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Message-ID: <20240826063659.15327-5-neilb@suse.de>
Date: Mon, 26 Aug 2024 16:31:01 +1000
From: NeilBrown <neilb@...e.de>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
linux-block@...r.kernel.org
Subject: [PATCH 4/7] 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@...e.de>
---
include/linux/wait_bit.h | 71 ++++++++++++++++++++++++++++++++++++++++
kernel/sched/wait_bit.c | 30 +++++++++++++++++
2 files changed, 101 insertions(+)
diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
index b792a92a036e..ca5c6e70f908 100644
--- a/include/linux/wait_bit.h
+++ b/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; \
diff --git a/kernel/sched/wait_bit.c b/kernel/sched/wait_bit.c
index 247997e1c9c4..d7ac2ec09f8f 100644
--- a/kernel/sched/wait_bit.c
+++ b/kernel/sched/wait_bit.c
@@ -199,6 +199,36 @@ 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
+ * condtion 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. Othwewise
+ * 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 update 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);
--
2.44.0
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