Messages in this thread | | | Date | Tue, 5 Oct 2021 10:58:36 +0200 | From | Boris Brezillon <> | Subject | Re: [RFC PATCH] mtd: rawnand: use mutex to protect access while in suspend |
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On Tue, 5 Oct 2021 10:49:38 +0200 Sean Nyekjaer <sean@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@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@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.
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