[PATCH] lib: fix data race in rhashtable_rehash_one

[Dmitry Vyukov]

rhashtable_rehash_one() uses plain writes to update entry->next,
while it is being concurrently accessed by readers.
Unfortunately, the compiler is within its rights to (for example) use
byte-at-a-time writes to update the pointer, which would fatally confuse
concurrent readers.

Use WRITE_ONCE to update entry->next in rhashtable_rehash_one().

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvy...@google.com>
---
KTSAN report for the record:

ThreadSanitizer: data-race in netlink_lookup

Atomic read at 0xffff880480443bd0 of size 8 by thread 2747 on CPU 11:
[< inline >] rhashtable_lookup_fast include/linux/rhashtable.h:543
[< inline >] __netlink_lookup net/netlink/af_netlink.c:1026
[<ffffffff81bd9a84>] netlink_lookup+0x134/0x1c0 net/netlink/af_netlink.c:1046
[< inline >] netlink_getsockbyportid net/netlink/af_netlink.c:1616
[<ffffffff81bdc701>] netlink_unicast+0x111/0x300 net/netlink/af_netlink.c:1812
[<ffffffff81bdcdb9>] netlink_sendmsg+0x4c9/0x5f0 net/netlink/af_netlink.c:2443
[< inline >] sock_sendmsg_nosec net/socket.c:610
[<ffffffff81b5d6f3>] sock_sendmsg+0x83/0x90 net/socket.c:620
[<ffffffff81b5e59f>] ___sys_sendmsg+0x3cf/0x3e0 net/socket.c:1952
[<ffffffff81b5f6ac>] __sys_sendmsg+0x4c/0xb0 net/socket.c:1986
[< inline >] SYSC_sendmsg net/socket.c:1997
[<ffffffff81b5f740>] SyS_sendmsg+0x30/0x50 net/socket.c:1993
[<ffffffff81ee3e11>] entry_SYSCALL_64_fastpath+0x31/0x95
arch/x86/entry/entry_64.S:188

Previous write at 0xffff880480443bd0 of size 8 by thread 213 on CPU 4:
[< inline >] rhashtable_rehash_one lib/rhashtable.c:193
[< inline >] rhashtable_rehash_chain lib/rhashtable.c:213
[< inline >] rhashtable_rehash_table lib/rhashtable.c:257
[<ffffffff8156f7e0>] rht_deferred_worker+0x3b0/0x6d0 lib/rhashtable.c:373
[<ffffffff810b1d6e>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
[<ffffffff810b22d0>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
[<ffffffff810bba40>] kthread+0x150/0x170 kernel/kthread.c:209
[<ffffffff81ee420f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:529

Mutexes locked by thread 213:
Mutex 217217 is locked here:
[<ffffffff81ee0407>] mutex_lock+0x57/0x70 kernel/locking/mutex.c:108
[<ffffffff8156f475>] rht_deferred_worker+0x45/0x6d0 lib/rhashtable.c:363
[<ffffffff810b1d6e>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
[<ffffffff810b22d0>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
[<ffffffff810bba40>] kthread+0x150/0x170 kernel/kthread.c:209
[<ffffffff81ee420f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:529

Mutex 431216 is locked here:
[< inline >] __raw_spin_lock_bh include/linux/spinlock_api_smp.h:149
[<ffffffff81ee3195>] _raw_spin_lock_bh+0x65/0x80 kernel/locking/spinlock.c:175
[< inline >] spin_lock_bh include/linux/spinlock.h:317
[< inline >] rhashtable_rehash_chain lib/rhashtable.c:212
[< inline >] rhashtable_rehash_table lib/rhashtable.c:257
[<ffffffff8156f616>] rht_deferred_worker+0x1e6/0x6d0 lib/rhashtable.c:373
[<ffffffff810b1d6e>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
[<ffffffff810b22d0>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
[<ffffffff810bba40>] kthread+0x150/0x170 kernel/kthread.c:209
[<ffffffff81ee420f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:529

