8.4. Specific Locking Strategies
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## 8.4. Specific Locking Strategies

### 8.4.1. Credentials

struct ucred is the kernel's internal credential structure, and is generally used as the basis for process-driven access control within the kernel. BSD-derived systems use a “copy-on-write” model for credential data: multiple references may exist for a credential structure, and when a change needs to be made, the structure is duplicated, modified, and then the reference replaced. Due to wide-spread caching of the credential to implement access control on open, this results in substantial memory savings. With a move to fine-grained SMP, this model also saves substantially on locking operations by requiring that modification only occur on an unshared credential, avoiding the need for explicit synchronization when consuming a known-shared credential.

Credential structures with a single reference are considered mutable; shared credential structures must not be modified or a race condition is risked. A mutex, `cr_mtxp` protects the reference count of struct ucred so as to maintain consistency. Any use of the structure requires a valid reference for the duration of the use, or the structure may be released out from under the illegitimate consumer.

The struct ucred mutex is a leaf mutex and is implemented via a mutex pool for performance reasons.

Usually, credentials are used in a read-only manner for access control decisions, and in this case `td_ucred` is generally preferred because it requires no locking. When a process' credential is updated the `proc` lock must be held across the check and update operations thus avoid races. The process credential `p_ucred` must be used for check and update operations to prevent time-of-check, time-of-use races.

If system call invocations will perform access control after an update to the process credential, the value of `td_ucred` must also be refreshed to the current process value. This will prevent use of a stale credential following a change. The kernel automatically refreshes the `td_ucred` pointer in the thread structure from the process `p_ucred` whenever a process enters the kernel, permitting use of a fresh credential for kernel access control.

### 8.4.2. File Descriptors and File Descriptor Tables

Details to follow.

### 8.4.3. Jail Structures

struct prison stores administrative details pertinent to the maintenance of jails created using the jail(2) API. This includes the per-jail hostname, IP address, and related settings. This structure is reference-counted since pointers to instances of the structure are shared by many credential structures. A single mutex, `pr_mtx` protects read and write access to the reference count and all mutable variables inside the struct jail. Some variables are set only when the jail is created, and a valid reference to the struct prison is sufficient to read these values. The precise locking of each entry is documented via comments in `sys/jail.h`.

### 8.4.4. MAC Framework

The TrustedBSD MAC Framework maintains data in a variety of kernel objects, in the form of struct label. In general, labels in kernel objects are protected by the same lock as the remainder of the kernel object. For example, the `v_label` label in struct vnode is protected by the vnode lock on the vnode.

In addition to labels maintained in standard kernel objects, the MAC Framework also maintains a list of registered and active policies. The policy list is protected by a global mutex (`mac_policy_list_lock`) and a busy count (also protected by the mutex). Since many access control checks may occur in parallel, entry to the framework for a read-only access to the policy list requires holding the mutex while incrementing (and later decrementing) the busy count. The mutex need not be held for the duration of the MAC entry operation--some operations, such as label operations on file system objects--are long-lived. To modify the policy list, such as during policy registration and de-registration, the mutex must be held and the reference count must be zero, to prevent modification of the list while it is in use.

A condition variable, `mac_policy_list_not_busy`, is available to threads that need to wait for the list to become unbusy, but this condition variable must only be waited on if the caller is holding no other locks, or a lock order violation may be possible. The busy count, in effect, acts as a form of shared/exclusive lock over access to the framework: the difference is that, unlike with an sx lock, consumers waiting for the list to become unbusy may be starved, rather than permitting lock order problems with regards to the busy count and other locks that may be held on entry to (or inside) the MAC Framework.

### 8.4.5. Modules

For the module subsystem there exists a single lock that is used to protect the shared data. This lock is a shared/exclusive (SX) lock and has a good chance of needing to be acquired (shared or exclusively), therefore there are a few macros that have been added to make access to the lock more easy. These macros can be located in `sys/module.h` and are quite basic in terms of usage. The main structures protected under this lock are the module_t structures (when shared) and the global modulelist_t structure, modules. One should review the related source code in `kern/kern_module.c` to further understand the locking strategy.

