Re: I2C from interrupt context?

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 07 Nov 2008 17:05:35 +0100 Krzysztof Halasa <khc@pm.waw.pl> wrote:

> Jean Delvare <khali@linux-fr.org> writes:
> 
> > The situation is far from perfect though. For one thing, I seem to
> > recall that Andrew Morton didn't like the approach taken in
> > i2c_transfer(). For another, i2c_smbus_xfer() was not yet modified so
> > at this point only I2C-level transactions can be non-sleeping,
> > SMBus-level transactions can't. But all this could be fixed by anyone
> > who cares about these specific issues.
> 
> Thanks, I'll look at it.

The problem (well: bug) is that in_atomic() returns false inside a
spinlock when CONFIG_PREEMPT=n.  The code as it stands can sleep
inside a spinlock, which is deadlockable if a scheduled-to task
tries to take the same spinlock.

There is no means like this by which a piece of code can determine
whether it can call schedule().  The pattern which we use in many many
places (most especially GFP_KERNEL/GFP_ATOMIC) is to pass a flag down
to callees telling them in some manner which context they were called from.