path: root/Documentation/locking/spinlocks.txt
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authorDavidlohr Bueso <davidlohr@hp.com>2014-07-30 13:41:55 -0700
committerIngo Molnar <mingo@kernel.org>2014-08-13 10:32:03 +0200
commit214e0aed639ef40987bf6159fad303171a6de31e (patch)
tree9f4c2eb1497a7377de93d619c05cf6c82fcfa0cb /Documentation/locking/spinlocks.txt
parent7608a43d8f2e02f8b532f8e11481d7ecf8b5d3f9 (diff)
locking/Documentation: Move locking related docs into Documentation/locking/
Specifically: Documentation/locking/lockdep-design.txt Documentation/locking/lockstat.txt Documentation/locking/mutex-design.txt Documentation/locking/rt-mutex-design.txt Documentation/locking/rt-mutex.txt Documentation/locking/spinlocks.txt Documentation/locking/ww-mutex-design.txt Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> Acked-by: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: jason.low2@hp.com Cc: aswin@hp.com Cc: Alexei Starovoitov <ast@plumgrid.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Chris Mason <clm@fb.com> Cc: Dan Streetman <ddstreet@ieee.org> Cc: David Airlie <airlied@linux.ie> Cc: Davidlohr Bueso <davidlohr@hp.com> Cc: David S. Miller <davem@davemloft.net> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Josef Bacik <jbacik@fusionio.com> Cc: Kees Cook <keescook@chromium.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Lubomir Rintel <lkundrak@v3.sk> Cc: Masanari Iida <standby24x7@gmail.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Randy Dunlap <rdunlap@infradead.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Vineet Gupta <vgupta@synopsys.com> Cc: fengguang.wu@intel.com Link: http://lkml.kernel.org/r/1406752916-3341-6-git-send-email-davidlohr@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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+Lesson 1: Spin locks
+The most basic primitive for locking is spinlock.
+static DEFINE_SPINLOCK(xxx_lock);
+ unsigned long flags;
+ spin_lock_irqsave(&xxx_lock, flags);
+ ... critical section here ..
+ spin_unlock_irqrestore(&xxx_lock, flags);
+The above is always safe. It will disable interrupts _locally_, but the
+spinlock itself will guarantee the global lock, so it will guarantee that
+there is only one thread-of-control within the region(s) protected by that
+lock. This works well even under UP also, so the code does _not_ need to
+worry about UP vs SMP issues: the spinlocks work correctly under both.
+ NOTE! Implications of spin_locks for memory are further described in:
+ Documentation/memory-barriers.txt
+ (5) LOCK operations.
+ (6) UNLOCK operations.
+The above is usually pretty simple (you usually need and want only one
+spinlock for most things - using more than one spinlock can make things a
+lot more complex and even slower and is usually worth it only for
+sequences that you _know_ need to be split up: avoid it at all cost if you
+aren't sure).
+This is really the only really hard part about spinlocks: once you start
+using spinlocks they tend to expand to areas you might not have noticed
+before, because you have to make sure the spinlocks correctly protect the
+shared data structures _everywhere_ they are used. The spinlocks are most
+easily added to places that are completely independent of other code (for
+example, internal driver data structures that nobody else ever touches).
+ NOTE! The spin-lock is safe only when you _also_ use the lock itself
+ to do locking across CPU's, which implies that EVERYTHING that
+ touches a shared variable has to agree about the spinlock they want
+ to use.
+Lesson 2: reader-writer spinlocks.
+If your data accesses have a very natural pattern where you usually tend
+to mostly read from the shared variables, the reader-writer locks
+(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
+readers to be in the same critical region at once, but if somebody wants
+to change the variables it has to get an exclusive write lock.
+ NOTE! reader-writer locks require more atomic memory operations than
+ simple spinlocks. Unless the reader critical section is long, you
+ are better off just using spinlocks.
+The routines look the same as above:
+ rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
+ unsigned long flags;
+ read_lock_irqsave(&xxx_lock, flags);
+ .. critical section that only reads the info ...
+ read_unlock_irqrestore(&xxx_lock, flags);
+ write_lock_irqsave(&xxx_lock, flags);
+ .. read and write exclusive access to the info ...
+ write_unlock_irqrestore(&xxx_lock, flags);
+The above kind of lock may be useful for complex data structures like
+linked lists, especially searching for entries without changing the list
+itself. The read lock allows many concurrent readers. Anything that
+_changes_ the list will have to get the write lock.
+ NOTE! RCU is better for list traversal, but requires careful
+ attention to design detail (see Documentation/RCU/listRCU.txt).
+Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
+time need to do any changes (even if you don't do it every time), you have
+to get the write-lock at the very beginning.
+ NOTE! We are working hard to remove reader-writer spinlocks in most
+ cases, so please don't add a new one without consensus. (Instead, see
+ Documentation/RCU/rcu.txt for complete information.)
+Lesson 3: spinlocks revisited.
+The single spin-lock primitives above are by no means the only ones. They
+are the most safe ones, and the ones that work under all circumstances,
+but partly _because_ they are safe they are also fairly slow. They are slower
+than they'd need to be, because they do have to disable interrupts
+(which is just a single instruction on a x86, but it's an expensive one -
+and on other architectures it can be worse).
+If you have a case where you have to protect a data structure across
+several CPU's and you want to use spinlocks you can potentially use
+cheaper versions of the spinlocks. IFF you know that the spinlocks are
+never used in interrupt handlers, you can use the non-irq versions:
+ spin_lock(&lock);
+ ...
+ spin_unlock(&lock);
+(and the equivalent read-write versions too, of course). The spinlock will
+guarantee the same kind of exclusive access, and it will be much faster.
+This is useful if you know that the data in question is only ever
+manipulated from a "process context", ie no interrupts involved.
+The reasons you mustn't use these versions if you have interrupts that
+play with the spinlock is that you can get deadlocks:
+ spin_lock(&lock);
+ ...
+ <- interrupt comes in:
+ spin_lock(&lock);
+where an interrupt tries to lock an already locked variable. This is ok if
+the other interrupt happens on another CPU, but it is _not_ ok if the
+interrupt happens on the same CPU that already holds the lock, because the
+lock will obviously never be released (because the interrupt is waiting
+for the lock, and the lock-holder is interrupted by the interrupt and will
+not continue until the interrupt has been processed).
+(This is also the reason why the irq-versions of the spinlocks only need
+to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
+on other CPU's, because an interrupt on another CPU doesn't interrupt the
+CPU that holds the lock, so the lock-holder can continue and eventually
+releases the lock).
+Note that you can be clever with read-write locks and interrupts. For
+example, if you know that the interrupt only ever gets a read-lock, then
+you can use a non-irq version of read locks everywhere - because they
+don't block on each other (and thus there is no dead-lock wrt interrupts.
+But when you do the write-lock, you have to use the irq-safe version.
+For an example of being clever with rw-locks, see the "waitqueue_lock"
+handling in kernel/sched/core.c - nothing ever _changes_ a wait-queue from
+within an interrupt, they only read the queue in order to know whom to
+wake up. So read-locks are safe (which is good: they are very common
+indeed), while write-locks need to protect themselves against interrupts.
+ Linus
+Reference information:
+For dynamic initialization, use spin_lock_init() or rwlock_init() as
+ spinlock_t xxx_lock;
+ rwlock_t xxx_rw_lock;
+ static int __init xxx_init(void)
+ {
+ spin_lock_init(&xxx_lock);
+ rwlock_init(&xxx_rw_lock);
+ ...
+ }
+ module_init(xxx_init);
+For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
+__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.

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