[Xen-devel] [PATCH] credit: fix race condition in csched_vcpu flags

[Igor Pavlikevich <ipavlikevich@xxxxxxxxx>]

When vcpu stops yielding, it's possible to overwrite CSCHED_FLAG_VCPU_PARKED 
flag after clearing CSCHED_FLAG_VCPU_YIELD.

 332.970218393 -------x d1v1 runstate_continue d1v1 running->running
]332.970248371 -------x d1v1   28005(2:8:5) 2 [ 1 1 ]                  
yielding, flags=0x0002
]332.970251968 -------x d1v1                                           clearing 
0x0002, flags=0
 332.970252175 -------x d1v1 runstate_continue d1v1 running->running
]332.970282574 -------x d1v1   28005(2:8:5) 2 [ 1 1 ]                  
yielding, flags=0x0002
]332.970283068 x------| d32767v0                                       entering 
csched_acct(), d1v1 flags=0x0002
]332.970285763 x------| d32767v0   28003(2:8:3) 2 [ 1 1 ]              calling 
vcpu_pause_nosync() for d1v1
]332.970286647 -------x d1v1                                           clearing 
0x0002, flags=0
]332.970286771 x------| d32767v0                                       flags |= 
CSCHED_FLAG_VCPU_PARKED, flags=0x0003
]332.970286990 -------x d1v1   2800e(2:8:e) 2 [ 1 1cacfca ]
]332.970287122 -------x d1v1   2800f(2:8:f) 3 [ 7fff 1cacfca ffffffff ]
]332.970287242 -------x d1v1   2800a(2:8:a) 4 [ 1 1 7fff 7 ]           exiting 
schedule(), flags=0
 332.970287417 -------x d1v1 runstate_change d1v1 running->offline
 332.970287629 -------x d?v? runstate_change d32767v7 runnable->running
]332.970288397 x------- d32767v0                                       exiting 
csched_acct(), flags=0x0003
]332.995349690 x------- d32767v0                                       entering 
csched_acct(), d1v1 flags=0x0000, flag lost.
]332.995352170 x------- d32767v0   28003(2:8:3) 2 [ 1 1 ]              calling 
vcpu_pause_nosync() for d1v1 again

>From this moment d1v1 has extra +1 in pause_count, so it goes offline right 
>after credit>prv->credits_per_tslice forever.

Signed-off-by: Igor Pavlikevich <ipavlikevich@xxxxxxxxx>

diff -r 66f563be41d9 -r 4d2fb69d362b xen/common/sched_credit.c
--- a/xen/common/sched_credit.c Tue Feb 19 10:49:53 2013 +0100
+++ b/xen/common/sched_credit.c Mon Feb 25 07:54:02 2013 +0000
@@ -1155,7 +1155,9 @@ csched_acct(void* dummy)
                 {
                     SCHED_STAT_CRANK(vcpu_park);
                     vcpu_pause_nosync(svc->vcpu);
+                    vcpu_schedule_lock_irq(svc->vcpu);
                     svc->flags |= CSCHED_FLAG_VCPU_PARKED;
+                    vcpu_schedule_unlock_irq(svc->vcpu);
                 }
 
                 /* Lower bound on credits */

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