Messages in this thread Patch in this message |  | | Date | Sat, 22 Oct 2016 11:07:37 -0000 | From | Thomas Gleixner <> | Subject | [patch 2/2] timers: Prevent base clock corruption when forwarding |
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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 therefor advancing base->clk after the next expiry value.
So we need to read jiffies once 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 <scoopta@gmail.com> Reported-by: Michal Necasek <michal.necasek@oracle.com> Cc: stable@vger.kernel.org Cc: stern@rowland.harvard.edu Cc: michael.thayer@oracle.com Cc: knut.osmundsen@oracle.com Cc: Peter Zijlstra <peterz@infradead.org> Cc: rt@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- kernel/time/timer.c | 23 ++++++++++------------- 1 file changed, 10 insertions(+), 13 deletions(-)
--- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -878,7 +878,7 @@ static inline struct timer_base *get_tim #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 @@ static inline struct timer_base * 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 @@ static inline struct timer_base * 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 @@ -1025,6 +1019,9 @@ static inline int } } + /* 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
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