RE: [PATCH 1/2] genirq: Fix the possible synchronize_irq() wait-forever

[Thomas Gleixner]

On Thu, 20 Feb 2014, Liu, Chuansheng wrote:
> Hello Thomas,
> 
> > -----Original Message-----
> > From: Thomas Gleixner [mailto:tglx@xxxxxxxxxxxxx]
> > Sent: Monday, February 10, 2014 4:58 PM
> > To: Liu, Chuansheng
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; Wang, Xiaoming
> > Subject: Re: [PATCH 1/2] genirq: Fix the possible synchronize_irq() wait-forever
> > 
> > On Mon, 10 Feb 2014, Chuansheng Liu wrote:
> > > There is below race between irq handler and irq thread:
> > > irq handler                             irq thread
> > >
> > > irq_wake_thread()                       irq_thread()
> > >   set bit RUNTHREAD
> > >   ...                                    clear bit RUNTHREAD
> > >                                          thread_fn()
> > >                                          [A]test_and_decrease
> > >                                                thread_active
> > >   [B]increase thread_active
> > >
> > > If action A is before action B, after that the thread_active
> > > will be always > 0, and for synchronize_irq() calling, which
> > > will be waiting there forever.
> > 
> > No. thread_active is 0, simply because after the atomic_dec_and_test()
> > it is -1 and the atomic_inc on the other side will bring it back to 0.
> > 
> Yes, you are right. The thread_active is back to 0 at last.
> 
> The case we meet is:
> 1/ T1: blocking at disable_irq() -- > sync_irq() -- > wait_event()
> [  142.678681]  [<c1a5b353>] schedule+0x23/0x60
> [  142.683466]  [<c12b24c5>] synchronize_irq+0x75/0xb0
> [  142.688931]  [<c125fad0>] ? wake_up_bit+0x30/0x30
> [  142.694201]  [<c12b33ab>] disable_irq+0x1b/0x20
> [  142.699278]  [<c17a79bc>] smb347_shutdown+0x2c/0x50
> [  142.704744]  [<c1789f7d>] i2c_device_shutdown+0x2d/0x40
> [  142.710597]  [<c1601734>] device_shutdown+0x14/0x140
> [  142.716161]  [<c12535f2>] kernel_restart_prepare+0x32/0x40
> [  142.722307]  [<c1253613>] kernel_restart+0x13/0x60
> 
> 2/ The corresponding irq thread is at sleep state:
> [  587.552408] irq/388-SMB0349 S f1c47620  7276   119      2 0x00000000
> [  587.552439]  f1d6bf20 00000046 f1c47a48 f1c47620 f1d6bec4 9e91731c 00000001 c1a5f3a5
> [  587.552468]  c20469c0 00000001 c20469c0 f36559c0 f1c47620 f307bde0 c20469c0 f1d6bef0
> [  587.552497]  00000296 00000000 00000296 f1d6bef0 c1a5bfa6 f1c47620 f1d6bf14 c126e329
> [  587.552501] Call Trace:
> [  587.552519]  [<c1a5f3a5>] ? sub_preempt_count+0x55/0xe0
> [  587.552535]  [<c1a5bfa6>] ? _raw_spin_unlock_irqrestore+0x26/0x50
> [  587.552548]  [<c126e329>] ? set_cpus_allowed_ptr+0x59/0xe0
> [  587.552563]  [<c1a5b093>] schedule+0x23/0x60
> [  587.552576]  [<c12b2ae1>] irq_thread+0xa1/0x130
> [  587.552588]  [<c12b27f0>] ? irq_thread_dtor+0xa0/0xa0
> 
> 3/ All the cpus are in the idle task;

Lets look at it again:

CPU 0                          CPU1

irq handler                    irq thread
  set IRQS_INPROGRESS
  ...
  irq_wake_thread()            irq_thread()
    set bit RUNTHREAD
    ...                          clear bit RUNTHREAD
                                 thread_fn()
  				 atomic_dec_and_test(threads_active) ( 0 -> -1)

    atomic_inc(threads_active) ( -1 -> 0)
  clr IRQS_INPROGRESS

Now synchronize_irq comes into play, that's what caused you to look
into this. 

synchronize_irq() can never observe the -1 state because it is
serialized against IRQS_INPROGESS. And when IRQS_INPROGRESS is
cleared, the threads_active state is back to 0.

I'm really not seing how this can happen. Any chance you can reproduce
this by executing the situation which led to this in a loop?

Thanks,

	tglx


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