Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754340AbbFEJI2 (ORCPT ); Fri, 5 Jun 2015 05:08:28 -0400 Received: from forward11h.cmail.yandex.net ([87.250.230.153]:49933 "EHLO forward11h.cmail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751748AbbFEJIZ (ORCPT ); Fri, 5 Jun 2015 05:08:25 -0400 X-Greylist: delayed 367 seconds by postgrey-1.27 at vger.kernel.org; Fri, 05 Jun 2015 05:08:24 EDT From: Kirill Tkhai To: Peter Zijlstra Cc: "umgwanakikbuti@gmail.com" , "mingo@elte.hu" , "ktkhai@parallels.com" , "rostedt@goodmis.org" , "tglx@linutronix.de" , "juri.lelli@gmail.com" , "pang.xunlei@linaro.org" , "oleg@redhat.com" , "wanpeng.li@linux.intel.com" , "linux-kernel@vger.kernel.org" In-Reply-To: <5883311433494921@web27h.yandex.ru> References: <20150603132903.203333087@infradead.org> <20150603134023.156059118@infradead.org> <214021433348760@web25g.yandex.ru> <20150603211324.GC3644@twins.programming.kicks-ass.net> <2134411433408823@web8j.yandex.ru> <20150604104902.GH3644@twins.programming.kicks-ass.net> <5883311433494921@web27h.yandex.ru> Subject: Re: [PATCH 8/9] hrtimer: Allow hrtimer::function() to free the timer MIME-Version: 1.0 Message-Id: <5894071433495014@web27h.yandex.ru> X-Mailer: Yamail [ http://yandex.ru ] 5.0 Date: Fri, 05 Jun 2015 12:03:34 +0300 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=koi8-r Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9366 Lines: 255 This message is too late, /me going to see new series :) 05.06.2015, 12:02, "Kirill Tkhai" : > ? ??, 04/06/2015 ? 12:49 +0200, Peter Zijlstra ?????: > On Thu, Jun 04, 2015 at 12:07:03PM +0300, Kirill Tkhai wrote: >>>> ?--- a/include/linux/hrtimer.h >>>> ?+++ b/include/linux/hrtimer.h >>>> ?@@ -391,11 +391,25 @@ extern u64 hrtimer_get_next_event(void); >>>> ???* A timer is active, when it is enqueued into the rbtree or the >>>> ???* callback function is running or it's in the state of being migrated >>>> ???* to another cpu. >>>> ?+ * >>>> ?+ * See __run_hrtimer(). >>>> ???*/ >>>> ?-static inline int hrtimer_active(const struct hrtimer *timer) >>>> ?+static inline bool hrtimer_active(const struct hrtimer *timer) >>>> ??{ >>>> ?- return timer->state != HRTIMER_STATE_INACTIVE || >>>> ?- timer->base->running == timer; >>>> ?+ if (timer->state != HRTIMER_STATE_INACTIVE) >>>> ?+ return true; >>>> ?+ >>>> ?+ smp_rmb(); /* C matches A */ >>>> ?+ >>>> ?+ if (timer->base->running == timer) >>>> ?+ return true; >>>> ?+ >>>> ?+ smp_rmb(); /* D matches B */ >>>> ?+ >>>> ?+ if (timer->state != HRTIMER_STATE_INACTIVE) >>>> ?+ return true; >>>> ?+ >>>> ?+ return false; >>> ?This races with two sequential timer handlers. hrtimer_active() >>> ?is preemptible everywhere, and no guarantees that all three "if" >>> ?conditions check the same timer tick. >> ?Indeed. >>> ?How about transformation of hrtimer_bases.lock: raw_spinlock_t --> seqlock_t? >> ?Ingo will like that because it means we already need to touch cpu_base. >> >> ?But I think there's a problem there on timer migration, the timer can >> ?migrate between bases while we do the seq read loop and then you can get >> ?false positives on the different seqcount numbers. >> >> ?We could of course do something like the below, but hrtimer_is_active() >> ?is turning into quite the monster. >> >> ?Needs more comments at the very least, its fully of trickery. > > Yeah, it's safe for now, but it may happen difficulties with a support > in the future, because barrier logic is not easy to review. But it seems > we may simplify it a little bit. Please, see the comments below. >> ?