summaryrefslogtreecommitdiff
path: root/kernel/locking
diff options
context:
space:
mode:
authorPeter Ziljstra <peterz@infradead.org>2018-06-15 11:07:12 +0300
committerThomas Hellstrom <thellstrom@vmware.com>2018-07-03 10:42:40 +0300
commit55f036ca7e74b85e34958af3d22121c656796413 (patch)
tree37bdbcea3384bb7d315f242b03ecc2bbff135f82 /kernel/locking
parenteab976693153b9854bfa83d131374748f6ca4280 (diff)
downloadlinux-55f036ca7e74b85e34958af3d22121c656796413.tar.xz
locking: WW mutex cleanup
Make the WW mutex code more readable by adding comments, splitting up functions and pointing out that we're actually using the Wait-Die algorithm. Cc: Ingo Molnar <mingo@redhat.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Gustavo Padovan <gustavo@padovan.org> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Sean Paul <seanpaul@chromium.org> Cc: David Airlie <airlied@linux.ie> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Kate Stewart <kstewart@linuxfoundation.org> Cc: Philippe Ombredanne <pombredanne@nexb.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: linux-doc@vger.kernel.org Cc: linux-media@vger.kernel.org Cc: linaro-mm-sig@lists.linaro.org Co-authored-by: Thomas Hellstrom <thellstrom@vmware.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> Acked-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking')
-rw-r--r--kernel/locking/mutex.c202
1 files changed, 125 insertions, 77 deletions
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index f44f658ae629..cfe48419b7d0 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -244,6 +244,17 @@ void __sched mutex_lock(struct mutex *lock)
EXPORT_SYMBOL(mutex_lock);
#endif
+/*
+ * Wait-Die:
+ * The newer transactions are killed when:
+ * It (the new transaction) makes a request for a lock being held
+ * by an older transaction.
+ */
+
+/*
+ * Associate the ww_mutex @ww with the context @ww_ctx under which we acquired
+ * it.
+ */
static __always_inline void
ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
{
@@ -282,26 +293,53 @@ ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
#endif
ww_ctx->acquired++;
+ ww->ctx = ww_ctx;
}
+/*
+ * Determine if context @a is 'after' context @b. IOW, @a is a younger
+ * transaction than @b and depending on algorithm either needs to wait for
+ * @b or die.
+ */
static inline bool __sched
__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
{
- return a->stamp - b->stamp <= LONG_MAX &&
- (a->stamp != b->stamp || a > b);
+
+ return (signed long)(a->stamp - b->stamp) > 0;
+}
+
+/*
+ * Wait-Die; wake a younger waiter context (when locks held) such that it can
+ * die.
+ *
+ * Among waiters with context, only the first one can have other locks acquired
+ * already (ctx->acquired > 0), because __ww_mutex_add_waiter() and
+ * __ww_mutex_check_kill() wake any but the earliest context.
+ */
+static bool __sched
+__ww_mutex_die(struct mutex *lock, struct mutex_waiter *waiter,
+ struct ww_acquire_ctx *ww_ctx)
+{
+ if (waiter->ww_ctx->acquired > 0 &&
+ __ww_ctx_stamp_after(waiter->ww_ctx, ww_ctx)) {
+ debug_mutex_wake_waiter(lock, waiter);
+ wake_up_process(waiter->task);
+ }
+
+ return true;
}
/*
- * Wake up any waiters that may have to back off when the lock is held by the
- * given context.
+ * We just acquired @lock under @ww_ctx, if there are later contexts waiting
+ * behind us on the wait-list, check if they need to die.
*
- * Due to the invariants on the wait list, this can only affect the first
- * waiter with a context.
+ * See __ww_mutex_add_waiter() for the list-order construction; basically the
+ * list is ordered by stamp, smallest (oldest) first.
*
* The current task must not be on the wait list.
