Re: [PATCH] rcu: avoid data-race in rcu_gp_fqs_check_wake()

From: Paul E. McKenney
Date: Wed Oct 09 2019 - 23:18:25 EST

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In reply to: Eric Dumazet: "[PATCH] rcu: avoid data-race in rcu_gp_fqs_check_wake()"
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On Wed, Oct 09, 2019 at 02:21:54PM -0700, Eric Dumazet wrote:
> rcu_gp_fqs_check_wake() uses rcu_preempt_blocked_readers_cgp()
> to read ->gp_tasks while otehr cpus might write over this field.
>
> We need READ_ONCE()/WRITE_ONCE() pairs to avoid compiler
> tricks and KCSAN splats like the following :
>
> BUG: KCSAN: data-race in rcu_gp_fqs_check_wake / rcu_preempt_deferred_qs_irqrestore
>
> write to 0xffffffff85a7f190 of 8 bytes by task 7317 on cpu 0:
> rcu_preempt_deferred_qs_irqrestore+0x43d/0x580 kernel/rcu/tree_plugin.h:507
> rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659
> __rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394
> rcu_read_unlock include/linux/rcupdate.h:645 [inline]
> __ip_queue_xmit+0x3b0/0xa40 net/ipv4/ip_output.c:533
> ip_queue_xmit+0x45/0x60 include/net/ip.h:236
> __tcp_transmit_skb+0xdeb/0x1cd0 net/ipv4/tcp_output.c:1158
> __tcp_send_ack+0x246/0x300 net/ipv4/tcp_output.c:3685
> tcp_send_ack+0x34/0x40 net/ipv4/tcp_output.c:3691
> tcp_cleanup_rbuf+0x130/0x360 net/ipv4/tcp.c:1575
> tcp_recvmsg+0x633/0x1a30 net/ipv4/tcp.c:2179
> inet_recvmsg+0xbb/0x250 net/ipv4/af_inet.c:838
> sock_recvmsg_nosec net/socket.c:871 [inline]
> sock_recvmsg net/socket.c:889 [inline]
> sock_recvmsg+0x92/0xb0 net/socket.c:885
> sock_read_iter+0x15f/0x1e0 net/socket.c:967
> call_read_iter include/linux/fs.h:1864 [inline]
> new_sync_read+0x389/0x4f0 fs/read_write.c:414
>
> read to 0xffffffff85a7f190 of 8 bytes by task 10 on cpu 1:
> rcu_gp_fqs_check_wake kernel/rcu/tree.c:1556 [inline]
> rcu_gp_fqs_check_wake+0x93/0xd0 kernel/rcu/tree.c:1546
> rcu_gp_fqs_loop+0x36c/0x580 kernel/rcu/tree.c:1611
> rcu_gp_kthread+0x143/0x220 kernel/rcu/tree.c:1768
> kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 10 Comm: rcu_preempt Not tainted 5.3.0+ #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>

Again, good catch, applied for review and testing, thank you!

I added another READ_ONCE() to dump_blkd_tasks(), which is not exercised
unless you get an RCU CPU stall warning or some such. The updated patch
is below, please let me know if I messed anything up.

Thanx, Paul

------------------------------------------------------------------------

commit 53a6fffb998fd442e4049b145d8843fecc2bb396
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Wed Oct 9 14:21:54 2019 -0700

rcu: Avoid data-race in rcu_gp_fqs_check_wake()

The rcu_gp_fqs_check_wake() function uses rcu_preempt_blocked_readers_cgp()
to read ->gp_tasks while other cpus might overwrite this field.

We need READ_ONCE()/WRITE_ONCE() pairs to avoid compiler
tricks and KCSAN splats like the following :

BUG: KCSAN: data-race in rcu_gp_fqs_check_wake / rcu_preempt_deferred_qs_irqrestore

write to 0xffffffff85a7f190 of 8 bytes by task 7317 on cpu 0:
rcu_preempt_deferred_qs_irqrestore+0x43d/0x580 kernel/rcu/tree_plugin.h:507
rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659
__rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394
rcu_read_unlock include/linux/rcupdate.h:645 [inline]
__ip_queue_xmit+0x3b0/0xa40 net/ipv4/ip_output.c:533
ip_queue_xmit+0x45/0x60 include/net/ip.h:236
__tcp_transmit_skb+0xdeb/0x1cd0 net/ipv4/tcp_output.c:1158
__tcp_send_ack+0x246/0x300 net/ipv4/tcp_output.c:3685
tcp_send_ack+0x34/0x40 net/ipv4/tcp_output.c:3691
tcp_cleanup_rbuf+0x130/0x360 net/ipv4/tcp.c:1575
tcp_recvmsg+0x633/0x1a30 net/ipv4/tcp.c:2179
inet_recvmsg+0xbb/0x250 net/ipv4/af_inet.c:838
sock_recvmsg_nosec net/socket.c:871 [inline]
sock_recvmsg net/socket.c:889 [inline]
sock_recvmsg+0x92/0xb0 net/socket.c:885
sock_read_iter+0x15f/0x1e0 net/socket.c:967
call_read_iter include/linux/fs.h:1864 [inline]
new_sync_read+0x389/0x4f0 fs/read_write.c:414

read to 0xffffffff85a7f190 of 8 bytes by task 10 on cpu 1:
rcu_gp_fqs_check_wake kernel/rcu/tree.c:1556 [inline]
rcu_gp_fqs_check_wake+0x93/0xd0 kernel/rcu/tree.c:1546
rcu_gp_fqs_loop+0x36c/0x580 kernel/rcu/tree.c:1611
rcu_gp_kthread+0x143/0x220 kernel/rcu/tree.c:1768
kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10 Comm: rcu_preempt Not tainted 5.3.0+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
[ paulmck: Added another READ_ONCE() for RCU CPU stall warnings. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d43f4e0..56ac8a6 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -220,7 +220,7 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
* blocked tasks.
*/
if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
- rnp->gp_tasks = &t->rcu_node_entry;
+ WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
}
if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
@@ -340,7 +340,7 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
*/
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
- return rnp->gp_tasks != NULL;
+ return READ_ONCE(rnp->gp_tasks) != NULL;
}

/* Bias and limit values for ->rcu_read_lock_nesting. */
@@ -493,7 +493,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
rnp->gp_seq, t->pid);
if (&t->rcu_node_entry == rnp->gp_tasks)
- rnp->gp_tasks = np;
+ WRITE_ONCE(rnp->gp_tasks, np);
if (&t->rcu_node_entry == rnp->exp_tasks)
rnp->exp_tasks = np;
if (IS_ENABLED(CONFIG_RCU_BOOST)) {
@@ -663,7 +663,7 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
dump_blkd_tasks(rnp, 10);
if (rcu_preempt_has_tasks(rnp) &&
(rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
- rnp->gp_tasks = rnp->blkd_tasks.next;
+ WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
t = container_of(rnp->gp_tasks, struct task_struct,
rcu_node_entry);
trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
@@ -757,7 +757,8 @@ dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
- __func__, rnp->gp_tasks, rnp->boost_tasks, rnp->exp_tasks);
+ __func__, READ_ONCE(rnp->gp_tasks), rnp->boost_tasks,
+ rnp->exp_tasks);
pr_info("%s: ->blkd_tasks", __func__);
i = 0;
list_for_each(lhp, &rnp->blkd_tasks) {

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In reply to: Eric Dumazet: "[PATCH] rcu: avoid data-race in rcu_gp_fqs_check_wake()"
Next in thread: Eric Dumazet: "Re: [PATCH] rcu: avoid data-race in rcu_gp_fqs_check_wake()"
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