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. The above sequence under UP
-essentially is just the same as doing
-
- 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 operation).
+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:
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.
+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
The routines look the same as above:
- rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
+ rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
unsigned long flags;
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).
+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
For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
-
-SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED are deprecated. These interfere
-with lockdep state tracking.
-
-Most of the time, you can simply turn:
- static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
-into:
- static DEFINE_SPINLOCK(xxx_lock);
-
-Static structure member variables go from:
-
- struct foo bar {
- .lock = SPIN_LOCK_UNLOCKED;
- };
-
-to:
-
- struct foo bar {
- .lock = __SPIN_LOCK_UNLOCKED(bar.lock);
- };
-
-Declaration of static rw_locks undergo a similar transformation.
projects / firefly-linux-kernel-4.4.55.git / blobdiff