71 lines
2.0 KiB
Diff
71 lines
2.0 KiB
Diff
Subject: seqlock: Prevent rt starvation
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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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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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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 | 23 +++++++++++++++++++++++
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include/net/neighbour.h | 2 +-
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2 files changed, 24 insertions(+), 1 deletion(-)
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Index: linux-3.2/include/linux/seqlock.h
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===================================================================
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--- linux-3.2.orig/include/linux/seqlock.h
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+++ linux-3.2/include/linux/seqlock.h
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@@ -177,10 +177,33 @@ 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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Index: linux-3.2/include/net/neighbour.h
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===================================================================
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--- linux-3.2.orig/include/net/neighbour.h
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+++ linux-3.2/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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