From 6bad6bccf2d717f652d37e63cf261eaa23466009 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sat, 22 Oct 2016 11:07:37 +0000 Subject: [PATCH] timers: Prevent base clock corruption when forwarding When a timer is enqueued we try to forward the timer base clock. This mechanism has two issues: 1) Forwarding a remote base unlocked The forwarding function is called from get_target_base() with the current timer base lock held. But if the new target base is a different base than the current base (can happen with NOHZ, sigh!) then the forwarding is done on an unlocked base. This can lead to corruption of base->clk. Solution is simple: Invoke the forwarding after the target base is locked. 2) Possible corruption due to jiffies advancing This is similar to the issue in get_net_timer_interrupt() which was fixed in the previous patch. jiffies can advance between check and assignement and therefore advancing base->clk beyond the next expiry value. So we need to read jiffies into a local variable once and do the checks and assignment with the local copy. Fixes: a683f390b93f("timers: Forward the wheel clock whenever possible") Reported-by: Ashton Holmes Reported-by: Michael Thayer Signed-off-by: Thomas Gleixner Cc: Michal Necasek Cc: Peter Zijlstra Cc: knut.osmundsen@oracle.com Cc: stable@vger.kernel.org Cc: stern@rowland.harvard.edu Cc: rt@linutronix.de Link: http://lkml.kernel.org/r/20161022110552.253640125@linutronix.de Signed-off-by: Thomas Gleixner --- kernel/time/timer.c | 23 ++++++++++------------- 1 file changed, 10 insertions(+), 13 deletions(-) diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 7c446fb5163a..c611c47de884 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -878,7 +878,7 @@ static inline struct timer_base *get_timer_base(u32 tflags) #ifdef CONFIG_NO_HZ_COMMON static inline struct timer_base * -__get_target_base(struct timer_base *base, unsigned tflags) +get_target_base(struct timer_base *base, unsigned tflags) { #ifdef CONFIG_SMP if ((tflags & TIMER_PINNED) || !base->migration_enabled) @@ -891,25 +891,27 @@ __get_target_base(struct timer_base *base, unsigned tflags) static inline void forward_timer_base(struct timer_base *base) { + unsigned long jnow = READ_ONCE(jiffies); + /* * We only forward the base when it's idle and we have a delta between * base clock and jiffies. */ - if (!base->is_idle || (long) (jiffies - base->clk) < 2) + if (!base->is_idle || (long) (jnow - base->clk) < 2) return; /* * If the next expiry value is > jiffies, then we fast forward to * jiffies otherwise we forward to the next expiry value. */ - if (time_after(base->next_expiry, jiffies)) - base->clk = jiffies; + if (time_after(base->next_expiry, jnow)) + base->clk = jnow; else base->clk = base->next_expiry; } #else static inline struct timer_base * -__get_target_base(struct timer_base *base, unsigned tflags) +get_target_base(struct timer_base *base, unsigned tflags) { return get_timer_this_cpu_base(tflags); } @@ -917,14 +919,6 @@ __get_target_base(struct timer_base *base, unsigned tflags) static inline void forward_timer_base(struct timer_base *base) { } #endif -static inline struct timer_base * -get_target_base(struct timer_base *base, unsigned tflags) -{ - struct timer_base *target = __get_target_base(base, tflags); - - forward_timer_base(target); - return target; -} /* * We are using hashed locking: Holding per_cpu(timer_bases[x]).lock means @@ -1037,6 +1031,9 @@ __mod_timer(struct timer_list *timer, unsigned long expires, bool pending_only) } } + /* Try to forward a stale timer base clock */ + forward_timer_base(base); + timer->expires = expires; /* * If 'idx' was calculated above and the base time did not advance