linux-kernel - Re: [PATCH] pci: lock the pci_cfg_wait queue for the consistency of data

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Date:   Tue, 29 Oct 2019 11:34:33 +0800
From:   Xiang Zheng <zhengxiang9@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <bhelgaas@...gle.com>, <wangxiongfeng2@...wei.com>,
        <wanghaibin.wang@...wei.com>, <guoheyi@...wei.com>,
        <yebiaoxiang@...wei.com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rjw@...ysocki.net>,
        <tglx@...utronix.de>, <guohanjun@...wei.com>,
        <yangyingliang@...wei.com>
Subject: Re: [PATCH] pci: lock the pci_cfg_wait queue for the consistency of
 data



On 2019/10/29 0:30, Matthew Wilcox wrote:
> On Mon, Oct 28, 2019 at 05:18:09PM +0800, Xiang Zheng wrote:
>> Commit "7ea7e98fd8d0" suggests that the "pci_lock" is sufficient,
>> and all the callers of pci_wait_cfg() are wrapped with the "pci_lock".
>>
>> However, since the commit "cdcb33f98244" merged, the accesses to
>> the pci_cfg_wait queue are not safe anymore. A "pci_lock" is
>> insufficient and we need to hold an additional queue lock while
>> read/write the wait queue.
>>
>> So let's use the add_wait_queue()/remove_wait_queue() instead of
>> __add_wait_queue()/__remove_wait_queue().
> 
> As I said earlier, this reintroduces the deadlock addressed by
> cdcb33f9824429a926b971bf041a6cec238f91ff
> 

Thanks Matthew, sorry for that I did not understand the way to reintroduce
the deadlock and sent this patch. If what I think is right, the possible
deadlock may be caused by the condition in which there are three processes:

   *Process*                          *Acquired*         *Wait For*
   wake_up_all()                      wq_head->lock      pi_lock
   snbep_uncore_pci_read_counter()    pi_lock            pci_lock
   pci_wait_cfg()                     pci_lock           wq_head->lock

These processes suffer from the nested locks.:)

But for this problem, what do you think about the solution below:

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index 2fccb5762c76..09342a74e5ea 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -207,14 +207,14 @@ static noinline void pci_wait_cfg(struct pci_dev *dev)
 {
        DECLARE_WAITQUEUE(wait, current);

-       __add_wait_queue(&pci_cfg_wait, &wait);
        do {
                set_current_state(TASK_UNINTERRUPTIBLE);
                raw_spin_unlock_irq(&pci_lock);
+               add_wait_queue(&pci_cfg_wait, &wait);
                schedule();
+               remove_wait_queue(&pci_cfg_wait, &wait);
                raw_spin_lock_irq(&pci_lock);
        } while (dev->block_cfg_access);
-       __remove_wait_queue(&pci_cfg_wait, &wait);
 }

 /* Returns 0 on success, negative values indicate error. */



> .
> 

-- 

Thanks,
Xiang