[PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc() invocation

[David Hildenbrand <david@redhat.com>]

Add a mutext 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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 util/oslib-posix.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 60d1da2d6c..181f6bbf1a 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -94,6 +94,7 @@ 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;
@@ -605,12 +606,17 @@ 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);
     size_t numpages = DIV_ROUND_UP(memory, hpagesize);
     bool use_madv_populate_write;
 
+    if (g_once_init_enter(&initialized)) {
+        qemu_mutex_init(&sigbus_mutex);
+    }
+
     /*
      * Sense on every invocation, as MADV_POPULATE_WRITE cannot be used for
      * some special mappings, such as mapping /dev/mem.
@@ -620,6 +626,7 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
     }
 
     if (!use_madv_populate_write) {
+        qemu_mutex_lock(&sigbus_mutex);
         memset(&act, 0, sizeof(act));
         act.sa_handler = &sigbus_handler;
         act.sa_flags = 0;
@@ -646,6 +653,7 @@ 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);
     }
 }
 
-- 
2.31.1