On 10/15/24 11:59 AM, 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(). Thanks Omar, nice debugging! -- Jens Axboe
