Regression from patch 'tty: hvc: hvc_poll() break hv read loop'

[Jason Gunthorpe]

Hi Nicholas,

I am testing 4.19-rc2 and I see bad behavior with my qemu hvc0
console..

Running interactive with qemu (qemu-2.11.2-1.fc28) on the console
providing hvc0, using options like:

        -nographic
        -chardev stdio,id=stdio,mux=on,signal=off
        -mon chardev=stdio
	-device isa-serial,chardev=stdio
        -device virtio-serial-pci
	-device virtconsole,chardev=stdio

I see the hvc0 console hang regularly, ie doing something like 'up
arrow' in bash causes the hvc0 console to hang. Prior kernels worked
OK.

Any ideas? I'm not familiar with this code.. Thanks!

git bisect says this patch is to blame:

# bad: [5b394b2ddf0347bef56e50c69a58773c94343ff3] Linux 4.19-rc1
# good: [845b397a7771f2d3504beff5521f452be0d22eec] IB/ucm: fix UCM link error
# bad: [60c1f89241d49bacf71035470684a8d7b4bb46ea] Merge tag 'dma-mapping-4.19-2' of git://git.infradead.org/users/hch/dma-mapping
# bad: [1290290c922fdcefbce8984e6e44b8f4e3a169b5] Merge tag 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rdma/rdma
# bad: [6eaac34ff30e189fda28110298ca9fbfb2f51e28] Merge tag 'linux-watchdog-4.19-rc1' of git://www.linux-watchdog.org/linux-watchdog
# good: [e4f6a44c4aeca9eda153302abb0c14d053914f72] staging:rtl8192u: Remove unused macro definitions - Style
# bad: [5695d5d1970f975de059bb6dec76941440f62488] Merge tag 'usb-4.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb
# bad: [f80a71b0c4111e26433744721ad68f05c169ad39] Merge tag 'drm-next-2018-08-17' of git://anongit.freedesktop.org/drm/drm
# good: [cf175dc315f90185128fb061dc05b6fbb211aa2f] powerpc/64: Disable the speculation barrier from the command line
# bad: [a2dc009afa9ae8b92305be7728676562a104cb40] powerpc/mm/book3s/radix: Add mapping statistics
# good: [d6690b1a9b0dd95a6fbd166d9657e0cc80afbf99] powerpc: Allow CPU selection of e300core variants
# good: [f2c6d0d1092e3da2bd36c768777e883fa3908548] powerpc/32: Include setup.h header file to fix warnings
# good: [b3124ec2f9970c7374d34b00843d9791fca66afc] Merge branch 'fixes' into next
# bad: [70b5c4ee8e3bf7ce107d6926d9c7d8ebb68578c2] Merge branch 'topic/hvc' into next
# bad: [cfb5946b55f1dfd19e042feae1fbff6041e25a98] tty: hvc: hvc_poll() may sleep
# bad: [ec97eaad1383ab2500fcf9a07ade6044fbcc67f5] tty: hvc: hvc_poll() break hv read loop
# good: [a9bf5c8a271b9a954709b7ada1bd258f5cadf7ff] tty: hvc: use mutex instead of spinlock for hvc_structs lock
# first bad commit: [ec97eaad1383ab2500fcf9a07ade6044fbcc67f5] tty: hvc: hvc_poll() break hv read loop