[PATCH RT 16/25][RFC 3.0.23-rt39-rc1] seqlock: Prevent rt starvation
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2012-03-06 16:20:10
Also in:
lkml
Subsystem:
locking primitives, networking [general], the rest · Maintainers:
Peter Zijlstra, Ingo Molnar, Will Deacon, Boqun Feng, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Thomas Gleixner <redacted> If a low prio writer gets preempted while holding the seqlock write locked, a high prio reader spins forever on RT. To prevent this let the reader grab the spinlock, so it blocks and eventually boosts the writer. This way the writer can proceed and endless spinning is prevented. Signed-off-by: Thomas Gleixner <redacted> Cc: stable-rt@vger.kernel.org Signed-off-by: Steven Rostedt <rostedt@goodmis.org> --- include/linux/seqlock.h | 23 +++++++++++++++++++++++ include/net/neighbour.h | 2 +- 2 files changed, 24 insertions(+), 1 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index b44048d..29ffd4f 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h@@ -177,10 +177,33 @@ typedef struct { /* * Read side functions for starting and finalizing a read side section. */ +#ifndef CONFIG_PREEMPT_RT_FULL static inline unsigned read_seqbegin(const seqlock_t *sl) { return read_seqcount_begin(&sl->seqcount); } +#else +/* + * Starvation safe read side for RT + */ +static inline unsigned read_seqbegin(seqlock_t *sl) +{ + unsigned ret; + +repeat: + ret = sl->seqcount.sequence; + if (unlikely(ret & 1)) { + /* + * Take the lock and let the writer proceed (i.e. evtl + * boost it), otherwise we could loop here forever. + */ + spin_lock(&sl->lock); + spin_unlock(&sl->lock); + goto repeat; + } + return ret; +} +#endif static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) {
diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 4014b62..9c1cb97 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h@@ -377,7 +377,7 @@ struct neighbour_cb { #define NEIGH_CB(skb) ((struct neighbour_cb *)(skb)->cb) -static inline void neigh_ha_snapshot(char *dst, const struct neighbour *n, +static inline void neigh_ha_snapshot(char *dst, struct neighbour *n, const struct net_device *dev) { unsigned int seq;
--
1.7.8.3