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From 8d5bef256e0b58bb6f45910d0e9e724da72e100c Mon Sep 17 00:00:00 2001
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Message-Id: <8d5bef256e0b58bb6f45910d0e9e724da72e100c@dist-git>
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From: Michal Privoznik <mprivozn@redhat.com>
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Date: Wed, 26 Feb 2014 10:05:24 +0100
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Subject: [PATCH] virNetDevVethCreate: Serialize callers
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https://bugzilla.redhat.com/show_bug.cgi?id=1014604
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Consider dozen of LXC domains, each of them having this type of interface:
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<interface type='network'>
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<mac address='52:54:00:a7:05:4b'/>
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<source network='default'/>
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</interface>
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When starting these domain in parallel, all workers may meet in
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virNetDevVethCreate() where a race starts. Race over allocating veth
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pairs because allocation requires two steps:
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1) find first nonexistent '/sys/class/net/vnet%d/'
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2) run 'ip link add ...' command
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Now consider two threads. Both of them find N as the first unused veth
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index but only one of them succeeds allocating it. The other one fails.
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For such cases, we are running the allocation in a loop with 10 rounds.
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However this is very flaky synchronization. It should be rather used
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when libvirt is competing with other process than when libvirt threads
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fight each other. Therefore, internally we should use mutex to serialize
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callers, and do the allocation in loop (just in case we are competing
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with a different process). By the way we have something similar already
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since 1cf97c87.
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Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
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(cherry picked from commit c0d162c68c2f19af8d55a435a9e372da33857048)
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Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
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---
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src/util/virnetdevveth.c | 18 ++++++++++++++++++
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1 file changed, 18 insertions(+)
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diff --git a/src/util/virnetdevveth.c b/src/util/virnetdevveth.c
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index 25eb282..e698ce2 100644
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--- a/src/util/virnetdevveth.c
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+++ b/src/util/virnetdevveth.c
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@@ -39,6 +39,19 @@
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/* Functions */
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+virMutex virNetDevVethCreateMutex;
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+
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+static int virNetDevVethCreateMutexOnceInit(void)
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+{
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+ if (virMutexInit(&virNetDevVethCreateMutex) < 0) {
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+ virReportSystemError(errno, "%s", _("unable to init mutex"));
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+ return -1;
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+ }
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+ return 0;
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+}
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+
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+VIR_ONCE_GLOBAL_INIT(virNetDevVethCreateMutex);
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+
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static int virNetDevVethExists(int devNum)
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{
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int ret;
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@@ -117,6 +130,10 @@ int virNetDevVethCreate(char** veth1, char** veth2)
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* We might race with other containers, but this is reasonably
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* unlikely, so don't do too many retries for device creation
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*/
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+ if (virNetDevVethCreateMutexInitialize() < 0)
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+ return -1;
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+
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+ virMutexLock(&virNetDevVethCreateMutex);
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#define MAX_VETH_RETRIES 10
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for (i = 0; i < MAX_VETH_RETRIES; i++) {
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@@ -179,6 +196,7 @@ int virNetDevVethCreate(char** veth1, char** veth2)
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MAX_VETH_RETRIES);
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cleanup:
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+ virMutexUnlock(&virNetDevVethCreateMutex);
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virCommandFree(cmd);
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VIR_FREE(veth1auto);
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VIR_FREE(veth2auto);
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--
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1.9.0
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