Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c

From: Andrew Morton <[email-masked]>
Date: Thu, 20 Mar 2008 18:08:02 -0700

On Thu, 20 Mar 2008 21:36:04 -0300 Henrique de Moraes Holschuh <[email-masked]> wrote:

> Well, so far so good for LEDs, but what about the other users of in_atomic
> that apparently should not be doing it either?

Ho hum.  Lots of cc's added.

./arch/x86/mm/pageattr.c

  Looks wrong.

./arch/m68k/atari/time.c

  Possibly buggy: deadlockable

./sound/core/seq/seq_virmidi.c

  Possibly buggy

./net/iucv/iucv.c
./kernel/power/process.c

  Just a debug check.

./drivers/s390/char/sclp_tty.c

  Possibly buggy: deadlockable
  
./drivers/s390/char/sclp_vt220.c

  Possibly buggy: deadlockable

./drivers/s390/net/netiucv.c

  Possibly buggy: deadlockable

./drivers/char/isicom.c

  Possibly buggy: deadlockable

./drivers/usb/misc/sisusbvga/sisusb_con.c

  Possibly buggy: deadlockable

./drivers/net/usb/pegasus.c

  Possibly buggy: deadlockable (I assume)

./drivers/net/wireless/airo.c

  Possibly buggy: deadlockable

./drivers/net/wireless/rt2x00/rt73usb.c

  Possibly buggy: deadlockable (I assume)

./drivers/net/wireless/rt2x00/rt2500usb.c

  Possibly buggy: deadlockable (I assume)

./drivers/net/wireless/hostap/hostap_ioctl.c

  Possibly buggy: deadlockable (I assume)

./drivers/net/wireless/zd1211rw/zd_usb.c

  Possibly buggy: deadlockable (I assume)

./drivers/net/irda/sir_dev.c

  Possibly buggy: deadlockable

./drivers/net/netxen/netxen_nic_niu.c

  Possibly buggy: deadlockable

./drivers/net/netxen/netxen_nic_init.c

  Possibly buggy: deadlockable

./drivers/ieee1394/ieee1394_transactions.c

  Possibly buggy: deadlockable

./drivers/video/amba-clcd.c

  Possibly buggy: deadlockable

./drivers/i2c/i2c-core.c

  Possibly buggy: deadlockable

The usual pattern for most of the above is

	if (!in_atomic())
		do_something_which_might_sleep();

problem is, in_atomic() returns false inside spinlock on non-preptible
kernels.  So if anyone calls those functions inside spinlock they will
incorrectly schedule and another task can then come in and try take the
already-held lock.

Now, it happens that in_atomic() returns true on non-preemtible kernels
when running in interrupt or softirq context.  But if the above code really
is using in_atomic() to detect am-i-called-from-interrupt and NOT
am-i-called-from-inside-spinlock, they should be using in_irq(),
in_softirq() or in_interrupt().