Mutex 432766 is locked here:
[< inline >] __raw_spin_lock include/linux/spinlock_api_smp.h:158
[<ffffffff81ee37d0>] _raw_spin_lock+0x50/0x70 kernel/locking/spinlock.c:151
[< inline >] rhashtable_rehash_one lib/rhashtable.c:186
[< inline >] rhashtable_rehash_chain lib/rhashtable.c:213
[< inline >] rhashtable_rehash_table lib/rhashtable.c:257
[<ffffffff8156f79b>] rht_deferred_worker+0x36b/0x6d0 lib/rhashtable.c:373
[<ffffffff810b1d6e>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
[<ffffffff810b22d0>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
[<ffffffff810bba40>] kthread+0x150/0x170 kernel/kthread.c:209
[<ffffffff81ee420f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:529
---
lib/rhashtable.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index cc0c697..978624d 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -188,9 +188,12 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
new_tbl, new_hash);

if (rht_is_a_nulls(head))
- INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
- else
- RCU_INIT_POINTER(entry->next, head);
+ head = (struct rhash_head *)rht_marker(ht, new_hash);
+ /* We don't insert any new nodes that were not previously accessible
+ * to readers, so we don't need to use rcu_assign_pointer().
+ * But entry is being concurrently accessed by readers, so we need to
+ * use at least WRITE_ONCE. */
+ WRITE_ONCE(entry->next, head);

rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
spin_unlock(new_bucket_lock);
--
2.6.0.rc0.131.gf624c3d

[Eric Dumazet]

This is bogus.

1) Linux is certainly not working if some arch or compiler is not doing
single word writes. WRITE_ONCE() would not help at all to enforce this.

2) If new node is not yet visible, we don't care if we write
entry->next using any kind of operation.

So the WRITE_ONCE() is not needed at all.



> + WRITE_ONCE(entry->next, head);


The rcu_assign_pointer() immediately following is enough in this case.

We have hundred of similar cases in the kernel.

[Eric Dumazet]

On Mon, 2015-09-21 at 06:31 -0700, Eric Dumazet wrote:
> On Mon, 2015-09-21 at 10:08 +0200, Dmitry Vyukov wrote:
> > rhashtable_rehash_one() uses plain writes to update entry->next,
> > while it is being concurrently accessed by readers.
> > Unfortunately, the compiler is within its rights to (for example) use
> > byte-at-a-time writes to update the pointer, which would fatally confuse
> > concurrent readers.
> >

> This is bogus.
>
> 1) Linux is certainly not working if some arch or compiler is not doing
> single word writes. WRITE_ONCE() would not help at all to enforce this.
>
> 2) If new node is not yet visible, we don't care if we write
> entry->next using any kind of operation.
>
> So the WRITE_ONCE() is not needed at all.
>
>
>
> > + WRITE_ONCE(entry->next, head);
>
>
> The rcu_assign_pointer() immediately following is enough in this case.
>
> We have hundred of similar cases in the kernel.
>
>

The changelog and comment are totally confusing.

Please remove the bogus parts in them, and/or rephrase.

The important part here is that we rehash an item, so we need to make
sure to maintain consistent ->next field, and need to prevent compiler
from using ->next as a temporary variable.

ptr->next = 1UL | ((base + offset) << 1);

Is dangerous because compiler could issue :

ptr->next = (base + offset);

ptr->next <<= 1;

ptr->next += 1UL;

Frankly, all this looks like an oversight in this code.

Not sure why the NULLS value is even recomputed.


diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index cc0c69710dcf..0a29f07ba45a 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -187,10 +187,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
head = rht_dereference_bucket(new_tbl->buckets[new_hash],
new_tbl, new_hash);

- if (rht_is_a_nulls(head))

- INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
- else
- RCU_INIT_POINTER(entry->next, head);

+ RCU_INIT_POINTER(entry->next, head);

[Dmitry Vyukov]

I have not looked in detail yet, but the NULLS recomputation uses
new_hash, which obviously wasn't available when the value was
previously computed. Don't know yet whether it is important or not.

