[xen master] xen/sched: Add missing memory barrier in vcpu_block()

[patchbot@xxxxxxx]

commit 109e8177fd4a225e7025c4c17d2c9537b550b4ed
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Sat Feb 20 19:22:34 2021 +0000
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Fri Feb 26 09:47:23 2021 +0000

    xen/sched: Add missing memory barrier in vcpu_block()
    
    The comment in vcpu_block() states that the events should be checked
    /after/ blocking to avoids wakeup waiting race. However, from a generic
    perspective, set_bit() doesn't prevent re-ordering. So the following
    could happen:
    
    CPU0  (blocking vCPU A)         |Â   CPU1 ( unblock vCPU A)
                                    |
    A <- read local events          |
                                    |   set local events
                                    |   test_and_clear_bit(_VPF_blocked)
                                    |       -> Bail out as the bit if not set
                                    |
    set_bit(_VFP_blocked)           |
                                    |
    check A                         |
    
    The variable A will be 0 and therefore the vCPU will be blocked when it
    should continue running.
    
    vcpu_block() is now gaining an smp_mb__after_atomic() to prevent the CPU
    to read any information about local events before the flag _VPF_blocked
    is set.
    
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Ash Wilding <ash.j.wilding@xxxxxxxxx>
    Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
    Acked-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    Release-Acked-by: Ian Jackson <iwj@xxxxxxxxxxxxxx>
---
 xen/common/sched/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 9745a77eee..2b974fd6f8 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -1418,6 +1418,8 @@ void vcpu_block(void)
 
     set_bit(_VPF_blocked, &v->pause_flags);
 
+    smp_mb__after_atomic();
+
     arch_vcpu_block(v);
 
     /* Check for events /after/ blocking: avoids wakeup waiting race. */
--
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