### 8.4.6. Newbus Device Tree

The newbus system will have one sx lock. Readers will hold a shared (read) lock (sx_slock(9)) and writers will hold an exclusive (write) lock (sx_xlock(9)). Internal functions will not do locking at all. Externally visible ones will lock as needed. Those items that do not matter if the race is won or lost will not be locked, since they tend to be read all over the place (e.g., device_get_softc(9)). There will be relatively few changes to the newbus data structures, so a single lock should be sufficient and not impose a performance penalty.

### 8.4.7. Pipes

...

### 8.4.8. Processes and Threads

- process hierarchy

- proc locks, references

- thread-specific copies of proc entries to freeze during system calls, including td_ucred

- inter-process operations

- process groups and sessions

### 8.4.9. Scheduler

Lots of references to `sched_lock` and notes pointing at specific primitives and related magic elsewhere in the document.

### 8.4.10. Select and Poll

The `select` and `poll` functions permit threads to block waiting on events on file descriptors--most frequently, whether or not the file descriptors are readable or writable.

...

### 8.4.11. SIGIO

The SIGIO service permits processes to request the delivery of a SIGIO signal to its process group when the read/write status of specified file descriptors changes. At most one process or process group is permitted to register for SIGIO from any given kernel object, and that process or group is referred to as the owner. Each object supporting SIGIO registration contains pointer field that is `NULL` if the object is not registered, or points to a struct sigio describing the registration. This field is protected by a global mutex, `sigio_lock`. Callers to SIGIO maintenance functions must pass in this field “by reference” so that local register copies of the field are not made when unprotected by the lock.

One struct sigio is allocated for each registered object associated with any process or process group, and contains back-pointers to the object, owner, signal information, a credential, and the general disposition of the registration. Each process or progress group contains a list of registered struct sigio structures, `p_sigiolst` for processes, and `pg_sigiolst` for process groups. These lists are protected by the process or process group locks respectively. Most fields in each struct sigio are constant for the duration of the registration, with the exception of the `sio_pgsigio` field which links the struct sigio into the process or process group list. Developers implementing new kernel objects supporting SIGIO will, in general, want to avoid holding structure locks while invoking SIGIO supporting functions, such as `fsetown` or `funsetown` to avoid defining a lock order between structure locks and the global SIGIO lock. This is generally possible through use of an elevated reference count on the structure, such as reliance on a file descriptor reference to a pipe during a pipe operation.

### 8.4.12. Sysctl

The `sysctl` MIB service is invoked from both within the kernel and from userland applications using a system call. At least two issues are raised in locking: first, the protection of the structures maintaining the namespace, and second, interactions with kernel variables and functions that are accessed by the sysctl interface. Since sysctl permits the direct export (and modification) of kernel statistics and configuration parameters, the sysctl mechanism must become aware of appropriate locking semantics for those variables. Currently, sysctl makes use of a single global sx lock to serialize use of `sysctl`; however, it is assumed to operate under Giant and other protections are not provided. The remainder of this section speculates on locking and semantic changes to sysctl.

- Need to change the order of operations for sysctl's that update values from read old, copyin and copyout, write new to copyin, lock, read old and write new, unlock, copyout. Normal sysctl's that just copyout the old value and set a new value that they copyin may still be able to follow the old model. However, it may be cleaner to use the second model for all of the sysctl handlers to avoid lock operations.

- To allow for the common case, a sysctl could embed a pointer to a mutex in the SYSCTL_FOO macros and in the struct. This would work for most sysctl's. For values protected by sx locks, spin mutexes, or other locking strategies besides a single sleep mutex, SYSCTL_PROC nodes could be used to get the locking right.

### 8.4.13. Taskqueue

The taskqueue's interface has two basic locks associated with it in order to protect the related shared data. The `taskqueue_queues_mutex` is meant to serve as a lock to protect the `taskqueue_queues` TAILQ. The other mutex lock associated with this system is the one in the struct taskqueue data structure. The use of the synchronization primitive here is to protect the integrity of the data in the struct taskqueue. It should be noted that there are no separate macros to assist the user in locking down his/her own work since these locks are most likely not going to be used outside of `kern/subr_taskqueue.c`.

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