--- >> ?--- a/include/linux/hrtimer.h >> ?+++ b/include/linux/hrtimer.h >> ?@@ -59,7 +59,9 @@ enum hrtimer_restart { >> ???* mean touching the timer after the callback, this makes it impossible to free >> ???* the timer from the callback function. >> ???* >> ?- * Therefore we track the callback state in timer->base->running == timer. >> ?+ * Therefore we track the callback state in: >> ?+ * >> ?+ * timer->base->cpu_base->running == timer >> ???* >> ???* On SMP it is possible to have a "callback function running and enqueued" >> ???* status. It happens for example when a posix timer expired and the callback >> ?@@ -144,7 +146,6 @@ struct hrtimer_clock_base { >> ??????????struct timerqueue_head active; >> ??????????ktime_t (*get_time)(void); >> ??????????ktime_t offset; >> ?- struct hrtimer *running; >> ??} __attribute__((__aligned__(HRTIMER_CLOCK_BASE_ALIGN))); >> >> ??enum ?hrtimer_base_type { >> ?@@ -159,6 +160,8 @@ enum ?hrtimer_base_type { >> ???* struct hrtimer_cpu_base - the per cpu clock bases >> ???* @lock: lock protecting the base and associated clock bases >> ???* and timers >> ?+ * @seq: seqcount around __run_hrtimer >> ?+ * @running: pointer to the currently running hrtimer >> ???* @cpu: cpu number >> ???* @active_bases: Bitfield to mark bases with active timers >> ???* @clock_was_set_seq: Sequence counter of clock was set events >> ?@@ -180,6 +183,8 @@ enum ?hrtimer_base_type { >> ???*/ >> ??struct hrtimer_cpu_base { >> ??????????raw_spinlock_t lock; >> ?+ seqcount_t seq; >> ?+ struct hrtimer *running; >> ??????????unsigned int cpu; >> ??????????unsigned int active_bases; >> ??????????unsigned int clock_was_set_seq; >> ?@@ -394,8 +399,24 @@ extern u64 hrtimer_get_next_event(void); >> ???*/ >> ??static inline int hrtimer_active(const struct hrtimer *timer) >> ??{ >> ?- return timer->state != HRTIMER_STATE_INACTIVE || >> ?- timer->base->running == timer; >> ?+ struct hrtimer_cpu_base *cpu_base; >> ?+ unsigned int seq; >> ?+ bool active; >> ?+ >> ?+ do { >> ?+ active = false; >> ?+ cpu_base = READ_ONCE(timer->base->cpu_base); >> ?+ seqcount_lockdep_reader_access(&cpu_base->seq); >> ?+ seq = raw_read_seqcount(&cpu_base->seq); >> ?+ >> ?+ if (timer->state != HRTIMER_STATE_INACTIVE || >> ?+ ???cpu_base->running == timer) >> ?+ active = true; >> ?+ >> ?+ } while (read_seqcount_retry(&cpu_base->seq, seq) || >> ?+ cpu_base != READ_ONCE(timer->base->cpu_base)); >> ?+ >> ?+ return active; >> ??} > > This may race with migrate_hrtimer_list(), so it needs write seqcounter > too. >> ??/* >> ?@@ -412,7 +433,7 @@ static inline int hrtimer_is_queued(stru >> ???*/ >> ??static inline int hrtimer_callback_running(struct hrtimer *timer) >> ??{ >> ?- return timer->base->running == timer; >> ?+ return timer->base->cpu_base->running == timer; >> ??} >> >> ??/* Forward a hrtimer so it expires after now: */ >> ?--- a/kernel/time/hrtimer.c >> ?+++ b/kernel/time/hrtimer.c >> ?@@ -67,6 +67,7 @@ >> ??DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) = >> ??{ >> ??????????.lock = __RAW_SPIN_LOCK_UNLOCKED(hrtimer_bases.lock), >> ?+ .seq = SEQCNT_ZERO(hrtimer_bases.seq), >> ??????????.clock_base = >> ??????????{ >> ??????????????????{ >> ?@@ -113,9 +114,15 @@ static inline int hrtimer_clockid_to_bas >> ??/* >> ???* We require the migration_base for lock_hrtimer_base()/switch_hrtimer_base() >> ???* such that hrtimer_callback_running() can unconditionally dereference >> ?- * timer->base. >> ?+ * timer->base->cpu_base >> ???*/ >> ?