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Subject[PATCH v2] lib: fix data race in llist_del_first
Date
llist_del_first reads entry->next, but it did not acquire
visibility over the entry node. As the result it can get
a stale value of entry->next (e.g. NULL or whatever garbage
was there before the appending thread wrote correct value).
And then commit that value as llist head with cmpxchg.
That will corrupt llist.

Note there is a control-dependency between read of head->first
and read of entry->next, but it does not make the code correct.
Kernel memory model unambiguously says:
"A load-load control dependency requires a full read memory barrier".

Use smp_load_acquire to acquire visibility over the entry node.

The data race was found with KernelThreadSanitizer (KTSAN).

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

---

Change from the previous patch: actually use smp_load_acquire
(instead of READ_ONCE_CTRL, which would order only stores).

Here is an example of KTSAN report:

ThreadSanitizer: data-race in llist_del_first

Read of size 1 by thread T389 (K2630, CPU0):
[<ffffffff8156b8a9>] llist_del_first+0x39/0x70 lib/llist.c:74
[< inlined >] tty_buffer_alloc drivers/tty/tty_buffer.c:181
[<ffffffff81664af4>] __tty_buffer_request_room+0xb4/0x250 drivers/tty/tty_buffer.c:292
[<ffffffff81664e6c>] tty_insert_flip_string_fixed_flag+0x6c/0x150 drivers/tty/tty_buffer.c:337
[< inlined >] tty_insert_flip_string include/linux/tty_flip.h:35
[<ffffffff81667422>] pty_write+0x72/0xc0 drivers/tty/pty.c:110
[< inlined >] process_output_block drivers/tty/n_tty.c:611
[<ffffffff8165c016>] n_tty_write+0x346/0x7f0 drivers/tty/n_tty.c:2401
[< inlined >] do_tty_write drivers/tty/tty_io.c:1159
[<ffffffff816568df>] tty_write+0x21f/0x3f0 drivers/tty/tty_io.c:1245
[<ffffffff8125f00f>] __vfs_write+0x5f/0x1f0 fs/read_write.c:489
[<ffffffff8125ff8f>] vfs_write+0xef/0x280 fs/read_write.c:538
[< inlined >] SYSC_write fs/read_write.c:585
[<ffffffff81261390>] SyS_write+0x70/0xe0 fs/read_write.c:577
[<ffffffff81ee862e>] entry_SYSCALL_64_fastpath+0x12/0x71 arch/x86/entry/entry_64.S:186

Previous write of size 8 by thread T226 (K761, CPU0):
[<ffffffff8156b832>] llist_add_batch+0x32/0x70 lib/llist.c:44 (discriminator 16)
[< inlined >] llist_add include/linux/llist.h:180
[<ffffffff816649fc>] tty_buffer_free+0x6c/0xb0 drivers/tty/tty_buffer.c:221
[<ffffffff816651e7>] flush_to_ldisc+0x107/0x300 drivers/tty/tty_buffer.c:514
[<ffffffff810b20ee>] process_one_work+0x47e/0x930 kernel/workqueue.c:2036
[<ffffffff810b2650>] worker_thread+0xb0/0x900 kernel/workqueue.c:2170
[<ffffffff810bbe20>] kthread+0x150/0x170 kernel/kthread.c:209
[<ffffffff81ee8a1f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:526
---
lib/llist.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/llist.c b/lib/llist.c
index 0b0e977..ae5872b 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -66,12 +66,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
{
struct llist_node *entry, *old_entry, *next;

- entry = head->first;
+ entry = smp_load_acquire(&head->first);
for (;;) {
if (entry == NULL)
return NULL;
old_entry = entry;
- next = entry->next;
+ next = READ_ONCE(entry->next);
entry = cmpxchg(&head->first, old_entry, next);
if (entry == old_entry)
break;
--
2.6.0.rc0.131.gf624c3d


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