172 lines
4.6 KiB
Diff
172 lines
4.6 KiB
Diff
From 4d679adbfa52d738a7e72a71c5b63582c55fb487 Mon Sep 17 00:00:00 2001
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From: Thomas Gleixner <tglx@linutronix.de>
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Date: Wed, 22 Feb 2012 12:03:30 +0100
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Subject: [PATCH 253/267] seqlock: Prevent rt starvation
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If a low prio writer gets preempted while holding the seqlock write
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locked, a high prio reader spins forever on RT.
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To prevent this let the reader grab the spinlock, so it blocks and
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eventually boosts the writer. This way the writer can proceed and
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endless spinning is prevented.
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For seqcount writers we disable preemption over the update code
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path. Thaanks to Al Viro for distangling some VFS code to make that
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possible.
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Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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Cc: stable-rt@vger.kernel.org
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---
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include/linux/seqlock.h | 55 ++++++++++++++++++++++++++++++++++++++---------
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include/net/neighbour.h | 2 +-
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2 files changed, 46 insertions(+), 11 deletions(-)
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diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
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index 3e1f3f9..a1dbf48 100644
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--- a/include/linux/seqlock.h
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+++ b/include/linux/seqlock.h
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@@ -125,18 +125,30 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
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* Sequence counter only version assumes that callers are using their
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* own mutexing.
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*/
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-static inline void write_seqcount_begin(seqcount_t *s)
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+static inline void __write_seqcount_begin(seqcount_t *s)
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{
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s->sequence++;
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smp_wmb();
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}
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-static inline void write_seqcount_end(seqcount_t *s)
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+static inline void write_seqcount_begin(seqcount_t *s)
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+{
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+ preempt_disable_rt();
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+ __write_seqcount_begin(s);
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+}
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+
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+static inline void __write_seqcount_end(seqcount_t *s)
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{
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smp_wmb();
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s->sequence++;
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}
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+static inline void write_seqcount_end(seqcount_t *s)
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+{
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+ __write_seqcount_end(s);
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+ preempt_enable_rt();
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+}
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+
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/**
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* write_seqcount_barrier - invalidate in-progress read-side seq operations
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* @s: pointer to seqcount_t
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@@ -177,10 +189,33 @@ typedef struct {
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/*
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* Read side functions for starting and finalizing a read side section.
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*/
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+#ifndef CONFIG_PREEMPT_RT_FULL
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static inline unsigned read_seqbegin(const seqlock_t *sl)
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{
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return read_seqcount_begin(&sl->seqcount);
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}
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+#else
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+/*
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+ * Starvation safe read side for RT
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+ */
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+static inline unsigned read_seqbegin(seqlock_t *sl)
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+{
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+ unsigned ret;
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+
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+repeat:
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+ ret = sl->seqcount.sequence;
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+ if (unlikely(ret & 1)) {
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+ /*
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+ * Take the lock and let the writer proceed (i.e. evtl
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+ * boost it), otherwise we could loop here forever.
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+ */
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+ spin_lock(&sl->lock);
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+ spin_unlock(&sl->lock);
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+ goto repeat;
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+ }
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+ return ret;
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+}
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+#endif
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static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
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{
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@@ -195,36 +230,36 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
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static inline void write_seqlock(seqlock_t *sl)
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{
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spin_lock(&sl->lock);
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- write_seqcount_begin(&sl->seqcount);
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+ __write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock(seqlock_t *sl)
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{
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- write_seqcount_end(&sl->seqcount);
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+ __write_seqcount_end(&sl->seqcount);
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spin_unlock(&sl->lock);
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}
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static inline void write_seqlock_bh(seqlock_t *sl)
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{
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spin_lock_bh(&sl->lock);
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- write_seqcount_begin(&sl->seqcount);
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+ __write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock_bh(seqlock_t *sl)
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{
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- write_seqcount_end(&sl->seqcount);
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+ __write_seqcount_end(&sl->seqcount);
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spin_unlock_bh(&sl->lock);
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}
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static inline void write_seqlock_irq(seqlock_t *sl)
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{
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spin_lock_irq(&sl->lock);
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- write_seqcount_begin(&sl->seqcount);
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+ __write_seqcount_begin(&sl->seqcount);
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}
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static inline void write_sequnlock_irq(seqlock_t *sl)
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{
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- write_seqcount_end(&sl->seqcount);
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+ __write_seqcount_end(&sl->seqcount);
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spin_unlock_irq(&sl->lock);
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}
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@@ -233,7 +268,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
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unsigned long flags;
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spin_lock_irqsave(&sl->lock, flags);
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- write_seqcount_begin(&sl->seqcount);
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+ __write_seqcount_begin(&sl->seqcount);
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return flags;
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}
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@@ -243,7 +278,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
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static inline void
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write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
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{
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- write_seqcount_end(&sl->seqcount);
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+ __write_seqcount_end(&sl->seqcount);
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spin_unlock_irqrestore(&sl->lock, flags);
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}
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diff --git a/include/net/neighbour.h b/include/net/neighbour.h
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index 2720884..6fda9fa 100644
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--- a/include/net/neighbour.h
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+++ b/include/net/neighbour.h
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@@ -385,7 +385,7 @@ struct neighbour_cb {
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#define NEIGH_CB(skb) ((struct neighbour_cb *)(skb)->cb)
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-static inline void neigh_ha_snapshot(char *dst, const struct neighbour *n,
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+static inline void neigh_ha_snapshot(char *dst, struct neighbour *n,
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const struct net_device *dev)
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{
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unsigned int seq;
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--
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1.7.10
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