>
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index cc0c69710dcf..0a29f07ba45a 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -187,10 +187,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
> head = rht_dereference_bucket(new_tbl->buckets[new_hash],
> new_tbl, new_hash);
>
> - if (rht_is_a_nulls(head))
> - INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
> - else
> - RCU_INIT_POINTER(entry->next, head);
> + RCU_INIT_POINTER(entry->next, head);
>
> rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
> spin_unlock(new_bucket_lock);
>
>

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--
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[Eric Dumazet]

Well, head already contains the right value, set in bucket_table_alloc()

for (i = 0; i < nbuckets; i++)
INIT_RHT_NULLS_HEAD(tbl->buckets[i], ht, i);

Think of this nulls value as a special NULL pointer.

If hash table is properly allocated/initialized, all the chains are
correctly ending with a proper NULL pointer.

[Thomas Graf]

On 09/21/15 at 07:51am, Eric Dumazet wrote:
> The important part here is that we rehash an item, so we need to make
> sure to maintain consistent ->next field, and need to prevent compiler
> from using ->next as a temporary variable.
>
> ptr->next = 1UL | ((base + offset) << 1);
>
> Is dangerous because compiler could issue :
>
> ptr->next = (base + offset);
>
> ptr->next <<= 1;
>
> ptr->next += 1UL;
>
> Frankly, all this looks like an oversight in this code.
>
> Not sure why the NULLS value is even recomputed.

The hash of the chain is part of the NULLS value. Since the
entry might have been moved to a different chain, the NULLS
value must be recalculated to contain the proper hash.

However, nobody is using the hash today as far as I can
see so we could as well just remove it and use the base
value only for the nulls marker.

[Eric Dumazet]

What I said is :

In @head you already have the correct nulls value, from hash table.

You do not need to recompute this value, and/or test if hash table chain
is empty.

If hash bucket is empty, it contains the appropriate NULLS value.

If you are paranoiac add this debugging check :

if (rht_is_a_nulls(head))
BUG_ON(head != (struct rhash_head *)rht_marker(ht, new_hash));


Therefore, simply fix the bug and unnecessary code with :

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index cc0c69710dcf..a54ff8949f91 100644

[Thomas Graf]

On 09/21/15 at 04:03pm, Eric Dumazet wrote:
> What I said is :
>
> In @head you already have the correct nulls value, from hash table.
>
> You do not need to recompute this value, and/or test if hash table chain
> is empty.
>
> If hash bucket is empty, it contains the appropriate NULLS value.
>
> If you are paranoiac add this debugging check :
>
> if (rht_is_a_nulls(head))
> BUG_ON(head != (struct rhash_head *)rht_marker(ht, new_hash));
>
>
> Therefore, simply fix the bug and unnecessary code with :

You are absolutely right Eric. Do you want to revise your patch Dmitry?
Eric's proposed fix absolutely the best way to fix this.

[Dmitry Vyukov]

rhashtable_rehash_one() uses complex logic to update entry->next field,
after INIT_RHT_NULLS_HEAD and NULLS_MARKER expansion:

entry->next = 1 | ((base + off) << 1)

This can be compiled along the lines of:

entry->next = base + off
entry->next <<= 1
entry->next |= 1

Which will break concurrent readers.

NULLS value recomputation is not needed here, so just remove
the complex logic.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvy...@google.com>
---

v2: Remove NULLS values recomputation as it is not needed.
Update commit description to clarify that the problem
is not with racy reads/writes per se but rather with
the complex update logic.

lib/rhashtable.c | 5 +----
1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index cc0c697..a54ff89 100644

--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -187,10 +187,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
head = rht_dereference_bucket(new_tbl->buckets[new_hash],
new_tbl, new_hash);

- if (rht_is_a_nulls(head))
- INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
- else
- RCU_INIT_POINTER(entry->next, head);
+ RCU_INIT_POINTER(entry->next, head);

rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
spin_unlock(new_bucket_lock);

--
2.6.0.rc0.131.gf624c3d

[Dmitry Vyukov]

Mailed v2 of the patch.

