* [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES
@ 2019-01-15 21:55 Waiman Long
2019-01-16 16:47 ` Will Deacon
0 siblings, 1 reply; 3+ messages in thread
From: Waiman Long @ 2019-01-15 21:55 UTC (permalink / raw)
To: Peter Zijlstra, Ingo Molnar, Will Deacon
Cc: linux-kernel, Zhenzhong Duan, James Morse, Borislav Petkov,
SRINIVAS, Waiman Long
On some architectures, it is possible to have nested NMIs taking
spinlocks nestedly. Even though the chance of having more than 4 nested
spinlocks with contention is extremely small, there could still be a
possibility that it may happen some days leading to system panic.
What we don't want is a silent corruption with system panic somewhere
else. So add a BUG_ON() check to make sure that a system panic caused
by this will show the correct root cause.
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/qspinlock.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 8a8c3c2..f823221 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
idx = node->count++;
tail = encode_tail(smp_processor_id(), idx);
+ /*
+ * 4 nodes are allocated based on the assumption that there will
+ * not be nested NMIs taking spinlocks. That may not be true in
+ * some architectures even though the chance of needing more than
+ * 4 nodes will still be extremely unlikely. Adding a bug check
+ * here to make sure there won't be a silent corruption in case
+ * this condition happens.
+ */
+ BUG_ON(idx >= MAX_NODES);
+
node = grab_mcs_node(node, idx);
/*
--
1.8.3.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES
2019-01-15 21:55 [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES Waiman Long
@ 2019-01-16 16:47 ` Will Deacon
2019-01-16 16:53 ` Waiman Long
0 siblings, 1 reply; 3+ messages in thread
From: Will Deacon @ 2019-01-16 16:47 UTC (permalink / raw)
To: Waiman Long
Cc: Peter Zijlstra, Ingo Molnar, linux-kernel, Zhenzhong Duan,
James Morse, Borislav Petkov, SRINIVAS
On Tue, Jan 15, 2019 at 04:55:44PM -0500, Waiman Long wrote:
> On some architectures, it is possible to have nested NMIs taking
> spinlocks nestedly. Even though the chance of having more than 4 nested
> spinlocks with contention is extremely small, there could still be a
> possibility that it may happen some days leading to system panic.
>
> What we don't want is a silent corruption with system panic somewhere
> else. So add a BUG_ON() check to make sure that a system panic caused
> by this will show the correct root cause.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/locking/qspinlock.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 8a8c3c2..f823221 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> idx = node->count++;
> tail = encode_tail(smp_processor_id(), idx);
>
> + /*
> + * 4 nodes are allocated based on the assumption that there will
> + * not be nested NMIs taking spinlocks. That may not be true in
> + * some architectures even though the chance of needing more than
> + * 4 nodes will still be extremely unlikely. Adding a bug check
> + * here to make sure there won't be a silent corruption in case
> + * this condition happens.
> + */
> + BUG_ON(idx >= MAX_NODES);
> +
Hmm, I really don't like the idea of putting a BUG_ON() on the spin_lock()
path. I'd prefer it if (a) we didn't add extra conditional code for the
common case and (b) didn't bring down the machine. Could we emit a
lockdep-style splat, instead?
Will
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES
2019-01-16 16:47 ` Will Deacon
@ 2019-01-16 16:53 ` Waiman Long
0 siblings, 0 replies; 3+ messages in thread
From: Waiman Long @ 2019-01-16 16:53 UTC (permalink / raw)
To: Will Deacon
Cc: Peter Zijlstra, Ingo Molnar, linux-kernel, Zhenzhong Duan,
James Morse, Borislav Petkov, SRINIVAS
On 01/16/2019 11:47 AM, Will Deacon wrote:
> On Tue, Jan 15, 2019 at 04:55:44PM -0500, Waiman Long wrote:
>> On some architectures, it is possible to have nested NMIs taking
>> spinlocks nestedly. Even though the chance of having more than 4 nested
>> spinlocks with contention is extremely small, there could still be a
>> possibility that it may happen some days leading to system panic.
>>
>> What we don't want is a silent corruption with system panic somewhere
>> else. So add a BUG_ON() check to make sure that a system panic caused
>> by this will show the correct root cause.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>> kernel/locking/qspinlock.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 8a8c3c2..f823221 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>> idx = node->count++;
>> tail = encode_tail(smp_processor_id(), idx);
>>
>> + /*
>> + * 4 nodes are allocated based on the assumption that there will
>> + * not be nested NMIs taking spinlocks. That may not be true in
>> + * some architectures even though the chance of needing more than
>> + * 4 nodes will still be extremely unlikely. Adding a bug check
>> + * here to make sure there won't be a silent corruption in case
>> + * this condition happens.
>> + */
>> + BUG_ON(idx >= MAX_NODES);
>> +
> Hmm, I really don't like the idea of putting a BUG_ON() on the spin_lock()
> path. I'd prefer it if (a) we didn't add extra conditional code for the
> common case and (b) didn't bring down the machine. Could we emit a
> lockdep-style splat, instead?
>
> Will
I am going to drop this patch. I am working on another one that will
handle the no MCS node available case by spinning directly on the lock
cacheline under this rare circumstance. Of course, that will incur a
little bit of performance overhead in the slow path which I am trying to
measure right now.
Cheers,
Longman
^ permalink raw reply [flat|nested] 3+ messages in thread
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2019-01-15 21:55 [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES Waiman Long
2019-01-16 16:47 ` Will Deacon
2019-01-16 16:53 ` Waiman Long
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