1This document provides options for those wishing to keep their 2memory-ordering lives simple, as is necessary for those whose domain 3is complex. After all, there are bugs other than memory-ordering bugs, 4and the time spent gaining memory-ordering knowledge is not available 5for gaining domain knowledge. Furthermore Linux-kernel memory model 6(LKMM) is quite complex, with subtle differences in code often having 7dramatic effects on correctness. 8 9The options near the beginning of this list are quite simple. The idea 10is not that kernel hackers don't already know about them, but rather 11that they might need the occasional reminder. 12 13Please note that this is a generic guide, and that specific subsystems 14will often have special requirements or idioms. For example, developers 15of MMIO-based device drivers will often need to use mb(), rmb(), and 16wmb(), and therefore might find smp_mb(), smp_rmb(), and smp_wmb() 17to be more natural than smp_load_acquire() and smp_store_release(). 18On the other hand, those coming in from other environments will likely 19be more familiar with these last two. 20 21 22Single-threaded code 23==================== 24 25In single-threaded code, there is no reordering, at least assuming 26that your toolchain and hardware are working correctly. In addition, 27it is generally a mistake to assume your code will only run in a single 28threaded context as the kernel can enter the same code path on multiple 29CPUs at the same time. One important exception is a function that makes 30no external data references. 31 32In the general case, you will need to take explicit steps to ensure that 33your code really is executed within a single thread that does not access 34shared variables. A simple way to achieve this is to define a global lock 35that you acquire at the beginning of your code and release at the end, 36taking care to ensure that all references to your code's shared data are 37also carried out under that same lock. Because only one thread can hold 38this lock at a given time, your code will be executed single-threaded. 39This approach is called "code locking". 40 41Code locking can severely limit both performance and scalability, so it 42should be used with caution, and only on code paths that execute rarely. 43After all, a huge amount of effort was required to remove the Linux 44kernel's old "Big Kernel Lock", so let's please be very careful about 45adding new "little kernel locks". 46 47One of the advantages of locking is that, in happy contrast with the 48year 1981, almost all kernel developers are very familiar with locking. 49The Linux kernel's lockdep (CONFIG_PROVE_LOCKING=y) is very helpful with 50the formerly feared deadlock scenarios. 51 52Please use the standard locking primitives provided by the kernel rather 53than rolling your own. For one thing, the standard primitives interact 54properly with lockdep. For another thing, these primitives have been 55tuned to deal better with high contention. And for one final thing, it is 56surprisingly hard to correctly code production-quality lock acquisition 57and release functions. After all, even simple non-production-quality 58locking functions must carefully prevent both the CPU and the compiler 59from moving code in either direction across the locking function. 60 61Despite the scalability limitations of single-threaded code, RCU 62takes this approach for much of its grace-period processing and also 63for early-boot operation. The reason RCU is able to scale despite 64single-threaded grace-period processing is use of batching, where all 65updates that accumulated during one grace period are handled by the 66next one. In other words, slowing down grace-period processing makes 67it more efficient. Nor is RCU unique: Similar batching optimizations 68are used in many I/O operations. 69 70 71Packaged code 72============= 73 74Even if performance and scalability concerns prevent your code from 75being completely single-threaded, it is often possible to use library 76functions that handle the concurrency nearly or entirely on their own. 77This approach delegates any LKMM worries to the library maintainer. 78 79In the kernel, what is the "library"? Quite a bit. It includes the 80contents of the lib/ directory, much of the include/linux/ directory along 81with a lot of other heavily used APIs. But heavily used examples include 82the list macros (for example, include/linux/{,rcu}list.h), workqueues, 83smp_call_function(), and the various hash tables and search trees. 84 85 86Data locking 87============ 88 89With code locking, we use single-threaded code execution to guarantee 90serialized access to the data that the code is accessing. However, 91we can also achieve this by instead associating the lock with specific 92instances of the data structures. This creates a "critical section" 93in the code execution that will execute as though it is single threaded. 94By placing all the accesses and modifications to a shared data structure 95inside a critical section, we ensure that the execution context that 96holds the lock has exclusive access to the shared data. 97 98The poster boy for this approach is the hash table, where placing a lock 99in each hash bucket allows operations on different buckets to proceed 100concurrently. This works because the buckets do not overlap with each 101other, so that an operation on one bucket does not interfere with any 102other bucket. 103 104As the number of buckets increases, data locking scales naturally. 105In particular, if the amount of data increases with the number of CPUs, 106increasing the number of buckets as the number of CPUs increase results 107in a naturally scalable data structure. 108 109 110Per-CPU processing 111================== 112 113Partitioning processing and data over CPUs allows each CPU to take 114a single-threaded approach while providing excellent performance and 115scalability. Of course, there is no free lunch: The dark side of this 116excellence is substantially increased memory footprint. 117 118In addition, it is sometimes necessary to occasionally update some global 119view of this processing and data, in which case something like locking 120must be used to protect this global view. This is the approach taken 121by the percpu_counter infrastructure. In many cases, there are already 122generic/library variants of commonly used per-cpu constructs available. 