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svcrpc: avoid memory-corruption on pool shutdown 

Socket callbacks use svc_xprt_enqueue() to add an xprt to a
pool->sp_sockets list.  In normal operation a server thread will later
come along and take the xprt off that list.  On shutdown, after all the
threads have exited, we instead manually walk the sv_tempsocks and
sv_permsocks lists to find all the xprt's and delete them.

So the sp_sockets lists don't really matter any more.  As a result,
we've mostly just ignored them and hoped they would go away.

Which has gotten us into trouble; witness for example ebc63e531c
"svcrpc: fix list-corrupting race on nfsd shutdown", the result of Ben
Greear noticing that a still-running svc_xprt_enqueue() could re-add an
xprt to an sp_sockets list just before it was deleted.  The fix was to
remove it from the list at the end of svc_delete_xprt().  But that only
made corruption less likely--I can see nothing that prevents a
svc_xprt_enqueue() from adding another xprt to the list at the same
moment that we're removing this xprt from the list.  In fact, despite
the earlier xpo_detach(), I don't even see what guarantees that
svc_xprt_enqueue() couldn't still be running on this xprt.

So, instead, note that svc_xprt_enqueue() essentially does:
	lock sp_lock
		if XPT_BUSY unset
			add to sp_sockets
	unlock sp_lock

So, if we do:

	set XPT_BUSY on every xprt.
	Empty every sp_sockets list, under the sp_socks locks.

Then we're left knowing that the sp_sockets lists are all empty and will
stay that way, since any svc_xprt_enqueue() will check XPT_BUSY under
the sp_lock and see it set.

And *then* we can continue deleting the xprt's.

(Thanks to Jeff Layton for being correctly suspicious of this code....)

Cc: Ben Greear <greearb@candelatech.com>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
This commit is contained in:
J. Bruce Fields 2011-11-29 17:00:26 -05:00
parent 2fefb8a09e
commit b4f36f88b3
2 changed files with 38 additions and 20 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
net/sunrpc/svc.c
View File

@ -527,7 +527,15 @@ svc_destroy(struct svc_serv *serv)
printk("svc_destroy: no threads for serv=%p!\n", serv);
del_timer_sync(&serv->sv_temptimer);
/*
* The set of xprts (contained in the sv_tempsocks and
* sv_permsocks lists) is now constant, since it is modified
* only by accepting new sockets (done by service threads in
* svc_recv) or aging old ones (done by sv_temptimer), or
* configuration changes (excluded by whatever locking the
* caller is using--nfsd_mutex in the case of nfsd). So it's
* safe to traverse those lists and shut everything down:
*/
svc_close_all(serv);
if (serv->sv_shutdown)

48
net/sunrpc/svc_xprt.c
View File

@ -894,14 +894,7 @@ static void svc_delete_xprt(struct svc_xprt *xprt)
spin_lock_bh(&serv->sv_lock);
if (!test_and_set_bit(XPT_DETACHED, &xprt->xpt_flags))
list_del_init(&xprt->xpt_list);
/*
* The only time we're called while xpt_ready is still on a list
* is while the list itself is about to be destroyed (in
* svc_destroy). BUT svc_xprt_enqueue could still be attempting
* to add new entries to the sp_sockets list, so we can't leave
* a freed xprt on it.
*/
list_del_init(&xprt->xpt_ready);
BUG_ON(!list_empty(&xprt->xpt_ready));
if (test_bit(XPT_TEMP, &xprt->xpt_flags))
serv->sv_tmpcnt--;
spin_unlock_bh(&serv->sv_lock);
@ -932,28 +925,45 @@ EXPORT_SYMBOL_GPL(svc_close_xprt);
static void svc_close_list(struct list_head *xprt_list)
{
struct svc_xprt *xprt;
struct svc_xprt *tmp;
/*
* The server is shutting down, and no more threads are running.
* svc_xprt_enqueue() might still be running, but at worst it
* will re-add the xprt to sp_sockets, which will soon get
* freed. So we don't bother with any more locking, and don't
* leave the close to the (nonexistent) server threads:
*/
list_for_each_entry_safe(xprt, tmp, xprt_list, xpt_list) {
list_for_each_entry(xprt, xprt_list, xpt_list) {
set_bit(XPT_CLOSE, &xprt->xpt_flags);
svc_delete_xprt(xprt);
set_bit(XPT_BUSY, &xprt->xpt_flags);
}
}
void svc_close_all(struct svc_serv *serv)
{
struct svc_pool *pool;
struct svc_xprt *xprt;
struct svc_xprt *tmp;
int i;
svc_close_list(&serv->sv_tempsocks);
svc_close_list(&serv->sv_permsocks);
for (i = 0; i < serv->sv_nrpools; i++) {
pool = &serv->sv_pools[i];
spin_lock_bh(&pool->sp_lock);
while (!list_empty(&pool->sp_sockets)) {
xprt = list_first_entry(&pool->sp_sockets, struct svc_xprt, xpt_ready);
list_del_init(&xprt->xpt_ready);
}
spin_unlock_bh(&pool->sp_lock);
}
/*
* At this point the sp_sockets lists will stay empty, since
* svc_enqueue will not add new entries without taking the
* sp_lock and checking XPT_BUSY.
*/
list_for_each_entry_safe(xprt, tmp, &serv->sv_tempsocks, xpt_list)
svc_delete_xprt(xprt);
list_for_each_entry_safe(xprt, tmp, &serv->sv_permsocks, xpt_list)
svc_delete_xprt(xprt);
BUG_ON(!list_empty(&serv->sv_permsocks));
BUG_ON(!list_empty(&serv->sv_tempsocks));
}
/*