*/
static void __sched
-__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
+__ww_mutex_check_waiters(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
{
struct mutex_waiter *cur;
@@ -311,30 +349,23 @@ __ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
if (!cur->ww_ctx)
continue;
- if (cur->ww_ctx->acquired > 0 &&
- __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
- debug_mutex_wake_waiter(lock, cur);
- wake_up_process(cur->task);
- }
-
- break;
+ if (__ww_mutex_die(lock, cur, ww_ctx))
+ break;
}
}
/*
- * After acquiring lock with fastpath or when we lost out in contested
- * slowpath, set ctx and wake up any waiters so they can recheck.
+ * After acquiring lock with fastpath, where we do not hold wait_lock, set ctx
+ * and wake up any waiters so they can recheck.
*/
static __always_inline void
ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
ww_mutex_lock_acquired(lock, ctx);
- lock->ctx = ctx;
-
/*
* The lock->ctx update should be visible on all cores before
- * the atomic read is done, otherwise contended waiters might be
+ * the WAITERS check is done, otherwise contended waiters might be
* missed. The contended waiters will either see ww_ctx == NULL
* and keep spinning, or it will acquire wait_lock, add itself
* to waiter list and sleep.
@@ -348,29 +379,14 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
return;
/*
- * Uh oh, we raced in fastpath, wake up everyone in this case,
- * so they can see the new lock->ctx.
+ * Uh oh, we raced in fastpath, check if any of the waiters need to
+ * die.
*/
spin_lock(&lock->base.wait_lock);
- __ww_mutex_wakeup_for_backoff(&lock->base, ctx);
+ __ww_mutex_check_waiters(&lock->base, ctx);
spin_unlock(&lock->base.wait_lock);
}
-/*
- * After acquiring lock in the slowpath set ctx.
- *
- * Unlike for the fast path, the caller ensures that waiters are woken up where
- * necessary.
- *
- * Callers must hold the mutex wait_lock.
- */
-static __always_inline void
-ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
-{
- ww_mutex_lock_acquired(lock, ctx);
- lock->ctx = ctx;
-}
-
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
static inline
@@ -646,37 +662,73 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock)
}
EXPORT_SYMBOL(ww_mutex_unlock);
+
+static __always_inline int __sched
+__ww_mutex_kill(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
+{
+ if (ww_ctx->acquired > 0) {
+#ifdef CONFIG_DEBUG_MUTEXES
+ struct ww_mutex *ww;
+
+ ww = container_of(lock, struct ww_mutex, base);
+ DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
+ ww_ctx->contending_lock = ww;
+#endif
+ return -EDEADLK;
+ }
+
+ return 0;
+}
+
+
+/*
+ * Check whether we need to kill the transaction for the current lock acquire.
+ *
+ * Wait-Die: If we're trying to acquire a lock already held by an older
+ * context, kill ourselves.
+ *
+ * Since __ww_mutex_add_waiter() orders the wait-list on stamp, we only have to
+ * look at waiters before us in the wait-list.
+ */
static inline int __sched
-__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
- struct ww_acquire_ctx *ctx)
+__ww_mutex_check_kill(struct mutex *lock, struct mutex_waiter *waiter,
+ struct ww_acquire_ctx *ctx)
{
struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
struct mutex_waiter *cur;
+ if (ctx->acquired == 0)
+ return 0;
+
if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
- goto deadlock;
+ return __ww_mutex_kill(lock, ctx);
/*
* If there is a waiter in front of us that has a context, then its
- * stamp is earlier than ours and we must back off.
+ * stamp is earlier than ours and we must kill ourself.
*/
cur = waiter;
list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
- if (cur->ww_ctx)
- goto deadlock;
+ if (!cur->ww_ctx)
+ continue;
+
+ return __ww_mutex_kill(lock, ctx);
}
return 0;
-
-deadlock:
-#ifdef CONFIG_DEBUG_MUTEXES
- DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
- ctx->contending_lock = ww;
-#endif
- return -EDEADLK;
}
+/*
+ * Add @waiter to the wait-list, keep the wait-list ordered by stamp, smallest
+ * first. Such that older contexts are preferred to acquire the lock over
+ * younger contexts.
