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a5cc43414b
1. cgrp_cap_budget() used __sync_fetch_and_sub(&cgc->cvtime_delta,
cgc->cvtime_delta) to atomically read and clear cvtime_delta. However,
this is not a true atomic read-clear operation: the second argument
(cgc->cvtime_delta) is evaluated as a normal read before the atomic
fetch_and_sub executes. If a concurrent __sync_fetch_and_add() happens
between the read and the sub, the added value gets included in the
returned delta AND remains in cvtime_delta, causing double charging.
Example:
CPU 0 runs cgrp_cap_budget(), CPU 1 runs fcg_stopping().
Assume cvtime_delta = 100 initially.
T1 CPU 0: sub_val = cvtime_delta = 100 cvtime_delta = 100
T2 CPU 1: __sync_fetch_and_add(&cvtime_delta, 10) cvtime_delta = 110
T3 CPU 0: __sync_fetch_and_sub(&cvtime_delta, sub_val) cvtime_delta = 10
returns old=110
delta = 110 (includes the 10 from CPU 1), but cvtime_delta = 10
(the 10 also remains). The 10 is charged twice: once in delta
(applied to cgv_node->cvtime) and once in the residual cvtime_delta
(fetched again next time).
Fix by using __sync_fetch_and_and(&cgc->cvtime_delta, 0).
Disassembly comparison:
(1) delta = __sync_fetch_and_sub(&cgc->cvtime_delta, cgc->cvtime_delta);
228: (79) r7 = *(u64 *)(r9 +40)
229: (87) r7 = -r7
230: (db) r7 = atomic64_fetch_add((u64 *)(r9 +40), r7) //r9 may be changed
(2) delta = __sync_fetch_and_and(&cgc->cvtime_delta, 0);
228: (b7) r8 = 0
229: (db) r8 = atomic64_xchg((u64 *)(r9 +40), r8)
2. The bypass charging path in fcg_stopping() charges raw execution time
to cvtime_delta without scaling by the inverse of the cgroup hweight.
Since cvtime_delta is eventually applied to cgv_node->cvtime which is
in vtime space (weight-scaled), the bypass path should also scale by
FCG_HWEIGHT_ONE / hweight to match the units used by the dispatch path.
Fixes:
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