Re: [Qemu-devel] [PATCH v2 08/12] cpus: always call seqlock_write in cpu_update_icount

[Paolo Bonzini]

On 24/09/2018 20:46, Emilio G. Cota wrote:
> Applying this on my local tree is deadlocking icount, since
> cpu_update_icount is called from cpu_get_icount_raw_locked:
> 
> #6  cpu_update_icount (cpu=<optimized out>) at /data/src/qemu/cpus.c:257
> #7  0x000055a6fbc7ae5c in cpu_get_icount_raw_locked () at 
> /data/src/qemu/cpus.c:271
> #8  0x000055a6fbc7ae99 in cpu_get_icount_locked () at 
> /data/src/qemu/cpus.c:279
> #9  0x000055a6fbc7b3ac in cpu_get_icount () at /data/src/qemu/cpus.c:302
> #10 0x000055a6fc0f3a05 in qemu_clock_get_ns (address@hidden) at 
> /data/src/qemu/util/qemu-timer.c:601
> 
> I am however not sure what Paolo's queued tree looks like, so I
> might be missing something.

No, you're not missing anything.

Looking at other callers of cpu_update_icount, this should be the fix:

diff --git a/cpus.c b/cpus.c
index b429b5a9e0..551076fb98 100644
--- a/cpus.c
+++ b/cpus.c
@@ -245,15 +245,25 @@ static int64_t cpu_get_icount_executed(CPUState *cpu)
  * account executed instructions. This is done by the TCG vCPU
  * thread so the main-loop can see time has moved forward.
  */
-void cpu_update_icount(CPUState *cpu)
+static void cpu_update_icount_locked(CPUState *cpu)
 {
     int64_t executed = cpu_get_icount_executed(cpu);
     cpu->icount_budget -= executed;
 
-    seqlock_write_lock(&timers_state.vm_clock_seqlock,
-                       &timers_state.vm_clock_lock);
     atomic_set_i64(&timers_state.qemu_icount,
                    timers_state.qemu_icount + executed);
+}
+
+/*
+ * Update the global shared timer_state.qemu_icount to take into
+ * account executed instructions. This is done by the TCG vCPU
+ * thread so the main-loop can see time has moved forward.
+ */
+void cpu_update_icount(CPUState *cpu)
+{
+    seqlock_write_lock(&timers_state.vm_clock_seqlock,
+                       &timers_state.vm_clock_lock);
+    cpu_update_icount_locked(cpu);
     seqlock_write_unlock(&timers_state.vm_clock_seqlock,
                          &timers_state.vm_clock_lock);
 }
@@ -268,7 +278,7 @@ static int64_t cpu_get_icount_raw_locked(void)
             exit(1);
         }
         /* Take into account what has run */
-        cpu_update_icount(cpu);
+        cpu_update_icount_locked(cpu);
     }
     /* The read is protected by the seqlock, but needs atomic64 to avoid UB */
     return atomic_read_i64(&timers_state.qemu_icount);

> Paolo: is that tree available anywhere?

Nope, unfortunately some patches I've queued broke on 32-bit and stuff so I
couldn't send out the pull request before leaving for Kernel Recipes...

Paolo