,7 +733,7 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
spin_unlock_bh(&serv->sv_lock);
svc_xprt_received(newxpt);
}
} else {
} else if (xprt->xpt_ops->xpo_has_wspace(xprt)) {
dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n",
rqstp, pool->sp_id, xprt,
atomic_read(&xprt->xpt_ref.refcount));