[Eric Dumazet]

On Tue, 2015-09-22 at 10:51 +0200, Dmitry Vyukov wrote:
> rhashtable_rehash_one() uses complex logic to update entry->next field,
> after INIT_RHT_NULLS_HEAD and NULLS_MARKER expansion:
>
> entry->next = 1 | ((base + off) << 1)
>
> This can be compiled along the lines of:
>
> entry->next = base + off
> entry->next <<= 1
> entry->next |= 1
>
> Which will break concurrent readers.
>
> NULLS value recomputation is not needed here, so just remove
> the complex logic.
>
> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Signed-off-by: Dmitry Vyukov <dvy...@google.com>
> ---

Thanks Dmitry

Acked-by: Eric Dumazet <edum...@google.com>

[Thomas Graf]

On 09/22/15 at 10:51am, Dmitry Vyukov wrote:
> rhashtable_rehash_one() uses complex logic to update entry->next field,
> after INIT_RHT_NULLS_HEAD and NULLS_MARKER expansion:
>
> entry->next = 1 | ((base + off) << 1)
>
> This can be compiled along the lines of:
>
> entry->next = base + off
> entry->next <<= 1
> entry->next |= 1
>
> Which will break concurrent readers.
>
> NULLS value recomputation is not needed here, so just remove
> the complex logic.
>
> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Signed-off-by: Dmitry Vyukov <dvy...@google.com>

Acked-by: Thomas Graf <tg...@suug.ch>

[Herbert Xu]

Eric Dumazet <eric.d...@gmail.com> wrote:
>
> What I said is :
>
> In @head you already have the correct nulls value, from hash table.
>
> You do not need to recompute this value, and/or test if hash table chain
> is empty.
>
> If hash bucket is empty, it contains the appropriate NULLS value.
>
> If you are paranoiac add this debugging check :
>
> if (rht_is_a_nulls(head))
> BUG_ON(head != (struct rhash_head *)rht_marker(ht, new_hash));
>
>
> Therefore, simply fix the bug and unnecessary code with :

Ack. I remember seeing this when I was working on it but never
got around to removing this bogosity.

Thanks,
--
Email: Herbert Xu <her...@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

[Herbert Xu]

Dmitry Vyukov <dvy...@google.com> wrote:
> rhashtable_rehash_one() uses complex logic to update entry->next field,
> after INIT_RHT_NULLS_HEAD and NULLS_MARKER expansion:
>
> entry->next = 1 | ((base + off) << 1)
>
> This can be compiled along the lines of:
>
> entry->next = base + off
> entry->next <<= 1
> entry->next |= 1
>
> Which will break concurrent readers.
>
> NULLS value recomputation is not needed here, so just remove
> the complex logic.
>
> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Signed-off-by: Dmitry Vyukov <dvy...@google.com>

Acked-by: Herbert Xu <her...@gondor.apana.org.au>

[David Miller]

From: Dmitry Vyukov <dvy...@google.com>
Date: Tue, 22 Sep 2015 10:51:52 +0200

> rhashtable_rehash_one() uses complex logic to update entry->next field,
> after INIT_RHT_NULLS_HEAD and NULLS_MARKER expansion:
>
> entry->next = 1 | ((base + off) << 1)
>
> This can be compiled along the lines of:
>
> entry->next = base + off
> entry->next <<= 1
> entry->next |= 1
>
> Which will break concurrent readers.
>
> NULLS value recomputation is not needed here, so just remove
> the complex logic.
>
> The data race was found with KernelThreadSanitizer (KTSAN).
>
> Signed-off-by: Dmitry Vyukov <dvy...@google.com>

Applied, thanks.