The patch titled
Subject: lib/llist.c: fix data race in llist_del_first
has been added to the -mm tree. Its filename is
lib-fix-data-race-in-llist_del_first.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/lib-fix-data-race-in-llist_del_first.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/lib-fix-data-race-in-llist_del_first.patch
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------------------------------------------------------
From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Subject: lib/llist.c: fix data race in llist_del_first
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).
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
Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Konstantin Serebryany <kcc@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/llist.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff -puN lib/llist.c~lib-fix-data-race-in-llist_del_first lib/llist.c
--- a/lib/llist.c~lib-fix-data-race-in-llist_del_first
+++ a/lib/llist.c
@@ -66,12 +66,12 @@ struct llist_node *llist_del_first(struc
{
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;
_
Patches currently in -mm which might be from dvyukov@xxxxxxxxxx are
lib-fix-data-race-in-llist_del_first.patch
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