Re: 2.5.50: sleeping function called from illegal context

[Takashi Iwai <tiwai@suse.de>]

At Tue, 3 Dec 2002 04:55:15 +0100,
Duncan Sands wrote:
> 
> On Tuesday 03 December 2002 13:01, Takashi Iwai wrote:
> > At Tue, 3 Dec 2002 04:07:52 +0100,
> >
> > Duncan Sands wrote:
> > > Got this with today 2.5 BK tree:
> > >
> > > Debug: sleeping function called from illegal context at
> > > include/asm/semaphore.h:119 Call Trace:
> > >  [<c0113f1a>] __might_sleep+0x52/0x58
> > >  [<c024291a>] snd_cs46xx_iec958_put+0x36/0xf8
> > >  [<c0217f28>] snd_ctl_elem_write+0xe0/0x1a4
> > >  [<c0218360>] snd_ctl_ioctl+0x184/0x2c8
> > >  [<c01462e6>] sys_ioctl+0x1fa/0x244
> > >  [<c01088f7>] syscall_call+0x7/0xb
> >
> > ouch, we are using rwlock in the control ioctls.
> >
> > mutex is necessary for many controls, so we cannot suppress the use of
> > mutex in control callbacks.
> > but temporary unlocking looks ad-hoc, too...
> 
> If I understand right, the problem is that snd_ctl_elem_write
> acquires control_rwlock, which is a rw spinlock.  It then calls
> snd_cs46xx_iec958_put which acquires chip->spos_mutex,
> which is a semaphore.  Thus the message.  Now I deduce
> from the fact that  you don't use read_lock_irqsave that the
> data structure is not read from interrupt context.  That means
> you are only protecting against other CPUs.  So why not use
> a semaphore instead of a spinlock?

hmm, there are some places calling with irq lock.
for example, snd_ctl_notify() can be called from the interrupt handler
for some interrupts like h/w volume change.

however, it seems that a single rwlock is used for the management of
two different lists.  and snd_ctl_elem_write() uses card->controls
only, whereas snd_ctl_notify() uses card->ctl_files only.
hence, we can merge two locks, rwsem for card->controls and rwlock for
card->ctl_files.  i'll give a try.


ciao,

Takashi


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