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+UPDATE March 21 2005 Amit Gud <gud@eth.net>
+
+Macros SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED are deprecated and will be
+removed soon. So for any new code dynamic initialization should be used:
+
+   spinlock_t xxx_lock;
+   rwlock_t xxx_rw_lock;
+
+   static int __init xxx_init(void)
+   {
+        spin_lock_init(&xxx_lock);
+        rw_lock_init(&xxx_rw_lock);
+        ...
+   }
+
+   module_init(xxx_init);
+
+Reasons for deprecation
+  - it hurts automatic lock validators
+  - it becomes intrusive for the realtime preemption patches
+
+Following discussion is still valid, however, with the dynamic initialization
+of spinlocks instead of static.
+
+-----------------------
+
+On Fri, 2 Jan 1998, Doug Ledford wrote:
+> 
+> I'm working on making the aic7xxx driver more SMP friendly (as well as
+> importing the latest FreeBSD sequencer code to have 7895 support) and wanted
+> to get some info from you.  The goal here is to make the various routines
+> SMP safe as well as UP safe during interrupts and other manipulating
+> routines.  So far, I've added a spin_lock variable to things like my queue
+> structs.  Now, from what I recall, there are some spin lock functions I can
+> use to lock these spin locks from other use as opposed to a (nasty)
+> save_flags(); cli(); stuff; restore_flags(); construct.  Where do I find
+> these routines and go about making use of them?  Do they only lock on a
+> per-processor basis or can they also lock say an interrupt routine from
+> mucking with a queue if the queue routine was manipulating it when the
+> interrupt occurred, or should I still use a cli(); based construct on that
+> one?
+
+See <asm/spinlock.h>. The basic version is:
+
+   spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
+
+
+        unsigned long flags;
+
+        spin_lock_irqsave(&xxx_lock, flags);
+        ... critical section here ..
+        spin_unlock_irqrestore(&xxx_lock, flags);
+
+and 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. 
+
+Note that it works well even under UP - the above sequence under UP
+essentially is just the same as doing a
+
+        unsigned long flags;
+
+        save_flags(flags); cli();
+         ... critical section ...
+        restore_flags(flags);
+
+so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
+work correctly under both (and spinlocks are actually more efficient on
+architectures that allow doing the "save_flags + cli" in one go because I
+don't export that interface normally).
+
+NOTE NOTE NOTE! The reason the spinlock is so much faster than a global
+interrupt lock under SMP is exactly because it disables interrupts only on
+the local CPU. 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.
+
+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). HOWEVER, it _does_ mean that if you have some code that does
+
+        cli();
+        .. critical section ..
+        sti();
+
+and another sequence that does
+
+        spin_lock_irqsave(flags);
+        .. critical section ..
+        spin_unlock_irqrestore(flags);
+
+then they are NOT mutually exclusive, and the critical regions can happen
+at the same time on two different CPU's. That's fine per se, but the
+critical regions had better be critical for different things (ie they
+can't stomp on each other). 
+
+The above is a problem mainly if you end up mixing code - for example the
+routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
+their actions, and if a driver uses spinlocks instead then you should
+think about issues like the above..
+
+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 (ie
+internal driver data structures that nobody else ever touches, for
+example). 
+
+----
+
+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 often nicer. 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. The
+routines look the same as above:
+
+   rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
+
+
+        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 is useful for complex data structures like linked
+lists etc, especially when you know that most of the work is to just
+traverse the list searching for entries without changing the list itself,
+for example. Then you can use the read lock for that kind of list
+traversal, which allows many concurrent readers. Anything that _changes_
+the list will have to get the write lock. 
+
+Note: 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. I could fairly easily add a
+primitive to create a "upgradeable" read-lock, but it hasn't been an issue
+yet. Tell me if you'd want one. 
+
+----
+
+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
+much faster than a generic global cli/sti pair, but 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.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
+
+