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>

5 files changed, 113 insertions, 102 deletions

diff --git a/fs/timerfd.c b/fs/timerfd.c
index 7e14c9e7c4ee..f67acbdda5e8 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.
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index eda4ccde0730..925c8c01db7b 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -155,7 +155,6 @@ enum  hrtimer_base_type {
         HRTIMER_BASE_REALTIME,
         HRTIMER_BASE_MONOTONIC,
         HRTIMER_BASE_BOOTTIME,
-        HRTIMER_BASE_REALTIME_COS,
         HRTIMER_MAX_CLOCK_BASES,
 };
 
@@ -306,6 +305,11 @@ static inline int hrtimer_is_hres_active(struct hrtimer *timer)
 #endif
 
 extern void clock_was_set(void);
+#ifdef CONFIG_TIMERFD
+extern void timerfd_clock_was_set(void);
+#else
+static inline void timerfd_clock_was_set(void) { }
+#endif
 extern void hrtimers_resume(void);
 
 extern ktime_t ktime_get(void);
diff --git a/include/linux/time.h b/include/linux/time.h
index a9242773eb24..b3061782dec3 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -302,12 +302,6 @@ struct itimerval {
  * The IDs of various hardware clocks:
  */
 #define CLOCK_SGI_CYCLE                 10
-
-#ifdef __KERNEL__
-/* This clock is not exposed to user space */
-#define CLOCK_REALTIME_COS              15
-#endif
-
 #define MAX_CLOCKS                      16
 #define CLOCKS_MASK                     (CLOCK_REALTIME | CLOCK_MONOTONIC)
 #define CLOCKS_MONO                     CLOCK_MONOTONIC
diff --git a/include/linux/timerfd.h b/include/linux/timerfd.h
index e9571fc8f1a0..d3b57fa12225 100644
--- a/include/linux/timerfd.h
+++ b/include/linux/timerfd.h
@@ -19,7 +19,7 @@
  * shared O_* flags.
  */
 #define TFD_TIMER_ABSTIME (1 << 0)
-#define TFD_TIMER_CANCELON_SET (1 << 1)
+#define TFD_TIMER_CANCEL_ON_SET (1 << 1)
 #define TFD_CLOEXEC O_CLOEXEC
 #define TFD_NONBLOCK O_NONBLOCK
 
@@ -27,6 +27,6 @@
 /* Flags for timerfd_create.  */
 #define TFD_CREATE_FLAGS TFD_SHARED_FCNTL_FLAGS
 /* Flags for timerfd_settime.  */
-#define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_TIMER_CANCELON_SET)
+#define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET)
 
 #endif /* _LINUX_TIMERFD_H */
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index eabcbd781433..26dd32f9f6b2 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -78,11 +78,6 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) =
                         .get_time = &ktime_get_boottime,
                         .resolution = KTIME_LOW_RES,
                 },
-                {
-                        .index = CLOCK_REALTIME_COS,
-                        .get_time = &ktime_get_real,
-                        .resolution = KTIME_LOW_RES,
-                },
         }
 };
 
@@ -90,7 +85,6 @@ static const int hrtimer_clock_to_base_table[MAX_CLOCKS] = {
         [CLOCK_REALTIME]        = HRTIMER_BASE_REALTIME,
         [CLOCK_MONOTONIC]       = HRTIMER_BASE_MONOTONIC,
         [CLOCK_BOOTTIME]        = HRTIMER_BASE_BOOTTIME,
-        [CLOCK_REALTIME_COS]    = HRTIMER_BASE_REALTIME_COS,
 };
 
 static inline int hrtimer_clockid_to_base(clockid_t clock_id)
@@ -116,7 +110,6 @@ static void hrtimer_get_softirq_time(struct hrtimer_cpu_base *base)
         base->clock_base[HRTIMER_BASE_REALTIME].softirq_time = xtim;
         base->clock_base[HRTIMER_BASE_MONOTONIC].softirq_time = mono;
         base->clock_base[HRTIMER_BASE_BOOTTIME].softirq_time = boot;
-        base->clock_base[HRTIMER_BASE_REALTIME_COS].softirq_time = xtim;
 }
 
 /*
@@ -486,8 +479,6 @@ static inline void debug_deactivate(struct hrtimer *timer)
         trace_hrtimer_cancel(timer);
 }
 
-static void hrtimer_expire_cancelable(struct hrtimer_cpu_base *cpu_base);
-
 /* High resolution timer related functions */
 #ifdef CONFIG_HIGH_RES_TIMERS
 
@@ -663,7 +654,33 @@ static inline int hrtimer_enqueue_reprogram(struct hrtimer *timer,
         return 0;
 }
 
