1=============== 2Locking lessons 3=============== 4 5Lesson 1: Spin locks 6==================== 7 8The most basic primitive for locking is spinlock:: 9 10 static DEFINE_SPINLOCK(xxx_lock); 11 12 unsigned long flags; 13 14 spin_lock_irqsave(&xxx_lock, flags); 15 ... critical section here .. 16 spin_unlock_irqrestore(&xxx_lock, flags); 17 18The above is always safe. It will disable interrupts _locally_, but the 19spinlock itself will guarantee the global lock, so it will guarantee that 20there is only one thread-of-control within the region(s) protected by that 21lock. This works well even under UP also, so the code does _not_ need to 22worry about UP vs SMP issues: the spinlocks work correctly under both. 23 24 NOTE! Implications of spin_locks for memory are further described in: 25 26 Documentation/memory-barriers.txt 27 28 (5) ACQUIRE operations. 29 30 (6) RELEASE operations. 31 32The above is usually pretty simple (you usually need and want only one 33spinlock for most things - using more than one spinlock can make things a 34lot more complex and even slower and is usually worth it only for 35sequences that you **know** need to be split up: avoid it at all cost if you 36aren't sure). 37 38This is really the only really hard part about spinlocks: once you start 39using spinlocks they tend to expand to areas you might not have noticed 40before, because you have to make sure the spinlocks correctly protect the 41shared data structures **everywhere** they are used. The spinlocks are most 42easily added to places that are completely independent of other code (for 43example, internal driver data structures that nobody else ever touches). 44 45 NOTE! The spin-lock is safe only when you **also** use the lock itself 46 to do locking across CPU's, which implies that EVERYTHING that 47 touches a shared variable has to agree about the spinlock they want 48 to use. 49 50---- 51 52Lesson 2: reader-writer spinlocks. 53================================== 54 55If your data accesses have a very natural pattern where you usually tend 56to mostly read from the shared variables, the reader-writer locks 57(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple 58readers to be in the same critical region at once, but if somebody wants 59to change the variables it has to get an exclusive write lock. 60 61 NOTE! reader-writer locks require more atomic memory operations than 62 simple spinlocks. Unless the reader critical section is long, you 63 are better off just using spinlocks. 64 65The routines look the same as above:: 66 67 rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock); 68 69 unsigned long flags; 70 71 read_lock_irqsave(&xxx_lock, flags); 72 .. critical section that only reads the info ... 73 read_unlock_irqrestore(&xxx_lock, flags); 74 75 write_lock_irqsave(&xxx_lock, flags); 76 .. read and write exclusive access to the info ... 77 write_unlock_irqrestore(&xxx_lock, flags); 78 79The above kind of lock may be useful for complex data structures like 80linked lists, especially searching for entries without changing the list 81itself. The read lock allows many concurrent readers. Anything that 82**changes** the list will have to get the write lock. 83 84 NOTE! RCU is better for list traversal, but requires careful 85 attention to design detail (see Documentation/RCU/listRCU.rst). 86 87Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_ 88time need to do any changes (even if you don't do it every time), you have 89to get the write-lock at the very beginning. 90 91 NOTE! We are working hard to remove reader-writer spinlocks in most 92 cases, so please don't add a new one without consensus. (Instead, see 93 Documentation/RCU/rcu.rst for complete information.) 94 95---- 96 97Lesson 3: spinlocks revisited. 98============================== 99 100The single spin-lock primitives above are by no means the only ones. They 101are the most safe ones, and the ones that work under all circumstances, 102but partly **because** they are safe they are also fairly slow. They are slower 103than they'd need to be, because they do have to disable interrupts 104(which is just a single instruction on a x86, but it's an expensive one - 105and on other architectures it can be worse). 106 107If you have a case where you have to protect a data structure across 108several CPU's and you want to use spinlocks you can potentially use 109cheaper versions of the spinlocks. IFF you know that the spinlocks are 110never used in interrupt handlers, you can use the non-irq versions:: 111 112 spin_lock(&lock); 113 ... 114 spin_unlock(&lock); 115 116(and the equivalent read-write versions too, of course). The spinlock will 117guarantee the same kind of exclusive access, and it will be much faster. 118This is useful if you know that the data in question is only ever 119manipulated from a "process context", ie no interrupts involved. 120 121The reasons you mustn't use these versions if you have interrupts that 122play with the spinlock is that you can get deadlocks:: 123 124 spin_lock(&lock); 125 ... 126 <- interrupt comes in: 127 spin_lock(&lock); 128 129where an interrupt tries to lock an already locked variable. This is ok if 130the other interrupt happens on another CPU, but it is _not_ ok if the 131interrupt happens on the same CPU that already holds the lock, because the 132lock will obviously never be released (because the interrupt is waiting 133for the lock, and the lock-holder is interrupted by the interrupt and will 134not continue until the interrupt has been processed). 135 136(This is also the reason why the irq-versions of the spinlocks only need 137to disable the _local_ interrupts - it's ok to use spinlocks in interrupts 138on other CPU's, because an interrupt on another CPU doesn't interrupt the 139CPU that holds the lock, so the lock-holder can continue and eventually 140releases the lock). 141 142 Linus 143 144---- 145 146Reference information: 147====================== 148 149For dynamic initialization, use spin_lock_init() or rwlock_init() as 150appropriate:: 151 152 spinlock_t xxx_lock; 153 rwlock_t xxx_rw_lock; 154 155 static int __init xxx_init(void) 156 { 157 spin_lock_init(&xxx_lock); 158 rwlock_init(&xxx_rw_lock); 159 ... 160 } 161 162 module_init(xxx_init); 163 164For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or 165__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate. 166