timerfd: Manage cancelable timers in timerfd

Peter is concerned about the extra scan of CLOCK_REALTIME_COS in the
timer interrupt. Yes, I did not think about it, because the solution
was so elegant. I didn't like the extra list in timerfd when it was
proposed some time ago, but with a rcu based list the list walk it's
less horrible than the original global lock, which was held over the
list iteration.

Requested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Peter Zijlstra <peterz@infradead.org>

1 files changed, 74 insertions, 31 deletions

diff --git a/fs/timerfd.c b/fs/timerfd.c
index 7e14c9e..f67acbd 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -22,6 +22,7 @@
 #include <linux/anon_inodes.h>
 #include <linux/timerfd.h>
 #include <linux/syscalls.h>
+#include <linux/rcupdate.h>
 
 struct timerfd_ctx {
 	struct hrtimer tmr;
@@ -31,9 +32,14 @@ struct timerfd_ctx {
 	u64 ticks;
 	int expired;
 	int clockid;
+	struct rcu_head rcu;
+	struct list_head clist;
 	bool might_cancel;
 };
 
+static LIST_HEAD(cancel_list);
+static DEFINE_SPINLOCK(cancel_lock);
+
 /*
  * This gets called when the timer event triggers. We set the "expired"
  * flag, but we do not re-arm the timer (in case it's necessary,
@@ -53,28 +59,69 @@ static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
 	return HRTIMER_NORESTART;
 }
 
-static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
+/*
+ * Called when the clock was set to cancel the timers in the cancel
+ * list.
+ */
+void timerfd_clock_was_set(void)
 {
-	ktime_t remaining;
+	ktime_t moffs = ktime_get_monotonic_offset();
+	struct timerfd_ctx *ctx;
+	unsigned long flags;
 
-	remaining = hrtimer_expires_remaining(&ctx->tmr);
-	return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
+	rcu_read_lock();
+	list_for_each_entry_rcu(ctx, &cancel_list, clist) {
+		if (!ctx->might_cancel)
+			continue;
+		spin_lock_irqsave(&ctx->wqh.lock, flags);
+		if (ctx->moffs.tv64 != moffs.tv64) {
+			ctx->moffs.tv64 = KTIME_MAX;
+			wake_up_locked(&ctx->wqh);
+		}
+		spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+	}
+	rcu_read_unlock();
 }
 
-static bool timerfd_canceled(struct timerfd_ctx *ctx)
+static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
 {
-	ktime_t moffs;
+	if (ctx->might_cancel) {
+		ctx->might_cancel = false;
+		spin_lock(&cancel_lock);
+		list_del_rcu(&ctx->clist);
+		spin_unlock(&cancel_lock);
+	}
+}
 
-	if (!ctx->might_cancel)
+static bool timerfd_canceled(struct timerfd_ctx *ctx)
+{
+	if (!ctx->might_cancel || ctx->moffs.tv64 != KTIME_MAX)
 		return false;
+	ctx->moffs = ktime_get_monotonic_offset();
+	return true;
+}
 
-	moffs = ktime_get_monotonic_offset();
+static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
+{
+	if (ctx->clockid == CLOCK_REALTIME && (flags & TFD_TIMER_ABSTIME) &&
+	    (flags & TFD_TIMER_CANCEL_ON_SET)) {
+		if (!ctx->might_cancel) {
+			ctx->might_cancel = true;
+			spin_lock(&cancel_lock);
+			list_add_rcu(&ctx->clist, &cancel_list);
+			spin_unlock(&cancel_lock);
+		}
+	} else if (ctx->might_cancel) {
+		timerfd_remove_cancel(ctx);
+	}
+}
 
-	if (moffs.tv64 == ctx->moffs.tv64)
-		return false;
+static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
+{
+	ktime_t remaining;
 
-	ctx->moffs = moffs;
-	return true;
+	remaining = hrtimer_expires_remaining(&ctx->tmr);
+	return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
 }
 
 static int timerfd_setup(struct timerfd_ctx *ctx, int flags,
@@ -87,13 +134,6 @@ static int timerfd_setup(struct timerfd_ctx *ctx, int flags,
 	htmode = (flags & TFD_TIMER_ABSTIME) ?
 		HRTIMER_MODE_ABS: HRTIMER_MODE_REL;
 
-	ctx->might_cancel = false;
-	if (htmode == HRTIMER_MODE_ABS && ctx->clockid == CLOCK_REALTIME &&
-	    (flags & TFD_TIMER_CANCELON_SET)) {
-		clockid = CLOCK_REALTIME_COS;
-		ctx->might_cancel = true;
-	}
-
 	texp = timespec_to_ktime(ktmr->it_value);
 	ctx->expired = 0;
 	ctx->ticks = 0;
@@ -113,8 +153,9 @@ static int timerfd_release(struct inode *inode, struct file *file)
 {
 	struct timerfd_ctx *ctx = file->private_data;
 
+	timerfd_remove_cancel(ctx);
 	hrtimer_cancel(&ctx->tmr);
-	kfree(ctx);
+	kfree_rcu(ctx, rcu);
 	return 0;
 }
 
@@ -149,20 +190,20 @@ static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
 	else
 		res = wait_event_interruptible_locked_irq(ctx->wqh, ctx->ticks);
 
+	/*
+	 * If clock has changed, we do not care about the
+	 * ticks and we do not rearm the timer. Userspace must
+	 * reevaluate anyway.
+	 */
+	if (timerfd_canceled(ctx)) {
+		ctx->ticks = 0;
+		ctx->expired = 0;
+		res = -ECANCELED;
+	}
+
 	if (ctx->ticks) {
 		ticks = ctx->ticks;
 
-		/*
-		 * If clock has changed, we do not care about the
-		 * ticks and we do not rearm the timer. Userspace must
-		 * reevaluate anyway.
-		 */
-		if (timerfd_canceled(ctx)) {
-			ticks = 0;
-			ctx->expired = 0;
-			res = -ECANCELED;
-		}
-
 		if (ctx->expired && ctx->tintv.tv64) {
 			/*
 			 * If tintv.tv64 != 0, this is a periodic timer that
@@ -258,6 +299,8 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
 		return PTR_ERR(file);
 	ctx = file->private_data;
 
+	timerfd_setup_cancel(ctx, flags);
+
 	/*
 	 * We need to stop the existing timer before reprogramming
 	 * it to the new values.