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authorLinus Torvalds <torvalds@linux-foundation.org>2017-09-15 15:43:55 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-15 15:43:55 -0700
commit9db59599ae502b38b27cff6462273f84acd59927 (patch)
tree96d90a2f7bcddc837987579ad2d3e58b891db716 /include/linux
parentb38923a068c10fc36ca8f596d650d095ce390b85 (diff)
parent4f350c6dbcb9000e18907515ec8a7b205ac33c69 (diff)
Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm
Pull more KVM updates from Paolo Bonzini: - PPC bugfixes - RCU splat fix - swait races fix - pointless userspace-triggerable BUG() fix - misc fixes for KVM_RUN corner cases - nested virt correctness fixes + one host DoS - some cleanups - clang build fix - fix AMD AVIC with default QEMU command line options - x86 bugfixes * tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm: (28 commits) kvm: nVMX: Handle deferred early VMLAUNCH/VMRESUME failure properly kvm: vmx: Handle VMLAUNCH/VMRESUME failure properly kvm: nVMX: Remove nested_vmx_succeed after successful VM-entry kvm,mips: Fix potential swait_active() races kvm,powerpc: Serialize wq active checks in ops->vcpu_kick kvm: Serialize wq active checks in kvm_vcpu_wake_up() kvm,x86: Fix apf_task_wake_one() wq serialization kvm,lapic: Justify use of swait_active() kvm,async_pf: Use swq_has_sleeper() sched/wait: Add swq_has_sleeper() KVM: VMX: Do not BUG() on out-of-bounds guest IRQ KVM: Don't accept obviously wrong gsi values via KVM_IRQFD kvm: nVMX: Don't allow L2 to access the hardware CR8 KVM: trace events: update list of exit reasons KVM: async_pf: Fix #DF due to inject "Page not Present" and "Page Ready" exceptions simultaneously KVM: X86: Don't block vCPU if there is pending exception KVM: SVM: Add irqchip_split() checks before enabling AVIC KVM: Add struct kvm_vcpu pointer parameter to get_enable_apicv() KVM: SVM: Refactor AVIC vcpu initialization into avic_init_vcpu() KVM: x86: fix clang build ...
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/swait.h58
1 files changed, 56 insertions, 2 deletions
diff --git a/include/linux/swait.h b/include/linux/swait.h
index 4a4e180d0a35..73e97a08d3d0 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -79,9 +79,63 @@ extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name
DECLARE_SWAIT_QUEUE_HEAD(name)
#endif
-static inline int swait_active(struct swait_queue_head *q)
+/**
+ * swait_active -- locklessly test for waiters on the queue
+ * @wq: the waitqueue to test for waiters
+ *
+ * returns true if the wait list is not empty
+ *
+ * NOTE: this function is lockless and requires care, incorrect usage _will_
+ * lead to sporadic and non-obvious failure.
+ *
+ * NOTE2: this function has the same above implications as regular waitqueues.
+ *
+ * Use either while holding swait_queue_head::lock or when used for wakeups
+ * with an extra smp_mb() like:
+ *
+ * CPU0 - waker CPU1 - waiter
+ *
+ * for (;;) {
+ * @cond = true; prepare_to_swait(&wq_head, &wait, state);
+ * smp_mb(); // smp_mb() from set_current_state()
+ * if (swait_active(wq_head)) if (@cond)
+ * wake_up(wq_head); break;
+ * schedule();
+ * }
+ * finish_swait(&wq_head, &wait);
+ *
+ * Because without the explicit smp_mb() it's possible for the
+ * swait_active() load to get hoisted over the @cond store such that we'll
+ * observe an empty wait list while the waiter might not observe @cond.
+ * This, in turn, can trigger missing wakeups.
+ *
+ * Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
+ * which (when the lock is uncontended) are of roughly equal cost.
+ */
+static inline int swait_active(struct swait_queue_head *wq)
+{
+ return !list_empty(&wq->task_list);
+}
+
+/**
+ * swq_has_sleeper - check if there are any waiting processes
+ * @wq: the waitqueue to test for waiters
+ *
+ * Returns true if @wq has waiting processes
+ *
+ * Please refer to the comment for swait_active.
+ */
+static inline bool swq_has_sleeper(struct swait_queue_head *wq)
{
- return !list_empty(&q->task_list);
+ /*
+ * We need to be sure we are in sync with the list_add()
+ * modifications to the wait queue (task_list).
+ *
+ * This memory barrier should be paired with one on the
+ * waiting side.
+ */
+ smp_mb();
+ return swait_active(wq);
}
extern void swake_up(struct swait_queue_head *q);