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>
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 @@
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,
@@ -53,6 +59,63 @@
return HRTIMER_NORESTART;
}
+/*
+ * Called when the clock was set to cancel the timers in the cancel
+ * list.
+ */
+void timerfd_clock_was_set(void)
+{
+ ktime_t moffs = ktime_get_monotonic_offset();
+ struct timerfd_ctx *ctx;
+ unsigned long flags;
+
+ 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 void timerfd_remove_cancel(struct timerfd_ctx *ctx)
+{
+ if (ctx->might_cancel) {
+ ctx->might_cancel = false;
+ spin_lock(&cancel_lock);
+ list_del_rcu(&ctx->clist);
+ spin_unlock(&cancel_lock);
+ }
+}
+
+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;
+}
+
+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);
+ }
+}
+
static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
{
ktime_t remaining;
@@ -61,22 +124,6 @@
return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}
-static bool timerfd_canceled(struct timerfd_ctx *ctx)
-{
- ktime_t moffs;
-
- if (!ctx->might_cancel)
- return false;
-
- moffs = ktime_get_monotonic_offset();
-
- if (moffs.tv64 == ctx->moffs.tv64)
- return false;
-
- ctx->moffs = moffs;
- return true;
-}
-
static int timerfd_setup(struct timerfd_ctx *ctx, int flags,
const struct itimerspec *ktmr)
{
@@ -87,13 +134,6 @@
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 @@
{
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 @@
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 @@
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.