On Thu, Nov 03, 2022 at 02:36 PM -07, John Fastabend wrote:
> Jakub Sitnicki wrote:
>> On Tue, Nov 01, 2022 at 01:01 PM -07, John Fastabend wrote:
>> > Jakub Sitnicki wrote:
>> >> On Fri, Oct 28, 2022 at 12:16 PM -07, Cong Wang wrote:
>> >> > On Mon, Oct 24, 2022 at 03:33:13PM +0200, Jakub Sitnicki wrote:
>> >> >> On Tue, Oct 18, 2022 at 11:13 AM -07, sdf@xxxxxxxxxx wrote:
>> >> >> > On 10/17, Cong Wang wrote:
>> >> >> >> From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
>> >> >> >
>> >> >> >> Technically we don't need lock the sock in the psock work, but we
>> >> >> >> need to prevent this work running in parallel with sock_map_close().
>> >> >> >
>> >> >> >> With this, we no longer need to wait for the psock->work synchronously,
>> >> >> >> because when we reach here, either this work is still pending, or
>> >> >> >> blocking on the lock_sock(), or it is completed. We only need to cancel
>> >> >> >> the first case asynchronously, and we need to bail out the second case
>> >> >> >> quickly by checking SK_PSOCK_TX_ENABLED bit.
>> >> >> >
>> >> >> >> Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
>> >> >> >> Reported-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
>> >> >> >> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
>> >> >> >> Cc: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
>> >> >> >> Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
>> >> >> >
>> >> >> > This seems to remove the splat for me:
>> >> >> >
>> >> >> > Tested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
>> >> >> >
>> >> >> > The patch looks good, but I'll leave the review to Jakub/John.
>> >> >>
>> >> >> I can't poke any holes in it either.
>> >> >>
>> >> >> However, it is harder for me to follow than the initial idea [1].
>> >> >> So I'm wondering if there was anything wrong with it?
>> >> >
>> >> > It caused a warning in sk_stream_kill_queues() when I actually tested
>> >> > it (after posting).
>> >>
>> >> We must have seen the same warnings. They seemed unrelated so I went
>> >> digging. We have a fix for these [1]. They were present since 5.18-rc1.
>> >>
>> >> >> This seems like a step back when comes to simplifying locking in
>> >> >> sk_psock_backlog() that was done in 799aa7f98d53.
>> >> >
>> >> > Kinda, but it is still true that this sock lock is not for sk_socket
>> >> > (merely for closing this race condition).
>> >>
>> >> I really think the initial idea [2] is much nicer. I can turn it into a
>> >> patch, if you are short on time.
>> >>
>> >> With [1] and [2] applied, the dead lock and memory accounting warnings
>> >> are gone, when running `test_sockmap`.
>> >>
>> >> Thanks,
>> >> Jakub
>> >>
>> >> [1]
>> >> https://lore.kernel.org/netdev/1667000674-13237-1-git-send-email-wangyufen@xxxxxxxxxx/
>> >> [2] https://lore.kernel.org/netdev/Y0xJUc%2FLRu8K%2FAf8@pop-os.localdomain/
>> >
>> > Cong, what do you think? I tend to agree [2] looks nicer to me.
>> >
>> > @Jakub,
>> >
>> > Also I think we could simply drop the proposed cancel_work_sync in
>> > sock_map_close()?
>> >
>> > }
>> > @@ -1619,9 +1619,10 @@ void sock_map_close(struct sock *sk, long timeout)
>> > saved_close = psock->saved_close;
>> > sock_map_remove_links(sk, psock);
>> > rcu_read_unlock();
>> > - sk_psock_stop(psock, true);
>> > - sk_psock_put(sk, psock);
>> > + sk_psock_stop(psock);
>> > release_sock(sk);
>> > + cancel_work_sync(&psock->work);
>> > + sk_psock_put(sk, psock);
>> > saved_close(sk, timeout);
>> > }
>> >
>> > The sk_psock_put is going to cancel the work before destroying the psock,
>> >
>> > sk_psock_put()
>> > sk_psock_drop()
>> > queue_rcu_work(system_wq, psock->rwork)
>> >
>> > and then in callback we
>> >
>> > sk_psock_destroy()
>> > cancel_work_synbc(psock->work)
>> >
>> > although it might be nice to have the work cancelled earlier rather than
>> > latter maybe.
>>
>> Good point.
>>
>> I kinda like the property that once close() returns we know there is no
>> deferred work running for the socket.
>>
>> I find the APIs where a deferred cleanup happens sometimes harder to
>> write tests for.
>>
>> But I don't really have a strong opinion here.
>
> I don't either and Cong left it so I'm good with that.
>
> Reviewing backlog logic though I think there is another bug there, but
> I haven't been able to trigger it in any of our tests.
>
> The sk_psock_backlog() logic is,
>
> sk_psock_backlog(struct work_struct *work)
> mutex_lock()
> while (skb = ...)
> ...
> do {
> ret = sk_psock_handle_skb()
> if (ret <= 0) {
> if (ret == -EAGAIN) {
> sk_psock_skb_state()
> goto end;
> }
> ...
> } while (len);
> ...
