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Re: [Xen-devel] [PATCH v8 10/19] x86/VPMU: Interface for setting PMU mode and flags



On 07/28/2014 12:36 PM, Jan Beulich wrote:
On 28.07.14 at 18:29, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 07/28/2014 11:22 AM, Jan Beulich wrote:
On 01.07.14 at 16:37, <boris.ostrovsky@xxxxxxxxxx> wrote:
+    start = NOW();
+    /*
+     * Note that we may fail here if a CPU is hot-unplugged while we are
+     * waiting. We will then time out.
+     */
+    while ( atomic_read(&vpmu_sched_counter) != allbutself_num )
+    {
+        /* Give up after 5 seconds */
+        if ( NOW() > start + SECONDS(5) )
+        {
+            printk("vpmu_unload_all: failed to sync\n");
+            ret = -EBUSY;
+            break;
+        }
+        cpu_relax();
+        if ( hypercall_preempt_check() )
+            return hypercall_create_continuation(
+                __HYPERVISOR_xenpmu_op, "ih", XENPMU_mode_set, arg);
+    }
I wonder whether this is race free (wrt another CPU doing something
similar) and how you expect the 5s timeout above to ever be reached
(you're virtually guaranteed to get asked to preempt earlier).
Race-wise there is xenpmu_mode_lock in the caller (quoted below).
That wasn't my point: I said "something similar" - imagine another
hypercall behaving this same way, and both hypercalls getting
run concurrently.


Isn't it already possible to have two hypercalls doing continuations at the same time? (Assuming this was your concern)

-boris

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