*/
/*
- * Copyright (c) 2017 by Delphix. All rights reserved.
+ * Copyright (c) 2017, 2019 by Delphix. All rights reserved.
*/
/*
*
* 1] The operation needs to run over multiple txgs.
* 2] There is be a single point of reference in memory or on disk that
- * indicates whether the operation should run/is running or is
+ * indicates whether the operation should run/is running or has
* stopped.
*
* If the operation satisfies the above then the following rules guarantee
* during creation to wakeup on its own after a specified interval
* [see zthr_create_timer()].
*
+ * Note: ZTHR threads are NOT a replacement for generic threads! Please
+ * ensure that they fit your use-case well before using them.
+ *
* == ZTHR creation
*
* Every zthr needs three inputs to start running:
* 2] A user-defined ZTHR function (func) which the zthr executes when
* it is not sleeping. The function should adhere to the following
* signature type:
- * int func_name(void *args, zthr_t *t);
+ * void func_name(void *args, zthr_t *t);
*
* 3] A void args pointer that will be passed to checkfunc and func
* implicitly by the infrastructure.
*
* The reason why the above API needs two different functions,
* instead of one that both checks and does the work, has to do with
- * the zthr's internal lock (zthr_lock) and the allowed cancellation
- * windows. We want to hold the zthr_lock while running checkfunc
- * but not while running func. This way the zthr can be cancelled
- * while doing work and not while checking for work.
+ * the zthr's internal state lock (zthr_state_lock) and the allowed
+ * cancellation windows. We want to hold the zthr_state_lock while
+ * running checkfunc but not while running func. This way the zthr
+ * can be cancelled while doing work and not while checking for work.
*
* To start a zthr:
* zthr_t *zthr_pointer = zthr_create(checkfunc, func, args);
* args, max_sleep);
*
* After that you should be able to wakeup, cancel, and resume the
- * zthr from another thread using zthr_pointer.
+ * zthr from another thread using the zthr_pointer.
*
* NOTE: ZTHR threads could potentially wake up spuriously and the
* user should take this into account when writing a checkfunc.
* zthr_resume(zthr_pointer);
*
* A zthr will implicitly check if it has received a cancellation
- * signal every time func returns and everytime it wakes up [see ZTHR
- * state transitions below].
+ * signal every time func returns and every time it wakes up [see
+ * ZTHR state transitions below].
*
* At times, waiting for the zthr's func to finish its job may take
* time. This may be very time-consuming for some operations that
* while (!work_done && !zthr_iscancelled(t)) {
* ... <do more work> ...
* }
- * return (0);
* }
*
- * == ZTHR exit
- *
- * For the rare cases where the zthr wants to stop running voluntarily
- * while running its ZTHR function (func), we provide zthr_exit().
- * When a zthr has voluntarily stopped running, it can be resumed with
- * zthr_resume(), just like it would if it was cancelled by some other
- * thread.
- *
* == ZTHR cleanup
*
* Cancelling a zthr doesn't clean up its metadata (internal locks,
* v
* zthr stopped running
*
+ * == Implementation of ZTHR requests
+ *
+ * ZTHR wakeup, cancel, and resume are requests on a zthr to
+ * change its internal state. Requests on a zthr are serialized
+ * using the zthr_request_lock, while changes in its internal
+ * state are protected by the zthr_state_lock. A request will
+ * first acquire the zthr_request_lock and then immediately
+ * acquire the zthr_state_lock. We do this so that incoming
+ * requests are serialized using the request lock, while still
+ * allowing us to use the state lock for thread communication
+ * via zthr_cv.
*/
#include <sys/zfs_context.h>
#include <sys/zthr.h>
-void
-zthr_exit(zthr_t *t, int rc)
-{
- ASSERT3P(t->zthr_thread, ==, curthread);
- mutex_enter(&t->zthr_lock);
- t->zthr_thread = NULL;
- t->zthr_rc = rc;
- cv_broadcast(&t->zthr_cv);
- mutex_exit(&t->zthr_lock);
- thread_exit();
-}
+struct zthr {
+ /* running thread doing the work */
+ kthread_t *zthr_thread;
+
+ /* lock protecting internal data & invariants */
+ kmutex_t zthr_state_lock;
+
+ /* mutex that serializes external requests */
+ kmutex_t zthr_request_lock;
+
+ /* notification mechanism for requests */
+ kcondvar_t zthr_cv;
+
+ /* flag set to true if we are canceling the zthr */
+ boolean_t zthr_cancel;
+
+ /*
+ * maximum amount of time that the zthr is spent sleeping;
+ * if this is 0, the thread doesn't wake up until it gets
+ * signaled.