> end:
> mutex_unlock()
>
> what I'm not seeing is if we get an EAGAIN through sk_psock_handle_skb
> how do we schedule the backlog again. For egress we would set the
> SOCK_NOSPACE bit and then get a write space available callback which
> would do the schedule(). The ingress side could fail with EAGAIN
> through the alloc_sk_msg(GFP_ATOMIC) call. This is just a kzalloc,
>
> sk_psock_handle_skb()
> sk_psock_skb_ingress()
> sk_psock_skb_ingress_self()
> msg = alloc_sk_msg()
> kzalloc() <- this can return NULL
> if (!msg)
> return -EAGAIN <- could we stall now
>
>
> I think we could stall here if there was nothing else to kick it. I
> was thinking about this maybe,
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 1efdc47a999b..b96e95625027 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -624,13 +624,20 @@ static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb,
> static void sk_psock_skb_state(struct sk_psock *psock,
> struct sk_psock_work_state *state,
> struct sk_buff *skb,
> - int len, int off)
> + int len, int off, bool ingress)
> {
> spin_lock_bh(&psock->ingress_lock);
> if (sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED)) {
> state->skb = skb;
> state->len = len;
> state->off = off;
> + /* For ingress we may not have a wakeup callback to trigger
> + * the reschedule on so need to reschedule retry. For egress
> + * we will get TCP stack callback when its a good time to
> + * retry.
> + */
> + if (ingress)
> + schedule_work(&psock->work);
> } else {
> sock_drop(psock->sk, skb);
> }
> @@ -678,7 +685,7 @@ static void sk_psock_backlog(struct work_struct *work)
> if (ret <= 0) {
> if (ret == -EAGAIN) {
> sk_psock_skb_state(psock, state, skb,
> - len, off);
> + len, off, ingress);
> goto end;
> }
> /* Hard errors break pipe and stop xmit. */
>
>
> Its tempting to try and use the memory pressure callbacks but those are
> built for the skb cache so I think overloading them is not so nice. The
> drawback to above is its possible no memory is available even when we
> get back to the backlog. We could use a delayed reschedule but its not
> clear what delay makes sense here. Maybe some backoff...
>
> Any thoughts?
I don't have any thoughts on the fix yet, but I have a repro.
We can use fault injection [1]. For some reason it's been disabled on
x86-64 since 2007 (stack walking didn't work back then?), so we need to
patch the kernel slightly.
Also, to better target the failure, just for this case, I've de-inlined
alloc_sk_msg(). But in general testing we can just inject any alloc
under sk_psock_backlog().
Incantation looks like so:
#!/usr/bin/env bash
readonly TARGET_FUNC=alloc_sk_msg
readonly ADDR=($(grep -A1 ${TARGET_FUNC} /proc/kallsyms | awk '{print "0x" $1}'))
exec bash \
../../fault-injection/failcmd.sh \
--require-start=${ADDR[0]} --require-end=${ADDR[1]} \
--stacktrace-depth=32 \
--probability=50 --times=100 \
--ignore-gfp-wait=N --task-filter=N \
-- \
./test_sockmap
We won't get a message in dmesg (even with --verbosity=1 set) because
we're allocating with __GFP_NOWARN, and fault injection interface
doesn't provide a way to override that. But we can obseve the 'times'
count go down after ./test_sockmap blocks (also confirmed with a printk
added on -EAGAIN error path).
This is what I observe:
bash-5.1# ./repro.sh
# 1/ 6 sockmap::txmsg test passthrough:OK
# 2/ 6 sockmap::txmsg test redirect:OK
# 3/ 1 sockmap::txmsg test redirect wait send mem:OK
# 4/ 6 sockmap::txmsg test drop:OK
# 5/ 6 sockmap::txmsg test ingress redirect:OK <-- blocked here
^Z
[1]+ Stopped ./repro.sh
bash-5.1# cat /sys/kernel/debug/failslab/times
99
bash-5.1#
Kernel tweaks attached below.
-jkbs
[1] https://www.kernel.org/doc/html/latest/fault-injection/fault-injection.html
---8<---
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 3fc7abffc7aa..32c5329b0dd9 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1963,7 +1963,6 @@ config FAIL_SUNRPC
config FAULT_INJECTION_STACKTRACE_FILTER
bool "stacktrace filter for fault-injection capabilities"
depends on FAULT_INJECTION_DEBUG_FS && STACKTRACE_SUPPORT
- depends on !X86_64
select STACKTRACE
depends on FRAME_POINTER || MIPS || PPC || S390 || MICROBLAZE || ARM || ARC || X86
help
diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index e6b9ced3eda8..0f7dc67a3708 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -500,7 +500,7 @@ bool sk_msg_is_readable(struct sock *sk)
}
EXPORT_SYMBOL_GPL(sk_msg_is_readable);
-static struct sk_msg *alloc_sk_msg(gfp_t gfp)
+static noinline struct sk_msg *alloc_sk_msg(gfp_t gfp)
{
struct sk_msg *msg;
diff --git a/tools/testing/fault-injection/failcmd.sh b/tools/testing/fault-injection/failcmd.sh
index 78dac34264be..887dd4553cae 100644
--- a/tools/testing/fault-injection/failcmd.sh
+++ b/tools/testing/fault-injection/failcmd.sh
@@ -212,7 +212,7 @@ done
echo $oom_kill_allocating_task > /proc/sys/vm/oom_kill_allocating_task
echo $task_filter > $FAULTATTR/task-filter
echo $probability > $FAULTATTR/probability
-echo $times > $FAULTATTR/times
+printf "%#x" $times > $FAULTATTR/times
trap "restore_values" SIGINT SIGTERM EXIT
