From: Omar Sandoval <osandov@xxxxxx>
We're seeing crashes from rq_qos_wake_function that look like this:
BUG: unable to handle page fault for address: ffffafe180a40084
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 100000067 P4D 100000067 PUD 10027c067 PMD 10115d067 PTE 0
Oops: Oops: 0002 [#1] PREEMPT SMP PTI
CPU: 17 UID: 0 PID: 0 Comm: swapper/17 Not tainted 6.12.0-rc3-00013-geca631b8fe80 #11
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:_raw_spin_lock_irqsave+0x1d/0x40
Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 41 54 9c 41 5c fa 65 ff 05 62 97 30 4c 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 0a 4c 89 e0 41 5c c3 cc cc cc cc 89 c6 e8 2c 0b 00
RSP: 0018:ffffafe180580ca0 EFLAGS: 00010046
RAX: 0000000000000000 RBX: ffffafe180a3f7a8 RCX: 0000000000000011
RDX: 0000000000000001 RSI: 0000000000000003 RDI: ffffafe180a40084
RBP: 0000000000000000 R08: 00000000001e7240 R09: 0000000000000011
R10: 0000000000000028 R11: 0000000000000888 R12: 0000000000000002
R13: ffffafe180a40084 R14: 0000000000000000 R15: 0000000000000003
FS: 0000000000000000(0000) GS:ffff9aaf1f280000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffafe180a40084 CR3: 000000010e428002 CR4: 0000000000770ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<IRQ>
try_to_wake_up+0x5a/0x6a0
rq_qos_wake_function+0x71/0x80
__wake_up_common+0x75/0xa0
__wake_up+0x36/0x60
scale_up.part.0+0x50/0x110
wb_timer_fn+0x227/0x450
...
So rq_qos_wake_function() calls wake_up_process(data->task), which calls
try_to_wake_up(), which faults in raw_spin_lock_irqsave(&p->pi_lock).
p comes from data->task, and data comes from the waitqueue entry, which
is stored on the waiter's stack in rq_qos_wait(). Analyzing the core
dump with drgn, I found that the waiter had already woken up and moved
on to a completely unrelated code path, clobbering what was previously
data->task. Meanwhile, the waker was passing the clobbered garbage in
data->task to wake_up_process(), leading to the crash.
What's happening is that in between rq_qos_wake_function() deleting the
waitqueue entry and calling wake_up_process(), rq_qos_wait() is finding
that it already got a token and returning. The race looks like this:
rq_qos_wait() rq_qos_wake_function()
==============================================================
prepare_to_wait_exclusive()
data->got_token = true;
list_del_init(&curr->entry);
if (data.got_token)
break;
finish_wait(&rqw->wait, &data.wq);
^- returns immediately because
list_empty_careful(&wq_entry->entry)
is true
... return, go do something else ...
wake_up_process(data->task)
(NO LONGER VALID!)-^
Normally, finish_wait() is supposed to synchronize against the waker.
But, as noted above, it is returning immediately because the waitqueue
entry has already been removed from the waitqueue.
The bug is that rq_qos_wake_function() is accessing the waitqueue entry
AFTER deleting it. Note that autoremove_wake_function() wakes the waiter
and THEN deletes the waitqueue entry, which is the proper order.
Fix it by swapping the order. We also need to use
list_del_init_careful() to match the list_empty_careful() in
finish_wait().
Fixes: 38cfb5a45ee0 ("blk-wbt: improve waking of tasks")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Omar Sandoval <osandov@xxxxxx>
---
Hi,
I've only managed to reproduce this bug by inserting udelay() calls, so
I don't have a test case, unfortunately. I ran it through fio stress
tests with and without additional udelay() calls.
Note that the bug was introduced in 4.19 in wbt, which was then copied
to the generic rq-qos code in 5.0 in commit 84f603246db9 ("block: add
rq_qos_wait to rq_qos"). If this gets backported to 4.19 LTS, that will
need to be accounted for.
Based on Linus's master branch as of commit
eca631b8fe808748d7585059c4307005ca5c5820.
Thanks,
Omar
block/blk-rq-qos.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 2cfb297d9a62..058f92c4f9d5 100644
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -219,8 +219,8 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
data->got_token = true;
smp_wmb();
- list_del_init(&curr->entry);
wake_up_process(data->task);
+ list_del_init_careful(&curr->entry);
return 1;
}
--
2.47.0
