[PATCH v2 5/6] util/oslib-posix: Support concurrent os_mem_prealloc() invocation

[David Hildenbrand <david@redhat.com>]

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.

Add a comment that messing with the SIGBUS handler is frowned upon and
can result in problems we fortunately haven't seen so far. Note that
forwarding signals to the already installed SIGBUS handler isn't clean
either, as that one might modify the SIGBUS handler again.

Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 util/oslib-posix.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 1483e985c6..7c75848a67 100644
--- a/util/oslib-posix.c
+++ b/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;
@@ -664,6 +672,7 @@ 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