Convert ram_list to RCU

Allow "unlocked" reads of the ram_list by using an RCU-enabled QLIST.

The ramlist mutex is kept.  call_rcu callbacks are run with the iothread
lock taken, but that may change in the future.  Writers still take the
ramlist mutex, but they no longer need to assume that the iothread lock
is taken.

Readers of the list, instead, no longer require either the iothread
or ramlist mutex, but they need to use rcu_read_lock() and
rcu_read_unlock().

One place in arch_init.c was downgrading from write side to read side
like this:

    qemu_mutex_lock_iothread()
    qemu_mutex_lock_ramlist()
    ...
    qemu_mutex_unlock_iothread()
    ...
    qemu_mutex_unlock_ramlist()

and the equivalent idiom is:

    qemu_mutex_lock_ramlist()
    rcu_read_lock()
    ...
    qemu_mutex_unlock_ramlist()
    ...
    rcu_read_unlock()

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Mike Day <ncmike@ncultra.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index 87b865800c..ac06c6721c 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -279,9 +279,7 @@ struct RAMBlock {
     uint32_t flags;
     /* Protected by iothread lock.  */
     char idstr[256];
-    /* Reads can take either the iothread or the ramlist lock.
-     * Writes must take both locks.
-     */
+    /* RCU-enabled, writes protected by the ramlist lock */
     QLIST_ENTRY(RAMBlock) next;
     int fd;
 };
@@ -298,7 +296,7 @@ typedef struct RAMList {
     /* Protected by the iothread lock.  */
     unsigned long *dirty_memory[DIRTY_MEMORY_NUM];
     RAMBlock *mru_block;
-    /* Protected by the ramlist lock.  */
+    /* RCU-enabled, writes protected by the ramlist lock. */
     QLIST_HEAD(, RAMBlock) blocks;
     uint32_t version;
 } RAMList;