+ *
+ * Waiters without context are interspersed in FIFO order.
+ *
+ * Furthermore, for Wait-Die kill ourself immediately when possible (there are
+ * older contexts already waiting) to avoid unnecessary waiting.
+ */
static inline int __sched
__ww_mutex_add_waiter(struct mutex_waiter *waiter,
struct mutex *lock,
@@ -693,7 +745,7 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
/*
* Add the waiter before the first waiter with a higher stamp.
* Waiters without a context are skipped to avoid starving
- * them.
+ * them. Wait-Die waiters may die here.
*/
pos = &lock->wait_list;
list_for_each_entry_reverse(cur, &lock->wait_list, list) {
@@ -701,34 +753,27 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
continue;
if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
- /* Back off immediately if necessary. */
- if (ww_ctx->acquired > 0) {
-#ifdef CONFIG_DEBUG_MUTEXES
- struct ww_mutex *ww;
+ /*
+ * Wait-Die: if we find an older context waiting, there
+ * is no point in queueing behind it, as we'd have to
+ * die the moment it would acquire the lock.
+ */
+ int ret = __ww_mutex_kill(lock, ww_ctx);
- ww = container_of(lock, struct ww_mutex, base);
- DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
- ww_ctx->contending_lock = ww;
-#endif
- return -EDEADLK;
- }
+ if (ret)
+ return ret;
break;
}
pos = &cur->list;
- /*
- * Wake up the waiter so that it gets a chance to back
- * off.
- */
- if (cur->ww_ctx->acquired > 0) {
- debug_mutex_wake_waiter(lock, cur);
- wake_up_process(cur->task);
- }
+ /* Wait-Die: ensure younger waiters die. */
+ __ww_mutex_die(lock, cur, ww_ctx);
}
list_add_tail(&waiter->list, pos);
+
return 0;
}
@@ -772,7 +817,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
*/
if (__mutex_trylock(lock)) {
if (use_ww_ctx && ww_ctx)
- __ww_mutex_wakeup_for_backoff(lock, ww_ctx);
+ __ww_mutex_check_waiters(lock, ww_ctx);
goto skip_wait;
}
@@ -790,10 +835,13 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
waiter.ww_ctx = MUTEX_POISON_WW_CTX;
#endif
} else {
- /* Add in stamp order, waking up waiters that must back off. */
+ /*
+ * Add in stamp order, waking up waiters that must kill
+ * themselves.
+ */
ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
if (ret)
- goto err_early_backoff;
+ goto err_early_kill;
waiter.ww_ctx = ww_ctx;
}
@@ -815,7 +863,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
goto acquired;
/*
- * Check for signals and wound conditions while holding
+ * Check for signals and kill conditions while holding
* wait_lock. This ensures the lock cancellation is ordered
* against mutex_unlock() and wake-ups do not go missing.
*/
@@ -824,8 +872,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
goto err;
}
- if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
- ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
+ if (use_ww_ctx && ww_ctx) {
+ ret = __ww_mutex_check_kill(lock, &waiter, ww_ctx);
if (ret)
goto err;
}
@@ -870,7 +918,7 @@ skip_wait:
lock_acquired(&lock->dep_map, ip);
if (use_ww_ctx && ww_ctx)
- ww_mutex_set_context_slowpath(ww, ww_ctx);
+ ww_mutex_lock_acquired(ww, ww_ctx);
spin_unlock(&lock->wait_lock);
preempt_enable();
@@ -879,7 +927,7 @@ skip_wait:
err:
__set_current_state(TASK_RUNNING);
mutex_remove_waiter(lock, &waiter, current);
-err_early_backoff:
+err_early_kill:
spin_unlock(&lock->wait_lock);
debug_mutex_free_waiter(&waiter);
mutex_release(&lock->dep_map, 1, ip);