-static void retrigger_next_event(void *arg);
+/*
+ * Retrigger next event is called after clock was set
+ *
+ * Called with interrupts disabled via on_each_cpu()
+ */
+static void retrigger_next_event(void *arg)
+{
+        struct hrtimer_cpu_base *base = &__get_cpu_var(hrtimer_bases);
+        struct timespec realtime_offset, xtim, wtm, sleep;
+
+        if (!hrtimer_hres_active())
+                return;
+
+        /* Optimized out for !HIGH_RES */
+        get_xtime_and_monotonic_and_sleep_offset(&xtim, &wtm, &sleep);
+        set_normalized_timespec(&realtime_offset, -wtm.tv_sec, -wtm.tv_nsec);
+
+        /* Adjust CLOCK_REALTIME offset */
+        raw_spin_lock(&base->lock);
+        base->clock_base[HRTIMER_BASE_REALTIME].offset =
+                timespec_to_ktime(realtime_offset);
+        base->clock_base[HRTIMER_BASE_BOOTTIME].offset =
+                timespec_to_ktime(sleep);
+
+        hrtimer_force_reprogram(base, 0);
+        raw_spin_unlock(&base->lock);
+}
 
 /*
  * Switch to high resolution mode
@@ -711,46 +728,11 @@ static inline int hrtimer_enqueue_reprogram(struct hrtimer *timer,
         return 0;
 }
 static inline void hrtimer_init_hres(struct hrtimer_cpu_base *base) { }
+static inline void retrigger_next_event(void *arg) { }
 
 #endif /* CONFIG_HIGH_RES_TIMERS */
 
 /*
- * Retrigger next event is called after clock was set
- *
- * Called with interrupts disabled via on_each_cpu()
- */
-static void retrigger_next_event(void *arg)
-{
-        struct hrtimer_cpu_base *base = &__get_cpu_var(hrtimer_bases);
-        struct timespec realtime_offset, xtim, wtm, sleep;
-
-        if (!hrtimer_hres_active()) {
-                raw_spin_lock(&base->lock);
-                hrtimer_expire_cancelable(base);
-                raw_spin_unlock(&base->lock);
-                return;
-        }
-
-        /* Optimized out for !HIGH_RES */
-        get_xtime_and_monotonic_and_sleep_offset(&xtim, &wtm, &sleep);
-        set_normalized_timespec(&realtime_offset, -wtm.tv_sec, -wtm.tv_nsec);
-
-        /* Adjust CLOCK_REALTIME offset */
-        raw_spin_lock(&base->lock);
-        base->clock_base[HRTIMER_BASE_REALTIME].offset =
-                timespec_to_ktime(realtime_offset);
-        base->clock_base[HRTIMER_BASE_BOOTTIME].offset =
-                timespec_to_ktime(sleep);
-        base->clock_base[HRTIMER_BASE_REALTIME_COS].offset =
-                timespec_to_ktime(realtime_offset);
-
-        hrtimer_expire_cancelable(base);
-
-        hrtimer_force_reprogram(base, 0);
-        raw_spin_unlock(&base->lock);
-}
-
-/*
  * Clock realtime was set
  *
  * Change the offset of the realtime clock vs. the monotonic
@@ -763,8 +745,11 @@ static void retrigger_next_event(void *arg)
  */
 void clock_was_set(void)
 {
+#ifdef CONFIG_HIGHRES_TIMERS
         /* Retrigger the CPU local events everywhere */
         on_each_cpu(retrigger_next_event, NULL, 1);
+#endif
+        timerfd_clock_was_set();
 }
 
 /*
@@ -777,6 +762,7 @@ void hrtimers_resume(void)
                   KERN_INFO "hrtimers_resume() called with IRQs enabled!");
 
         retrigger_next_event(NULL);
+        timerfd_clock_was_set();
 }
 
 static inline void timer_stats_hrtimer_set_start_info(struct hrtimer *timer)
@@ -1240,22 +1226,6 @@ static void __run_hrtimer(struct hrtimer *timer, ktime_t *now)
         timer->state &= ~HRTIMER_STATE_CALLBACK;
 }
 
-static void hrtimer_expire_cancelable(struct hrtimer_cpu_base *cpu_base)
-{
-        struct timerqueue_node *node;
-        struct hrtimer_clock_base *base;
-        ktime_t now = ktime_get_real();
-
-        base = &cpu_base->clock_base[HRTIMER_BASE_REALTIME_COS];
-
-        while ((node = timerqueue_getnext(&base->active))) {
-                        struct hrtimer *timer;
-
-                        timer = container_of(node, struct hrtimer, node);
-                        __run_hrtimer(timer, &now);
-        }
-}
-
 #ifdef CONFIG_HIGH_RES_TIMERS
 
 /*