Linux-2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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Linus Torvalds
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Documentation/spinlocks.txt
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212
Documentation/spinlocks.txt
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UPDATE March 21 2005 Amit Gud <gud@eth.net>
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Macros SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED are deprecated and will be
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removed soon. So for any new code dynamic initialization should be used:
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spinlock_t xxx_lock;
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rwlock_t xxx_rw_lock;
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static int __init xxx_init(void)
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{
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spin_lock_init(&xxx_lock);
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rw_lock_init(&xxx_rw_lock);
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...
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}
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module_init(xxx_init);
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Reasons for deprecation
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- it hurts automatic lock validators
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- it becomes intrusive for the realtime preemption patches
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Following discussion is still valid, however, with the dynamic initialization
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of spinlocks instead of static.
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-----------------------
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On Fri, 2 Jan 1998, Doug Ledford wrote:
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>
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> I'm working on making the aic7xxx driver more SMP friendly (as well as
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> importing the latest FreeBSD sequencer code to have 7895 support) and wanted
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> to get some info from you. The goal here is to make the various routines
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> SMP safe as well as UP safe during interrupts and other manipulating
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> routines. So far, I've added a spin_lock variable to things like my queue
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> structs. Now, from what I recall, there are some spin lock functions I can
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> use to lock these spin locks from other use as opposed to a (nasty)
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> save_flags(); cli(); stuff; restore_flags(); construct. Where do I find
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> these routines and go about making use of them? Do they only lock on a
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> per-processor basis or can they also lock say an interrupt routine from
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> mucking with a queue if the queue routine was manipulating it when the
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> interrupt occurred, or should I still use a cli(); based construct on that
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> one?
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See <asm/spinlock.h>. The basic version is:
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spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
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unsigned long flags;
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spin_lock_irqsave(&xxx_lock, flags);
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... critical section here ..
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spin_unlock_irqrestore(&xxx_lock, flags);
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and the above is always safe. It will disable interrupts _locally_, but the
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spinlock itself will guarantee the global lock, so it will guarantee that
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there is only one thread-of-control within the region(s) protected by that
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lock.
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Note that it works well even under UP - the above sequence under UP
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essentially is just the same as doing a
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unsigned long flags;
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save_flags(flags); cli();
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... critical section ...
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restore_flags(flags);
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so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
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work correctly under both (and spinlocks are actually more efficient on
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architectures that allow doing the "save_flags + cli" in one go because I
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don't export that interface normally).
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NOTE NOTE NOTE! The reason the spinlock is so much faster than a global
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interrupt lock under SMP is exactly because it disables interrupts only on
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the local CPU. The spin-lock is safe only when you _also_ use the lock
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itself to do locking across CPU's, which implies that EVERYTHING that
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touches a shared variable has to agree about the spinlock they want to
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use.
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The above is usually pretty simple (you usually need and want only one
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spinlock for most things - using more than one spinlock can make things a
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lot more complex and even slower and is usually worth it only for
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sequences that you _know_ need to be split up: avoid it at all cost if you
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aren't sure). HOWEVER, it _does_ mean that if you have some code that does
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cli();
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.. critical section ..
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sti();
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and another sequence that does
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spin_lock_irqsave(flags);
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.. critical section ..
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spin_unlock_irqrestore(flags);
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then they are NOT mutually exclusive, and the critical regions can happen
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at the same time on two different CPU's. That's fine per se, but the
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critical regions had better be critical for different things (ie they
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can't stomp on each other).
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The above is a problem mainly if you end up mixing code - for example the
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routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
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their actions, and if a driver uses spinlocks instead then you should
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think about issues like the above..
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This is really the only really hard part about spinlocks: once you start
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using spinlocks they tend to expand to areas you might not have noticed
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before, because you have to make sure the spinlocks correctly protect the
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shared data structures _everywhere_ they are used. The spinlocks are most
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easily added to places that are completely independent of other code (ie
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internal driver data structures that nobody else ever touches, for
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example).
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----
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Lesson 2: reader-writer spinlocks.
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If your data accesses have a very natural pattern where you usually tend
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to mostly read from the shared variables, the reader-writer locks
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(rw_lock) versions of the spinlocks are often nicer. They allow multiple
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readers to be in the same critical region at once, but if somebody wants
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to change the variables it has to get an exclusive write lock. The
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routines look the same as above:
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rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
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unsigned long flags;
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read_lock_irqsave(&xxx_lock, flags);
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.. critical section that only reads the info ...
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read_unlock_irqrestore(&xxx_lock, flags);
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write_lock_irqsave(&xxx_lock, flags);
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.. read and write exclusive access to the info ...
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write_unlock_irqrestore(&xxx_lock, flags);
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The above kind of lock is useful for complex data structures like linked
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lists etc, especially when you know that most of the work is to just
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traverse the list searching for entries without changing the list itself,
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for example. Then you can use the read lock for that kind of list
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traversal, which allows many concurrent readers. Anything that _changes_
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the list will have to get the write lock.
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Note: you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
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time need to do any changes (even if you don't do it every time), you have
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to get the write-lock at the very beginning. I could fairly easily add a
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primitive to create a "upgradeable" read-lock, but it hasn't been an issue
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yet. Tell me if you'd want one.
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----
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Lesson 3: spinlocks revisited.
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The single spin-lock primitives above are by no means the only ones. They
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are the most safe ones, and the ones that work under all circumstances,
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but partly _because_ they are safe they are also fairly slow. They are
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much faster than a generic global cli/sti pair, but slower than they'd
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need to be, because they do have to disable interrupts (which is just a
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single instruction on a x86, but it's an expensive one - and on other
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architectures it can be worse).
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If you have a case where you have to protect a data structure across
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several CPU's and you want to use spinlocks you can potentially use
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cheaper versions of the spinlocks. IFF you know that the spinlocks are
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never used in interrupt handlers, you can use the non-irq versions:
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spin_lock(&lock);
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...
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spin_unlock(&lock);
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(and the equivalent read-write versions too, of course). The spinlock will
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guarantee the same kind of exclusive access, and it will be much faster.
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This is useful if you know that the data in question is only ever
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manipulated from a "process context", ie no interrupts involved.
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The reasons you mustn't use these versions if you have interrupts that
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play with the spinlock is that you can get deadlocks:
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spin_lock(&lock);
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...
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<- interrupt comes in:
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spin_lock(&lock);
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where an interrupt tries to lock an already locked variable. This is ok if
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the other interrupt happens on another CPU, but it is _not_ ok if the
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interrupt happens on the same CPU that already holds the lock, because the
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lock will obviously never be released (because the interrupt is waiting
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for the lock, and the lock-holder is interrupted by the interrupt and will
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not continue until the interrupt has been processed).
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(This is also the reason why the irq-versions of the spinlocks only need
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to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
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on other CPU's, because an interrupt on another CPU doesn't interrupt the
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CPU that holds the lock, so the lock-holder can continue and eventually
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releases the lock).
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Note that you can be clever with read-write locks and interrupts. For
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example, if you know that the interrupt only ever gets a read-lock, then
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you can use a non-irq version of read locks everywhere - because they
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don't block on each other (and thus there is no dead-lock wrt interrupts.
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But when you do the write-lock, you have to use the irq-safe version.
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For an example of being clever with rw-locks, see the "waitqueue_lock"
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handling in kernel/sched.c - nothing ever _changes_ a wait-queue from
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within an interrupt, they only read the queue in order to know whom to
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wake up. So read-locks are safe (which is good: they are very common
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indeed), while write-locks need to protect themselves against interrupts.
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Linus
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