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diff --git a/pkg/refsvfs2/README.md b/pkg/refsvfs2/README.md new file mode 100644 index 000000000..eca53c282 --- /dev/null +++ b/pkg/refsvfs2/README.md @@ -0,0 +1,66 @@ +# Reference Counting + +Go does not offer a reliable way to couple custom resource management with +object lifetime. As a result, we need to manually implement reference counting +for many objects in gVisor to make sure that resources are acquired and released +appropriately. For example, the filesystem has many reference-counted objects +(file descriptions, dentries, inodes, etc.), and it is important that each +object persists while anything holds a reference on it and is destroyed once all +references are dropped. + +We provide a template in `refs_template.go` that can be applied to most objects +in need of reference counting. It contains a simple `Refs` struct that can be +incremented and decremented, and once the reference count reaches zero, a +destructor can be called. Note that there are some objects (e.g. `gofer.dentry`, +`overlay.dentry`) that should not immediately be destroyed upon reaching zero +references; in these cases, this template cannot be applied. + +# Reference Checking + +Unfortunately, manually keeping track of reference counts is extremely error +prone, and improper accounting can lead to production bugs that are very +difficult to root cause. + +We have several ways of discovering reference count errors in gVisor. Any +attempt to increment/decrement a `Refs` struct with a count of zero will trigger +a sentry panic, since the object should have been destroyed and become +unreachable. This allows us to identify missing increments or extra decrements, +which cause the reference count to be lower than it should be: the count will +reach zero earlier than expected, and the next increment/decrement--which should +be valid--will result in a panic. + +It is trickier to identify extra increments and missing decrements, which cause +the reference count to be higher than expected (i.e. a “reference leak”). +Reference leaks prevent resources from being released properly and can translate +to various issues that are tricky to diagnose, such as memory leaks. The +following section discusses how we implement leak checking. + +## Leak Checking + +When leak checking is enabled, reference-counted objects are added to a global +map when constructed and removed when destroyed. Near the very end of sandbox +execution, once no reference-counted objects should still be reachable, we +report everything left in the map as having leaked. Leak-checking objects +implement the `CheckedObject` interface, which allows us to print informative +warnings for each of the leaked objects. + +Leak checking is provided by `refs_template`, but objects that do not use the +template will also need to implement `CheckedObject` and be manually +registered/unregistered from the map in order to be checked. + +Note that leak checking affects performance and memory usage, so it should only +be enabled in testing environments. + +## Debugging + +Even with the checks described above, it can be difficult to track down the +exact source of a reference counting error. The error may occur far before it is +discovered (for instance, a missing `IncRef` may not be discovered until a +future `DecRef` makes the count negative). To aid in debugging, `refs_template` +provides the `enableLogging` option to log every `IncRef`, `DecRef`, and leak +check registration/unregistration, along with the object address and a call +stack. This allows us to search a log for all of the changes to a particular +object's reference count, which makes it much easier to identify the absent or +extraneous operation(s). The reference-counted objects that do not use +`refs_template` also provide logging, and others defined in the future should do +so as well. |