Author: sephe
Date: Mon Feb 29 03:54:51 2016
New Revision: 296178
URL: https://svnweb.freebsd.org/changeset/base/296178
Log:
buf_ring/drbr: Add buf_ring_peek_clear_sc and use it in drbr_peek
Unlike buf_ring_peek, it only supports single consumer mode, and it
clears the cons_head if DEBUG_BUFRING/INVARIANTS is defined.
The normal use case of drbr_peek for network drivers is:
m = drbr_peek(br);
err = hw_spec_encap(&m); /* could m_defrag/m_collapse */
(*)
if (err) {
if (m == NULL)
drbr_advance(br);
else
drbr_putback(br, m);
/* break the loop */
}
drbr_advance(br);
The race is:
If hw_spec_encap() m_defrag or m_collapse the mbuf, i.e. the old mbuf
was freed, or like the Hyper-V's network driver, that transmission-
done does not even require the TX lock; then on the other CPU at the
(*) time, the freed mbuf could be recycled and being drbr_enqueue even
before the current CPU had the chance to call drbr_{advance,putback}.
This triggers a panic in drbr_enqueue duplicated element check, if
DEBUG_BUFRING/INVARIANTS is defined.
Use buf_ring_peek_clear_sc() in drbr_peek() to fix the above race.
This change is a NO-OP, if neither DEBUG_BUFRING nor INVARIANTS are
defined.
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5416
Modified:
head/sys/net/ifq.h
head/sys/sys/buf_ring.h
Modified: head/sys/net/ifq.h
==============================================================================
--- head/sys/net/ifq.h Mon Feb 29 03:38:00 2016 (r296177)
+++ head/sys/net/ifq.h Mon Feb 29 03:54:51 2016 (r296178)
@@ -369,7 +369,7 @@ drbr_peek(struct ifnet *ifp, struct buf_
return (m);
}
#endif
- return(buf_ring_peek(br));
+ return(buf_ring_peek_clear_sc(br));
}
static __inline void
Modified: head/sys/sys/buf_ring.h
==============================================================================
--- head/sys/sys/buf_ring.h Mon Feb 29 03:38:00 2016 (r296177)
+++ head/sys/sys/buf_ring.h Mon Feb 29 03:54:51 2016 (r296178)
@@ -268,6 +268,37 @@ buf_ring_peek(struct buf_ring *br)
return (br->br_ring[br->br_cons_head]);
}
+static __inline void *
+buf_ring_peek_clear_sc(struct buf_ring *br)
+{
+#ifdef DEBUG_BUFRING
+ void *ret;
+
+ if (!mtx_owned(br->br_lock))
+ panic("lock not held on single consumer dequeue");
+#endif
+ /*
+ * I believe it is safe to not have a memory barrier
+ * here because we control cons and tail is worst case
+ * a lagging indicator so we worst case we might
+ * return NULL immediately after a buffer has been enqueued
+ */
+ if (br->br_cons_head == br->br_prod_tail)
+ return (NULL);
+
+#ifdef DEBUG_BUFRING
+ /*
+ * Single consumer, i.e. cons_head will not move while we are
+ * running, so atomic_swap_ptr() is not necessary here.
+ */
+ ret = br->br_ring[br->br_cons_head];
+ br->br_ring[br->br_cons_head] = NULL;
+ return (ret);
+#else
+ return (br->br_ring[br->br_cons_head]);
+#endif
+}
+
static __inline int
buf_ring_full(struct buf_ring *br)
{
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