+ */
+ hrtime_t zthr_wait_time;
+
+ /* consumer-provided callbacks & data */
+ zthr_checkfunc_t *zthr_checkfunc;
+ zthr_func_t *zthr_func;
+ void *zthr_arg;
+};
static void
zthr_procedure(void *arg)
{
zthr_t *t = arg;
- int rc = 0;
- mutex_enter(&t->zthr_lock);
+ mutex_enter(&t->zthr_state_lock);
+ ASSERT3P(t->zthr_thread, ==, curthread);
+
while (!t->zthr_cancel) {
if (t->zthr_checkfunc(t->zthr_arg, t)) {
- mutex_exit(&t->zthr_lock);
- rc = t->zthr_func(t->zthr_arg, t);
- mutex_enter(&t->zthr_lock);
+ mutex_exit(&t->zthr_state_lock);
+ t->zthr_func(t->zthr_arg, t);
+ mutex_enter(&t->zthr_state_lock);
} else {
/* go to sleep */
if (t->zthr_wait_time == 0) {
- cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
+ cv_wait_sig(&t->zthr_cv, &t->zthr_state_lock);
} else {
(void) cv_timedwait_sig_hires(&t->zthr_cv,
- &t->zthr_lock, t->zthr_wait_time,
+ &t->zthr_state_lock, t->zthr_wait_time,
MSEC2NSEC(1), 0);
}
}
}
- mutex_exit(&t->zthr_lock);
- zthr_exit(t, rc);
+ /*
+ * Clear out the kernel thread metadata and notify the
+ * zthr_cancel() thread that we've stopped running.
+ */
+ t->zthr_thread = NULL;
+ t->zthr_cancel = B_FALSE;
+ cv_broadcast(&t->zthr_cv);
+
+ mutex_exit(&t->zthr_state_lock);
+ thread_exit();
}
zthr_t *
void *arg, hrtime_t max_sleep)
{
zthr_t *t = kmem_zalloc(sizeof (*t), KM_SLEEP);
- mutex_init(&t->zthr_lock, NULL, MUTEX_DEFAULT, NULL);
+ mutex_init(&t->zthr_state_lock, NULL, MUTEX_DEFAULT, NULL);
+ mutex_init(&t->zthr_request_lock, NULL, MUTEX_DEFAULT, NULL);
cv_init(&t->zthr_cv, NULL, CV_DEFAULT, NULL);
- mutex_enter(&t->zthr_lock);
+ mutex_enter(&t->zthr_state_lock);
t->zthr_checkfunc = checkfunc;
t->zthr_func = func;
t->zthr_arg = arg;
t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
0, &p0, TS_RUN, minclsyspri);
- mutex_exit(&t->zthr_lock);
+ mutex_exit(&t->zthr_state_lock);
return (t);
}
void
zthr_destroy(zthr_t *t)
{
+ ASSERT(!MUTEX_HELD(&t->zthr_state_lock));
+ ASSERT(!MUTEX_HELD(&t->zthr_request_lock));
VERIFY3P(t->zthr_thread, ==, NULL);
- mutex_destroy(&t->zthr_lock);
+ mutex_destroy(&t->zthr_request_lock);
+ mutex_destroy(&t->zthr_state_lock);
cv_destroy(&t->zthr_cv);
kmem_free(t, sizeof (*t));
}
/*
- * Note: If the zthr is not sleeping and misses the wakeup
- * (e.g it is running its ZTHR function), it will check if
- * there is work to do before going to sleep using its checker
- * function [see ZTHR state transition in ZTHR block comment].
- * Thus, missing the wakeup still yields the expected behavior.
+ * Wake up the zthr if it is sleeping. If the thread has been
+ * cancelled that does nothing.
*/
void
zthr_wakeup(zthr_t *t)
{
- mutex_enter(&t->zthr_lock);
+ mutex_enter(&t->zthr_request_lock);
+ mutex_enter(&t->zthr_state_lock);
+
+ /*
+ * There are 4 states that we can find the zthr when issuing
+ * this broadcast:
+ *
+ * [1] The common case of the thread being asleep, at which
+ * point the broadcast will wake it up.
+ * [2] The thread has been cancelled. Waking up a cancelled
+ * thread is a no-op. Any work that is still left to be
+ * done should be handled the next time the thread is
+ * resumed.
+ * [3] The thread is doing work and is already up, so this
+ * is basically a no-op.
+ * [4] The thread was just created/resumed, in which case the
+ * behavior is similar to [3].
+ */
cv_broadcast(&t->zthr_cv);
- mutex_exit(&t->zthr_lock);
+
+ mutex_exit(&t->zthr_state_lock);
+ mutex_exit(&t->zthr_request_lock);
}
/*
- * Note: If the zthr is not running (e.g. has been cancelled
+ * Sends a cancel request to the zthr and blocks until the zthr is
+ * cancelled. If the zthr is not running (e.g. has been cancelled
* already), this is a no-op.
