On Tue, Oct 15, 2024 at 10:59:46AM -0700, Omar Sandoval wrote:
> 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>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Thanks for debugging this!
--
tejun
