[Xen-devel] [Patch]Fix Linux smp guest hangs with complaint "BUG: soft lock detected on CPU#0"

["Huang, Xinmei" <xinmei.huang@xxxxxxxxx>]

Fix Linux smp guest hangs with complaint "BUG: soft lock detected on
CPU#0" when console=ttyS0 in guest grub configuration and serial='pty'
in vmx configure file.
The root cause to this bug is the characteristic of PTY emulator in
Qemu. PTY has write/read end with some buffer. Write to a buffer will be
failed after the buffer is full until read end reads the data from
buffer,
Previous to changeset 12026, write to serial port would fail quietly
when buffer for pty is full. With changeset 12026, write to serial port
would retry 3 times in 300ms if failed, even without notifying guest
using serial irq. Smp guest will hang, waiting for an interrupt. For SMP
guest, a watchdog thread should be executed periodically, otherwise soft
lockup is detected.
With this patch, an upper threshold of total consecutive retries is
added and serial interrupt would be sent after retry 3 times for each
write request, even if failed.

Signed-off-by: Xinmei Huang <xinmei.huang@xxxxxxxxx>

bug862-3.patch
Description: bug862-3.patch

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