[RFC PATCH] mtd: rawnand: use mutex to protect access while in suspend

[Boris Brezillon]

On Tue, 5 Oct 2021 10:49:38 +0200
Sean Nyekjaer <sean at geanix.com> wrote:

> On Tue, Oct 05, 2021 at 10:23:00AM +0200, Boris Brezillon wrote:
> > On Tue, 5 Oct 2021 09:09:30 +0200
> > Sean Nyekjaer <sean at geanix.com> wrote:  
> 
> [ ... ]
> 
> > > 
> > > Have you seen the reproducer script?  
> > 
> > How would I know about this script or your previous attempt (mentioned
> > at the end of this email) given I was not Cc-ed on the previous
> > discussion, and nothing mentions it in this RFC...
> >   
> 
> That's why I shared it here ;)
> Initially I thought this was a bug introduced by exec_op.
> 
> > > ---
> > > root at iwg26-v1:/data/root# cat /data/crash.sh
> > > #!/bin/sh -x
> > > 
> > > echo enabled > /sys/devices/platform/soc/2100000.bus/21f4000.serial/tty/ttymxc4/power/wakeup
> > > 
> > > rm /data/test50M
> > > dd if=/dev/urandom of=/tmp/test50M bs=1M count=50
> > > cp /tmp/test50M /data/ &
> > > sleep 1
> > > echo mem > /sys/power/state
> > > ---
> > > 
> > > As seen in the log above disk is synced before suspend.
> > > cp is continuing to copy data to ubifs.
> > > And then user space processes are frozen.
> > > At this point the kernel thread would have unwritten data.
> > > 
> > > We tried to solve this with:
> > > https://lkml.org/lkml/2021/9/1/280  
> > 
> > I see. It's still unclear to me when the write happens. Is it in the
> > suspend path (before the system is actually suspended), or in the
> > resume path (when the system is being resumed).
> > 
> > Anyway, let's admit writing to a storage device while it's suspended is
> > a valid use case and requires the storage layer to put this request on
> > old. This wait should not, IMHO, be handled at the NAND level, but at
> > the MTD level (using a waitqueue, and an atomic to make
> > suspended/resumed transitions safe). And abusing a mutex to implement
> > that is certainly not a good idea.  
> 
> I did't say this was the right solution ;) I actually asked in the RFC:
> "Should we introduce a new mutex? Or maybe a spin_lock?"
> 
> What are you proposing, a waitqueue in mtd_info? That gets checked in
> mtd_write()/mtd_read()?

Yes, and replacing the suspended state by an atomic, and providing a
helper to wait on the device readiness. Helper you will call in every
path involving a communication with the HW, not just mtd_read/write()
(you're missing erase at least, and I fear there are other hooks that
might lead to commands being issued to the device). But before we get
there, I think it's important to understand what the kernel expects.
IOW, if and when threads can do a request on a suspended device, and
when it's acceptable to wait (vs returning -EBUSY), otherwise I fear
we'll end up with deadlocks in the suspend/resume path.