-static struct hrtimer_clock_base migration_base; >> ?+static struct hrtimer_cpu_base migration_cpu_base = { >> ?+ .seq = SEQCNT_ZERO(migration_cpu_base), >> ?+}; >> ?+ >> ?+static struct hrtimer_clock_base migration_base { >> ?+ .cpu_base = &migration_cpu_base, >> ?+}; >> >> ??/* >> ???* We are using hashed locking: holding per_cpu(hrtimer_bases)[n].lock >> ?@@ -1118,10 +1125,16 @@ static void __run_hrtimer(struct hrtimer >> ??????????enum hrtimer_restart (*fn)(struct hrtimer *); >> ??????????int restart; >> >> ?- WARN_ON(!irqs_disabled()); >> ?+ lockdep_assert_held(&cpu_base->lock); >> >> ??????????debug_deactivate(timer); >> ?- base->running = timer; >> ?+ cpu_base->running = timer; > > My suggestion is do not use seqcounters for long parts of code, and implement > short primitives for changing timer state and cpu_base running timer. Something > like this: > > static inline void hrtimer_set_state(struct hrtimer *timer, unsigned long state) > { > ????????struct hrtimer_cpu_base *cpu_base = timer->base->cpu_base; > > ????????lockdep_assert_held(&cpu_base->lock); > > ????????write_seqcount_begin(&cpu_base->seq); > ????????timer->state = state; > ????????write_seqcount_end(&cpu_base->seq); > } > > static inline void cpu_base_set_running(struct hrtimer_cpu_base *cpu_base, > ????????????????????????????????????????struct hrtimer *timer) > { > ????????lockdep_assert_held(&cpu_base->lock); > > ????????write_seqcount_begin(&cpu_base->seq); > ????????cpu_base->running = timer; > ????????write_seqcount_end(&cpu_base->seq); > } > > Implemented this, we may less think about right barrier order, because > all changes are being made under seqcount. > > static inline int hrtimer_active(const struct hrtimer *timer) > { > ????????struct hrtimer_cpu_base *cpu_base; > ????????struct hrtimer_clock_base *base; > ????????unsigned int seq; > ????????bool active = false; > > ????????do { > ????????????????base = READ_ONCE(timer->base); > ????????????????if (base == &migration_base) { > ????????????????????????active = true; > ????????????????????????break; > ????????????????} > > ????????????????cpu_base = base->cpu_base; > ????????????????seqcount_lockdep_reader_access(&cpu_base->seq); > ????????????????seq = raw_read_seqcount(&cpu_base->seq); > > ????????????????if (timer->state != HRTIMER_STATE_INACTIVE || > ????????????????????cpu_base->running == timer) { > ????????????????????????active = true; > ????????????????????????break; > ????????????????} > ????????} while (read_seqcount_retry(&cpu_base->seq, seq) || > ?????????????????READ_ONCE(timer->base) != base); > > ????????return active; > } >> ?+ >> ?+ /* >> ?+ * separate the ->running assignment from the ->state assignment >> ?+ */ >> ?+ write_seqcount_begin(&cpu_base->seq); >> ?+ >> ??????????__remove_hrtimer(timer, base, HRTIMER_STATE_INACTIVE, 0); >> ??????????timer_stats_account_hrtimer(timer); >> ??????????fn = timer->function; >> ?@@ -1150,8 +1163,13 @@ static void __run_hrtimer(struct hrtimer >> ??????????????!(timer->state & HRTIMER_STATE_ENQUEUED)) >> ??????????????????enqueue_hrtimer(timer, base); >> >> ?- WARN_ON_ONCE(base->running != timer); >> ?- base->running = NULL; >> ?+ /* >> ?+ * separate the ->running assignment from the ->state assignment >> ?+ */ >> ?+ write_seqcount_end(&cpu_base->seq); >> ?+ >> ?+ WARN_ON_ONCE(cpu_base->running != timer); >> ?+ cpu_base->running = NULL; >> ??} >> >> ??static void __hrtimer_run_queues(struct hrtimer_cpu_base *cpu_base, ktime_t now) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/