123Please use them rather than rolling your own. 124 125RCU uses DEFINE_PER_CPU*() declaration to create a number of per-CPU 126data sets. For example, each CPU does private quiescent-state processing 127within its instance of the per-CPU rcu_data structure, and then uses data 128locking to report quiescent states up the grace-period combining tree. 129 130 131Packaged primitives: Sequence locking 132===================================== 133 134Lockless programming is considered by many to be more difficult than 135lock-based programming, but there are a few lockless design patterns that 136have been built out into an API. One of these APIs is sequence locking. 137Although this APIs can be used in extremely complex ways, there are simple 138and effective ways of using it that avoid the need to pay attention to 139memory ordering. 140 141The basic keep-things-simple rule for sequence locking is "do not write 142in read-side code". Yes, you can do writes from within sequence-locking 143readers, but it won't be so simple. For example, such writes will be 144lockless and should be idempotent. 145 146For more sophisticated use cases, LKMM can guide you, including use 147cases involving combining sequence locking with other synchronization 148primitives. (LKMM does not yet know about sequence locking, so it is 149currently necessary to open-code it in your litmus tests.) 150 151Additional information may be found in include/linux/seqlock.h. 152 153Packaged primitives: RCU 154======================== 155 156Another lockless design pattern that has been baked into an API 157is RCU. The Linux kernel makes sophisticated use of RCU, but the 158keep-things-simple rules for RCU are "do not write in read-side code" 159and "do not update anything that is visible to and accessed by readers", 160and "protect updates with locking". 161 162These rules are illustrated by the functions foo_update_a() and 163foo_get_a() shown in Documentation/RCU/whatisRCU.rst. Additional 164RCU usage patterns maybe found in Documentation/RCU and in the 165source code. 166 167 168Packaged primitives: Atomic operations 169====================================== 170 171Back in the day, the Linux kernel had three types of atomic operations: 172 1731. Initialization and read-out, such as atomic_set() and atomic_read(). 174 1752. Operations that did not return a value and provided no ordering, 176 such as atomic_inc() and atomic_dec(). 177 1783. Operations that returned a value and provided full ordering, such as 179 atomic_add_return() and atomic_dec_and_test(). Note that some 180 value-returning operations provide full ordering only conditionally. 181 For example, cmpxchg() provides ordering only upon success. 182 183More recent kernels have operations that return a value but do not 184provide full ordering. These are flagged with either a _relaxed() 185suffix (providing no ordering), or an _acquire() or _release() suffix 186(providing limited ordering). 187 188Additional information may be found in these files: 189 190Documentation/atomic_t.txt 191Documentation/atomic_bitops.txt 192Documentation/core-api/refcount-vs-atomic.rst 193 194Reading code using these primitives is often also quite helpful. 195 196 197Lockless, fully ordered 198======================= 199 200When using locking, there often comes a time when it is necessary 201to access some variable or another without holding the data lock 202that serializes access to that variable. 203 204If you want to keep things simple, use the initialization and read-out 205operations from the previous section only when there are no racing 206accesses. Otherwise, use only fully ordered operations when accessing 207or modifying the variable. This approach guarantees that code prior 208to a given access to that variable will be seen by all CPUs has having 209happened before any code following any later access to that same variable. 210 211Please note that per-CPU functions are not atomic operations and 212hence they do not provide any ordering guarantees at all. 213 214If the lockless accesses are frequently executed reads that are used 215only for heuristics, or if they are frequently executed writes that 216are used only for statistics, please see the next section. 217 218 219Lockless statistics and heuristics 220================================== 221 222Unordered primitives such as atomic_read(), atomic_set(), READ_ONCE(), and 223WRITE_ONCE() can safely be used in some cases. These primitives provide 224no ordering, but they do prevent the compiler from carrying out a number 225of destructive optimizations (for which please see the next section). 226One example use for these primitives is statistics, such as per-CPU 227counters exemplified by the rt_cache_stat structure's routing-cache 228statistics counters. Another example use case is heuristics, such as 229the jiffies_till_first_fqs and jiffies_till_next_fqs kernel parameters 230controlling how often RCU scans for idle CPUs. 231 232But be careful. "Unordered" really does mean "unordered". It is all 233too easy to assume ordering, and this assumption must be avoided when 234using these primitives. 235 236 237Don't let the compiler trip you up 238================================== 239 240It can be quite tempting to use plain C-language accesses for lockless 241loads from and stores to shared variables. Although this is both 242possible and quite common in the Linux kernel, it does require a 243surprising amount of analysis, care, and knowledge about the compiler. 244Yes, some decades ago it was not unfair to consider a C compiler to be 245an assembler with added syntax and better portability, but the advent of 246sophisticated optimizing compilers mean that those days are long gone. 247Today's optimizing compilers can profoundly rewrite your code during the 248translation process, and have long been ready, willing, and able to do so. 249 250Therefore, if you really need to use C-language assignments instead of 251READ_ONCE(), WRITE_ONCE(), and so on, you will need to have a very good 252understanding of both the C standard and your compiler. Here are some 253introductory references and some tooling to start you on this noble quest: 254 255Who's afraid of a big bad optimizing compiler? 256 https://lwn.net/Articles/793253/ 257Calibrating your fear of big bad optimizing compilers 258 https://lwn.net/Articles/799218/ 259Concurrency bugs should fear the big bad data-race detector (part 1) 260 https://lwn.net/Articles/816850/ 261Concurrency bugs should fear the big bad data-race detector (part 2) 262 https://lwn.net/Articles/816854/ 263 264 265More complex use cases 266====================== 267 268If the alternatives above do not do what you need, please look at the 269recipes.txt file to peel off the next layer of the memory-ordering 270onion. 271