*/
-int
+void
zthr_cancel(zthr_t *t)
{
- int rc = 0;
+ mutex_enter(&t->zthr_request_lock);
+ mutex_enter(&t->zthr_state_lock);
- mutex_enter(&t->zthr_lock);
+ /*
+ * Since we are holding the zthr_state_lock at this point
+ * we can find the state in one of the following 4 states:
+ *
+ * [1] The thread has already been cancelled, therefore
+ * there is nothing for us to do.
+ * [2] The thread is sleeping, so we broadcast the CV first
+ * to wake it up and then we set the flag and we are
+ * waiting for it to exit.
+ * [3] The thread is doing work, in which case we just set
+ * the flag and wait for it to finish.
+ * [4] The thread was just created/resumed, in which case
+ * the behavior is similar to [3].
+ *
+ * Since requests are serialized, by the time that we get
+ * control back we expect that the zthr is cancelled and
+ * not running anymore.
+ */
+ if (t->zthr_thread != NULL) {
+ t->zthr_cancel = B_TRUE;
- /* broadcast in case the zthr is sleeping */
- cv_broadcast(&t->zthr_cv);
+ /* broadcast in case the zthr is sleeping */
+ cv_broadcast(&t->zthr_cv);
- t->zthr_cancel = B_TRUE;
- while (t->zthr_thread != NULL)
- cv_wait(&t->zthr_cv, &t->zthr_lock);
- t->zthr_cancel = B_FALSE;
- rc = t->zthr_rc;
- mutex_exit(&t->zthr_lock);
+ while (t->zthr_thread != NULL)
+ cv_wait(&t->zthr_cv, &t->zthr_state_lock);
- return (rc);
+ ASSERT(!t->zthr_cancel);
+ }
+
+ mutex_exit(&t->zthr_state_lock);
+ mutex_exit(&t->zthr_request_lock);
}
+/*
+ * Sends a resume request to the supplied zthr. If the zthr is
+ * already running this is a no-op.
+ */
void
zthr_resume(zthr_t *t)
{
- ASSERT3P(t->zthr_thread, ==, NULL);
-
- mutex_enter(&t->zthr_lock);
+ mutex_enter(&t->zthr_request_lock);
+ mutex_enter(&t->zthr_state_lock);
ASSERT3P(&t->zthr_checkfunc, !=, NULL);
ASSERT3P(&t->zthr_func, !=, NULL);
ASSERT(!t->zthr_cancel);
- t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
- 0, &p0, TS_RUN, minclsyspri);
+ /*
+ * There are 4 states that we find the zthr in at this point
+ * given the locks that we hold:
+ *
+ * [1] The zthr was cancelled, so we spawn a new thread for
+ * the zthr (common case).
+ * [2] The zthr is running at which point this is a no-op.
+ * [3] The zthr is sleeping at which point this is a no-op.
+ * [4] The zthr was just spawned at which point this is a
+ * no-op.
+ */
+ if (t->zthr_thread == NULL) {
+ t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
+ 0, &p0, TS_RUN, minclsyspri);
+ }
- mutex_exit(&t->zthr_lock);
+ mutex_exit(&t->zthr_state_lock);
+ mutex_exit(&t->zthr_request_lock);
}
/*
* This function is intended to be used by the zthr itself
- * to check if another thread has signal it to stop running.
+ * (specifically the zthr_func callback provided) to check
+ * if another thread has signaled it to stop running before
+ * doing some expensive operation.
*
* returns TRUE if we are in the middle of trying to cancel
* this thread.
boolean_t
zthr_iscancelled(zthr_t *t)
{
- boolean_t cancelled;
-
ASSERT3P(t->zthr_thread, ==, curthread);
- mutex_enter(&t->zthr_lock);
- cancelled = t->zthr_cancel;
- mutex_exit(&t->zthr_lock);
-
+ /*
+ * The majority of the functions here grab zthr_request_lock
+ * first and then zthr_state_lock. This function only grabs
+ * the zthr_state_lock. That is because this function should
+ * only be called from the zthr_func to check if someone has
+ * issued a zthr_cancel() on the thread. If there is a zthr_cancel()
+ * happening concurrently, attempting to grab the request lock
+ * here would result in a deadlock.
+ *
+ * By grabbing only the zthr_state_lock this function is allowed
+ * to run concurrently with a zthr_cancel() request.
+ */
+ mutex_enter(&t->zthr_state_lock);
+ boolean_t cancelled = t->zthr_cancel;
+ mutex_exit(&t->zthr_state_lock);
return (cancelled);
}
-
-boolean_t
-zthr_isrunning(zthr_t *t)
-{
- boolean_t running;
-
- mutex_enter(&t->zthr_lock);
- running = (t->zthr_thread != NULL);
- mutex_exit(&t->zthr_lock);
-
- return (running);
-}
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