Bug#964839: Debugging results

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Felix,

Many thanks for your debugging work!

On Mon, Sep 07, 2020 at 03:39:25PM +0200, Felix Dörre wrote:
> Hi,
> 
> Thanks for the notes. I did not install the 5.8.7-kernel because it did not
> automatically arrive via apt (and I am not sure which packages exactly I
> should install). So I went directly into trying to bisecting:
> 
> Bisecting did not help at all. The first few kernels generated by the
> commits indicated by bisect did not boot at all (loading the initramfs
> hangs). So as I already had a linux kerrnel I tried compiling the versions
> that I tested previously (just to see if they would boot and show the same
> problem): 5.6.14 and 5.7.6. That worked.
> 
> So I went with the 5.7.6-kernel and added printk-calls to understand what
> happens:
> 
> The mechanism to send the notifications on PM_SUSPEND allows many modules to
> register handlers. Adding prints there showed, that the handler was not
> "completely" broken. Handlers from other modules were invoked. However the
> mechanism that allows handlers to "break" the notification-chain and
> preventing all subsequent handlers to be executed. This is done, by
> returning "NOTIFY_STOP" from the handler. Searching the diff between 5.6.14
> and 5.7.6 for that constant showed a suspicious line at the end of
> hci_suspend_notifier in net/bluetooth/hci_core.c:
> 
> return ret ? notifier_from_errno(-EBUSY) : NOTIFY_STOP;
> 
> This line was introduced with 9952d90ea2885d7cbf80cd233f694f09a9c0eaec
> (which is in 5.7.6 and not in 5.6.14).
> 
> The handler was modified and improved upon between 5.7.6 and 5.8.7, but is
> still present in 5.8.7, where the handler always returns NOTIFY_STOP. In
> master the bug seems resolved, as there the handler returns (correctly)
> NOTIFY_DONE. The fix was made in 24b065727ceba53cc5bec0e725672417154df24f.
> Currently that commit is only contained in the tags: v5.9-rc1, v5.9-rc2,
> v5.9-rc3. To verify that this is indeed the problem I took the 5.7.6-kernel
> and adjusted:
> 
> return ret ? notifier_from_errno(-EBUSY) : NOTIFY_STOP;
> 
> to
> 
> 
> return ret ? notifier_from_errno(-EBUSY) : NOTIFY_DONE;
> 
> which gave me a working 5.7.6-kernel.
> 
> Will you try to backport a fix or do I just need to wait until 5.9 for a
> kernel that might handle suspend properly again?

I cherry-picked the commit and applied it to our packaging branch for
the next upload to unstable:
https://salsa.debian.org/kernel-team/linux/-/commit/a00d070d0498b28b5236420faf976d46fdb74362

Regards,
Salvatore