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Blame SOURCES/kvm-virtiofsd-prevent-fv_queue_thread-vs-virtio_loop-rac.patch

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From 69c6a829f8136a8c95ccdf480f2fd0173d64b6ec Mon Sep 17 00:00:00 2001
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From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
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Date: Mon, 27 Jan 2020 19:02:05 +0100
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Subject: [PATCH 094/116] virtiofsd: prevent fv_queue_thread() vs virtio_loop()
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 races
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
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Message-id: <20200127190227.40942-91-dgilbert@redhat.com>
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Patchwork-id: 93544
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O-Subject: [RHEL-AV-8.2 qemu-kvm PATCH 090/112] virtiofsd: prevent fv_queue_thread() vs virtio_loop() races
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Bugzilla: 1694164
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RH-Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Sergio Lopez Pascual <slp@redhat.com>
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From: Stefan Hajnoczi <stefanha@redhat.com>
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We call into libvhost-user from the virtqueue handler thread and the
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vhost-user message processing thread without a lock.  There is nothing
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protecting the virtqueue handler thread if the vhost-user message
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processing thread changes the virtqueue or memory table while it is
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running.
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This patch introduces a read-write lock.  Virtqueue handler threads are
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readers.  The vhost-user message processing thread is a writer.  This
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will allow concurrency for multiqueue in the future while protecting
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against fv_queue_thread() vs virtio_loop() races.
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Note that the critical sections could be made smaller but it would be
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more invasive and require libvhost-user changes.  Let's start simple and
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improve performance later, if necessary.  Another option would be an
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RCU-style approach with lighter-weight primitives.
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
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Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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(cherry picked from commit e7b337326d594b71b07cd6dbb332c49c122c80a4)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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 tools/virtiofsd/fuse_virtio.c | 34 +++++++++++++++++++++++++++++++++-
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 1 file changed, 33 insertions(+), 1 deletion(-)
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diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c
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index fb8d6d1..f6242f9 100644
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--- a/tools/virtiofsd/fuse_virtio.c
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+++ b/tools/virtiofsd/fuse_virtio.c
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@@ -59,6 +59,18 @@ struct fv_VuDev {
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     struct fuse_session *se;
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     /*
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+     * Either handle virtqueues or vhost-user protocol messages.  Don't do
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+     * both at the same time since that could lead to race conditions if
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+     * virtqueues or memory tables change while another thread is accessing
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+     * them.
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+     *
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+     * The assumptions are:
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+     * 1. fv_queue_thread() reads/writes to virtqueues and only reads VuDev.
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+     * 2. virtio_loop() reads/writes virtqueues and VuDev.
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+     */
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+    pthread_rwlock_t vu_dispatch_rwlock;
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+
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+    /*
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      * The following pair of fields are only accessed in the main
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      * virtio_loop
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      */
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@@ -415,6 +427,8 @@ static void *fv_queue_thread(void *opaque)
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              qi->qidx, qi->kick_fd);
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     while (1) {
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         struct pollfd pf[2];
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+        int ret;
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+
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         pf[0].fd = qi->kick_fd;
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         pf[0].events = POLLIN;
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         pf[0].revents = 0;
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@@ -461,6 +475,9 @@ static void *fv_queue_thread(void *opaque)
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             fuse_log(FUSE_LOG_ERR, "Eventfd_read for queue: %m\n");
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             break;
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         }
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+        /* Mutual exclusion with virtio_loop() */
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+        ret = pthread_rwlock_rdlock(&qi->virtio_dev->vu_dispatch_rwlock);
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+        assert(ret == 0); /* there is no possible error case */
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         /* out is from guest, in is too guest */
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         unsigned int in_bytes, out_bytes;
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         vu_queue_get_avail_bytes(dev, q, &in_bytes, &out_bytes, ~0, ~0);
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@@ -469,6 +486,7 @@ static void *fv_queue_thread(void *opaque)
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                  "%s: Queue %d gave evalue: %zx available: in: %u out: %u\n",
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                  __func__, qi->qidx, (size_t)evalue, in_bytes, out_bytes);
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+
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         while (1) {
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             bool allocated_bufv = false;
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             struct fuse_bufvec bufv;
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@@ -597,6 +615,8 @@ static void *fv_queue_thread(void *opaque)
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             free(elem);
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             elem = NULL;
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         }
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+
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+        pthread_rwlock_unlock(&qi->virtio_dev->vu_dispatch_rwlock);
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     }
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 out:
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     pthread_mutex_destroy(&ch.lock);
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@@ -711,6 +731,8 @@ int virtio_loop(struct fuse_session *se)
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     while (!fuse_session_exited(se)) {
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         struct pollfd pf[1];
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+        bool ok;
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+        int ret;
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         pf[0].fd = se->vu_socketfd;
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         pf[0].events = POLLIN;
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         pf[0].revents = 0;
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@@ -735,7 +757,15 @@ int virtio_loop(struct fuse_session *se)
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         }
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         assert(pf[0].revents & POLLIN);
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         fuse_log(FUSE_LOG_DEBUG, "%s: Got VU event\n", __func__);
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-        if (!vu_dispatch(&se->virtio_dev->dev)) {
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+        /* Mutual exclusion with fv_queue_thread() */
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+        ret = pthread_rwlock_wrlock(&se->virtio_dev->vu_dispatch_rwlock);
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+        assert(ret == 0); /* there is no possible error case */
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+
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+        ok = vu_dispatch(&se->virtio_dev->dev);
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+
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+        pthread_rwlock_unlock(&se->virtio_dev->vu_dispatch_rwlock);
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+
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+        if (!ok) {
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             fuse_log(FUSE_LOG_ERR, "%s: vu_dispatch failed\n", __func__);
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             break;
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         }
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@@ -877,6 +907,7 @@ int virtio_session_mount(struct fuse_session *se)
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     se->vu_socketfd = data_sock;
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     se->virtio_dev->se = se;
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+    pthread_rwlock_init(&se->virtio_dev->vu_dispatch_rwlock, NULL);
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     vu_init(&se->virtio_dev->dev, 2, se->vu_socketfd, fv_panic, fv_set_watch,
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             fv_remove_watch, &fv_iface);
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@@ -892,6 +923,7 @@ void virtio_session_close(struct fuse_session *se)
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     }
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     free(se->virtio_dev->qi);
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+    pthread_rwlock_destroy(&se->virtio_dev->vu_dispatch_rwlock);
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     free(se->virtio_dev);
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     se->virtio_dev = NULL;
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 }
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-- 
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1.8.3.1
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