[Xen-devel] [PATCH 3/3] watchdog/crash: Always disable watchdog in console_force_unlock()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Depending on the state of the conring and serial_tx_buffer,
console_force_unlock() can be a long running operation, usually because of
serial_start_sync()

XenServer testing has found a reliable case where console_force_unlock() on
one PCPU takes long enough for another PCPU to timeout due to the watchdog
(such as waiting for a tlb flush callin).

The watchdog timeout causes the second PCPU to repeat the
console_force_unlock(), at which point the first PCPU typically fails an
assertion in spin_unlock_irqrestore(&port->tx_lock) (because the tx_lock has
been unlocked behind itself).

console_force_unlock() is only on emergency paths, so one way or another the
host is going down.  Disable the watchdog before forcing the console lock to
help prevent having pcpus completing with each other to bring the host down.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/x86_64/traps.c |    2 --
 xen/drivers/char/console.c  |    1 +
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index ea2ffb6..1cc977c 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -227,8 +227,6 @@ void do_double_fault(struct cpu_user_regs *regs)
     unsigned int cpu;
     unsigned long crs[8];
 
-    watchdog_disable();
-
     console_force_unlock();
 
     asm ( "lsll %1, %0" : "=r" (cpu) : "rm" (PER_CPU_GDT_ENTRY << 3) );
diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 52ffa70..e47ddf2 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -737,6 +737,7 @@ void console_end_log_everything(void)
 
 void console_force_unlock(void)
 {
+    watchdog_disable();
     spin_lock_init(&console_lock);
     serial_force_unlock(sercon_handle);
     console_locks_busted = 1;
-- 
1.7.10.4


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