On 06/15/2014 06:17 PM, Peter Zijlstra wrote:
Signed-off-by: Peter Zijlstra<peterz@xxxxxxxxxxxxx>
---
[...]
+
+void kvm_wait(int *ptr, int val)
+{
+ unsigned long flags;
+
+ if (in_nmi())
+ return;
+
+ /*
+ * Make sure an interrupt handler can't upset things in a
+ * partially setup state.
+ */
I am seeing hang with even 2 cpu guest (with patches on top of 3.15-rc6 ).
looking further with gdb I see one cpu is stuck with native_halt with
slowpath flag(_Q_LOCKED_SLOW) set when it was called.
(gdb) bt
#0 native_halt () at /test/master/arch/x86/include/asm/irqflags.h:55
#1 0xffffffff81033118 in halt (ptr=0xffffffff81eb0e58, val=524291) at
/test/master/arch/x86/include/asm/paravirt.h:116
#2 kvm_wait (ptr=0xffffffff81eb0e58, val=524291) at
arch/x86/kernel/kvm.c:835
#3 kvm_wait (ptr=0xffffffff81eb0e58, val=524291) at
arch/x86/kernel/kvm.c:809
#4 0xffffffff810a2d8e in pv_wait (lock=0xffffffff81eb0e58) at
/test/master/arch/x86/include/asm/paravirt.h:744
#5 __pv_wait_head (lock=0xffffffff81eb0e58) at
kernel/locking/qspinlock.c:352
Value of lock seem to be 524288 (means already unlocked?)
So apart from races Waiman mentioned, are we also in need of smp_mb()
here and/or native_queue_unlock()?.
Interestingly I see other cpu stuck at multi_cpu_stop().
(gdb) thr 1
[Switching to thread 1 (Thread 1)]#0 multi_cpu_stop
(data=0xffff8802140d1da0) at kernel/stop_machine.c:192
192 if (msdata->state != curstate) {
Or is it I am missing something.
please let me know if .config need to be shared.
+ local_irq_save(flags);
+
+ /*
+ * check again make sure it didn't become free while
+ * we weren't looking.
+ */
+ if (ACCESS_ONCE(*ptr) != val)
+ goto out;
+
+ /*
+ * halt until it's our turn and kicked. Note that we do safe halt
+ * for irq enabled case to avoid hang when lock info is overwritten
+ * in irq spinlock slowpath and no spurious interrupt occur to save us.
+ */
+ if (arch_irqs_disabled_flags(flags))
+ halt();
+ else
+ safe_halt();
+
+out:
+ local_irq_restore(flags);
+}
+#endif /* QUEUE_SPINLOCK */
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