============================
By: David Howells <dhowells@redhat.com>
+ Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Contents:
- Locking functions.
- Interrupt disabling functions.
+ - Sleep and wake-up functions.
- Miscellaneous functions.
(*) Inter-CPU locking barrier effects.
- And then there's the Alpha.
+ (*) Example uses.
+
+ - Circular buffers.
+
(*) References.
other means.
+SLEEP AND WAKE-UP FUNCTIONS
+---------------------------
+
+Sleeping and waking on an event flagged in global data can be viewed as an
+interaction between two pieces of data: the task state of the task waiting for
+the event and the global data used to indicate the event. To make sure that
+these appear to happen in the right order, the primitives to begin the process
+of going to sleep, and the primitives to initiate a wake up imply certain
+barriers.
+
+Firstly, the sleeper normally follows something like this sequence of events:
+
+ for (;;) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (event_indicated)
+ break;
+ schedule();
+ }
+
+A general memory barrier is interpolated automatically by set_current_state()
+after it has altered the task state:
+
+ CPU 1
+ ===============================
+ set_current_state();
+ set_mb();
+ STORE current->state
+ <general barrier>
+ LOAD event_indicated
+
+set_current_state() may be wrapped by:
+
+ prepare_to_wait();
+ prepare_to_wait_exclusive();
+
+which therefore also imply a general memory barrier after setting the state.
+The whole sequence above is available in various canned forms, all of which
+interpolate the memory barrier in the right place:
+
+ wait_event();
+ wait_event_interruptible();
+ wait_event_interruptible_exclusive();
+ wait_event_interruptible_timeout();
+ wait_event_killable();
+ wait_event_timeout();
+ wait_on_bit();
+ wait_on_bit_lock();
+
+
+Secondly, code that performs a wake up normally follows something like this:
+
+ event_indicated = 1;
+ wake_up(&event_wait_queue);
+
+or:
+
+ event_indicated = 1;
+ wake_up_process(event_daemon);
+
+A write memory barrier is implied by wake_up() and co. if and only if they wake
+something up. The barrier occurs before the task state is cleared, and so sits
+between the STORE to indicate the event and the STORE to set TASK_RUNNING:
+
+ CPU 1 CPU 2
+ =============================== ===============================
+ set_current_state(); STORE event_indicated
+ set_mb(); wake_up();
+ STORE current->state <write barrier>
+ <general barrier> STORE current->state
+ LOAD event_indicated
+
+The available waker functions include:
+
+ complete();
+ wake_up();
+ wake_up_all();
+ wake_up_bit();
+ wake_up_interruptible();
+ wake_up_interruptible_all();
+ wake_up_interruptible_nr();
+ wake_up_interruptible_poll();
+ wake_up_interruptible_sync();
+ wake_up_interruptible_sync_poll();
+ wake_up_locked();
+ wake_up_locked_poll();
+ wake_up_nr();
+ wake_up_poll();
+ wake_up_process();
+
+
+[!] Note that the memory barriers implied by the sleeper and the waker do _not_
+order multiple stores before the wake-up with respect to loads of those stored
+values after the sleeper has called set_current_state(). For instance, if the
+sleeper does:
+
+ set_current_state(TASK_INTERRUPTIBLE);
+ if (event_indicated)
+ break;
+ __set_current_state(TASK_RUNNING);
+ do_something(my_data);
+
+and the waker does:
+
+ my_data = value;
+ event_indicated = 1;
+ wake_up(&event_wait_queue);
+
+there's no guarantee that the change to event_indicated will be perceived by
+the sleeper as coming after the change to my_data. In such a circumstance, the
+code on both sides must interpolate its own memory barriers between the
+separate data accesses. Thus the above sleeper ought to do:
+
+ set_current_state(TASK_INTERRUPTIBLE);
+ if (event_indicated) {
+ smp_rmb();
+ do_something(my_data);
+ }
+
+and the waker should do:
+
+ my_data = value;
+ smp_wmb();
+ event_indicated = 1;
+ wake_up(&event_wait_queue);
+
+
MISCELLANEOUS FUNCTIONS
-----------------------
Under normal operation, memory operation reordering is generally not going to
be a problem as a single-threaded linear piece of code will still appear to
-work correctly, even if it's in an SMP kernel. There are, however, three
+work correctly, even if it's in an SMP kernel. There are, however, four
circumstances in which reordering definitely _could_ be a problem:
(*) Interprocessor interaction.
See the subsection on "Cache Coherency" above.
+============
+EXAMPLE USES
+============
+
+CIRCULAR BUFFERS
+----------------
+
+Memory barriers can be used to implement circular buffering without the need
+of a lock to serialise the producer with the consumer. See:
+
+ Documentation/circular-buffers.txt
+
+for details.
+
+
==========
REFERENCES
==========
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff