sched: Document wait_var_event() family of functions and wake_up_var()

wake_up_var(), wait_var_event() and related interfaces are not
documented but have important ordering requirements.  This patch adds
documentation and makes these requirements explicit.

The return values for those wait_var_event_* functions which return a
value are documented.  Note that these are, perhaps surprisingly,
sometimes different from comparable wait_on_bit() functions.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240925053405.3960701-4-neilb@suse.de

include/linux/wait_bit.h

@@ -282,6 +282,22 @@ __out:	__ret;								\
 	___wait_var_event(var, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
 			  schedule())
 
+/**
+ * wait_var_event - wait for a variable to be updated and notified
+ * @var: the address of variable being waited on
+ * @condition: the condition to wait for
+ *
+ * Wait for a @condition to be true, only re-checking when a wake up is
+ * received for the given @var (an arbitrary kernel address which need
+ * not be directly related to the given condition, but usually is).
+ *
+ * The process will wait on a waitqueue selected by hash from a shared
+ * pool.  It will only be woken on a wake_up for the given address.
+ *
+ * The condition should normally use smp_load_acquire() or a similarly
+ * ordered access to ensure that any changes to memory made before the
+ * condition became true will be visible after the wait completes.
+ */
 #define wait_var_event(var, condition)					\
 do {									\
 	might_sleep();							\
@@ -294,6 +310,24 @@ do {									\
 	___wait_var_event(var, condition, TASK_KILLABLE, 0, 0,		\
 			  schedule())
 
+/**
+ * wait_var_event_killable - wait for a variable to be updated and notified
+ * @var: the address of variable being waited on
+ * @condition: the condition to wait for
+ *
+ * Wait for a @condition to be true or a fatal signal to be received,
+ * only re-checking the condition when a wake up is received for the given
+ * @var (an arbitrary kernel address which need not be directly related
+ * to the given condition, but usually is).
+ *
+ * This is similar to wait_var_event() but returns a value which is
+ * 0 if the condition became true, or %-ERESTARTSYS if a fatal signal
+ * was received.
+ *
+ * The condition should normally use smp_load_acquire() or a similarly
+ * ordered access to ensure that any changes to memory made before the
+ * condition became true will be visible after the wait completes.
+ */
 #define wait_var_event_killable(var, condition)				\
 ({									\
 	int __ret = 0;							\
@@ -308,6 +342,26 @@ do {									\
 			  TASK_UNINTERRUPTIBLE, 0, timeout,		\
 			  __ret = schedule_timeout(__ret))
 
+/**
+ * wait_var_event_timeout - wait for a variable to be updated or a timeout to expire
+ * @var: the address of variable being waited on
+ * @condition: the condition to wait for
+ * @timeout: maximum time to wait in jiffies
+ *
+ * Wait for a @condition to be true or a timeout to expire, only
+ * re-checking the condition when a wake up is received for the given
+ * @var (an arbitrary kernel address which need not be directly related
+ * to the given condition, but usually is).
+ *
+ * This is similar to wait_var_event() but returns a value which is 0 if
+ * the timeout expired and the condition was still false, or the
+ * remaining time left in the timeout (but at least 1) if the condition
+ * was found to be true.
+ *
+ * The condition should normally use smp_load_acquire() or a similarly
+ * ordered access to ensure that any changes to memory made before the
+ * condition became true will be visible after the wait completes.
+ */
 #define wait_var_event_timeout(var, condition, timeout)			\
 ({									\
 	long __ret = timeout;						\
@@ -321,6 +375,23 @@ do {									\
 	___wait_var_event(var, condition, TASK_INTERRUPTIBLE, 0, 0,	\
 			  schedule())
 
+/**
+ * wait_var_event_killable - wait for a variable to be updated and notified
+ * @var: the address of variable being waited on
+ * @condition: the condition to wait for
+ *
+ * Wait for a @condition to be true or a signal to be received, only
+ * re-checking the condition when a wake up is received for the given
+ * @var (an arbitrary kernel address which need not be directly related
+ * to the given condition, but usually is).
+ *
+ * This is similar to wait_var_event() but returns a value which is 0 if
+ * the condition became true, or %-ERESTARTSYS if a signal was received.
+ *
+ * The condition should normally use smp_load_acquire() or a similarly
+ * ordered access to ensure that any changes to memory made before the
+ * condition became true will be visible after the wait completes.
+ */
 #define wait_var_event_interruptible(var, condition)			\
 ({									\
 	int __ret = 0;							\

kernel/sched/wait_bit.c

@@ -196,6 +196,36 @@ void init_wait_var_entry(struct wait_bit_queue_entry *wbq_entry, void *var, int
 }
 EXPORT_SYMBOL(init_wait_var_entry);
 
+/**
+ * wake_up_var - wake up waiters on a variable (kernel address)
+ * @var: the address of the variable being waited on
+ *
+ * Wake up any process waiting in wait_var_event() or similar for the
+ * given variable to change.  wait_var_event() can be waiting for an
+ * arbitrary condition to be true and associates that condition with an
+ * address.  Calling wake_up_var() suggests that the condition has been
+ * made true, but does not strictly require the condtion to use the
+ * address given.
+ *
+ * The wake-up is sent to tasks in a waitqueue selected by hash from a
+ * shared pool.  Only those tasks on that queue which have requested
+ * wake_up on this specific address will be woken.
+ *
+ * In order for this to function properly there must be a full memory
+ * barrier after the variable is updated (or more accurately, after the
+ * condition waited on has been made to be true) and before this function
+ * is called.  If the variable was updated atomically, such as a by
+ * atomic_dec() then smb_mb__after_atomic() can be used.  If the
+ * variable was updated by a fully ordered operation such as
+ * atomic_dec_and_test() then no extra barrier is required.  Otherwise
+ * smb_mb() is needed.
+ *
+ * Normally the variable should be updated (the condition should be made
+ * to be true) by an operation with RELEASE semantics such as
+ * smp_store_release() so that any changes to memory made before the
+ * variable was updated are guaranteed to be visible after the matching
+ * wait_var_event() completes.
+ */
 void wake_up_var(void *var)
 {
 	__wake_up_bit(__var_waitqueue(var), var, -1);