util/oslib-posix: Support concurrent os_mem_prealloc() invocation

Add a mutex to protect the SIGBUS case, as we cannot mess concurrently
with the sigbus handler and we have to manage the global variable
sigbus_memset_context. The MADV_POPULATE_WRITE path can run
concurrently.

Note that page_mutex and page_cond are shared between concurrent
invocations, which shouldn't be a problem.

This is a preparation for future virtio-mem prealloc code, which will call
os_mem_prealloc() asynchronously from an iothread when handling guest
requests.

Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20211217134611.31172-7-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

util/oslib-posix.c

@@ -95,6 +95,7 @@ typedef struct MemsetThread MemsetThread;
 
 /* used by sigbus_handler() */
 static MemsetContext *sigbus_memset_context;
+static QemuMutex sigbus_mutex;
 
 static QemuMutex page_mutex;
 static QemuCond page_cond;
@@ -625,6 +626,7 @@ static bool madv_populate_write_possible(char *area, size_t pagesize)
 void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
                      Error **errp)
 {
+    static gsize initialized;
     int ret;
     struct sigaction act, oldact;
     size_t hpagesize = qemu_fd_getpagesize(fd);
@@ -638,6 +640,12 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
     use_madv_populate_write = madv_populate_write_possible(area, hpagesize);
 
     if (!use_madv_populate_write) {
+        if (g_once_init_enter(&initialized)) {
+            qemu_mutex_init(&sigbus_mutex);
+            g_once_init_leave(&initialized, 1);
+        }
+
+        qemu_mutex_lock(&sigbus_mutex);
         memset(&act, 0, sizeof(act));
         act.sa_handler = &sigbus_handler;
         act.sa_flags = 0;
@@ -665,6 +673,7 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
             perror("os_mem_prealloc: failed to reinstall signal handler");
             exit(1);
         }
+        qemu_mutex_unlock(&sigbus_mutex);
     }
 }