From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: I2C from interrupt context? Date: Mon, 10 Nov 2008 23:57:34 -0800 Message-ID: <20081110235734.f7666725.akpm@linux-foundation.org> References: <20081106.154754.02648408.davem@davemloft.net> <20081107101118.286f3713@hyperion.delvare> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-i2c-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Krzysztof Halasa Cc: Jean Delvare , David Miller , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-i2c@vger.kernel.org On Fri, 07 Nov 2008 17:05:35 +0100 Krzysztof Halasa wrote: > Jean Delvare 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.