Bug#964839: Debugging results

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Felix,

On Mon, Sep 07, 2020 at 09:55:02PM +0200, Salvatore Bonaccorso wrote:
> 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

It has additionally been queued for v5.8.y stable series:
https://lore.kernel.org/stable/20200907222024.GP8670@sasha-vm/T/#t

Regards,
Salvatore