On 2/16/21 9:18 PM, Alexander V. Chernikov wrote:
The branch main has been updated by melifaro:
URL:
https://cgit.FreeBSD.org/src/commit/?id=600eade2fb4faacfcd408a01140ef15f85f0c817
commit 600eade2fb4faacfcd408a01140ef15f85f0c817
Author: Alexander V. Chernikov <[email protected]>
AuthorDate: 2021-02-16 20:12:58 +0000
Commit: Alexander V. Chernikov <[email protected]>
CommitDate: 2021-02-16 20:14:50 +0000
Add ifa_try_ref() to simplify ifa handling inside epoch.
More and more code migrates from lock-based protection to the NET_EPOCH
umbrella. It requires some logic changes, including, notably, refcount
handling.
When we have an `ifa` pointer and we're running inside epoch we're
guaranteed that this pointer will not be freed.
However, the following case can still happen:
* in thread 1 we drop to 0 refcount for ifa and schedule its deletion.
* in thread 2 we use this ifa and reference it
* destroy callout kicks in
* unhappy user reports bug
To address it, new `ifa_try_ref()` function is added, allowing to return
failure when we try to reference `ifa` with 0 refcount.
Additionally, existing `ifa_ref()` is enforced with `KASSERT` to provide
cleaner error in such scenarious.
Reviewed By: rstone, donner
Differential Revision: https://reviews.freebsd.org/D28639
MFC after: 1 week
---
sys/net/if.c | 12 +++++++++++-
sys/net/if_var.h | 1 +
2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/sys/net/if.c b/sys/net/if.c
index c85cfab19bf6..948be6876b65 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1857,8 +1857,18 @@ fail:
void
ifa_ref(struct ifaddr *ifa)
{
+ u_int old;
- refcount_acquire(&ifa->ifa_refcnt);
+ old = refcount_acquire(&ifa->ifa_refcnt);
+ KASSERT(old > 0, ("%s: ifa %p has 0 refs", __func__, ifa));
+}
+
+int
+ifa_try_ref(struct ifaddr *ifa)
+{
+
+ NET_EPOCH_ASSERT();
+ return (refcount_acquire_if_not_zero(&ifa->ifa_refcnt));
}
static void
diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index 9ecdfb684296..291a7781d73c 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -577,6 +577,7 @@ struct ifaddr {
struct ifaddr * ifa_alloc(size_t size, int flags);
void ifa_free(struct ifaddr *ifa);
void ifa_ref(struct ifaddr *ifa);
+int ifa_try_ref(struct ifaddr *ifa);
I think you should add __result_use_check for this prototype!?
--HPS
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