Fix race condition in ext2fs when remounting

On some systems, ext2fs.static would regularly hang at startup, as a
race condition meant it would process paging requests while remounting.
To fix this, libpager has been altered to allow inhibiting and resuming
its worker threads, and ext2fs uses this to inhibit paging while
remounting.

* console/pager.c (pager_requests): New variable.
(user_pager_init): Updated call to pager_start_workers to use new
pager_requests variable.
* daemons/runsystem.sh: Removed artificial delay working around the race
condition.
* ext2fs/ext2fs.c (diskfs_reload_global_state): Call new
inhibit_ext2_pager and resume_ext2_pager functions, and leave sblock as
non-NULL so it will be munmapped.
* ext2fs/ext2fs.h (inhibit_ext2_pager,resume_ext2_pager): New functions.
* ext2fs/pager.c (file_pager_requests): New variable.
(create_disk_pager): Updated call to pager_start_workers to use new
file_pager_requests variable.
(inhibit_ext2_pager,resume_ext2_pager): New functions.
* fatfs/fatfs.h (inhibit_fat_pager,resume_fat_pager): New functions.
* fatfs/pager.c (file_pager_requests): New variable.
(create_fat_pager): Updated call to pager_start_workers to use new
file_pager_requests variable.
(inhibit_fat_pager,resume_fat_pager): New functions.
* libdiskfs/disk-pager.c (diskfs_disk_pager_requests): New variable.
(diskfs_start_disk_pager): Updated call to pager_start_workers to use
new diskfs_disk_pager_requests variable.
* libdiskfs/diskfs-pager.h (diskfs_disk_pager_requests): New variable.
* libpager/demuxer.c (struct pager_requests): Renamed struct requests to
struct pager_requests. Replaced queue with queue_in and queue_out
pointers. Added inhibit_wakeup field.
(pager_demuxer): Updated to use new queue_in/queue_out pointers. Only
wake up workers if not inhibited.
(worker_func): Updated to use new queue_in/queue_out pointers. Final
worker thread to sleep notifies the inhibit_wakeup condition variable.
(pager_start_workers): Added out parameter for the requests instance.
Allocate heap space shared by both queues. Initialise new inhibit_wakeup
condition.
(pager_inhibit_workers,pager_resume_workers): New functions.
* libpager/pager.h (struct pager_requests): Public forward definition.
(pager_start_workers): Added out parameter for the requests instance.
(pager_inhibit_workers,pager_resume_workers): New functions.
* libpager/queue.h (queue_empty): New function.
* storeio/pager.c (pager_requests): New variable.
(init_dev_paging): Updated call to pager_start_workers to use new
pager_requests variable.

3 files changed, 49 insertions, 2 deletions

diff --git a/ext2fs/ext2fs.c b/ext2fs/ext2fs.c
index d0fdfe7a..03c9eedc 100644
--- a/ext2fs/ext2fs.c
+++ b/ext2fs/ext2fs.c
@@ -207,10 +207,20 @@ main (int argc, char **argv)
 error_t
 diskfs_reload_global_state ()
 {
+  error_t err;
+
   pokel_flush (&global_pokel);
   pager_flush (diskfs_disk_pager, 1);
-  sblock = NULL;
+
+  /* libdiskfs is not responsible for inhibiting paging.  */
+  err = inhibit_ext2_pager ();
+  if (err)
+    return err;
+
   get_hypermetadata ();
   map_hypermetadata ();
+
+  resume_ext2_pager ();
+
   return 0;
 }
diff --git a/ext2fs/ext2fs.h b/ext2fs/ext2fs.h
index 72b7b09d..42717c52 100644
--- a/ext2fs/ext2fs.h
+++ b/ext2fs/ext2fs.h
@@ -202,6 +202,12 @@ struct user_pager_info
 /* Set up the disk pager.  */
 void create_disk_pager (void);
 
+/* Inhibit the disk pager.  */
+error_t inhibit_ext2_pager (void);
+
+/* Resume the disk pager.  */
+void resume_ext2_pager (void);
+
 /* Call this when we should turn off caching so that unused memory object
    ports get freed.  */
 void drop_pager_softrefs (struct node *node);
diff --git a/ext2fs/pager.c b/ext2fs/pager.c
index b56c923a..3e080f87 100644
--- a/ext2fs/pager.c
+++ b/ext2fs/pager.c
@@ -34,6 +34,10 @@ struct port_bucket *disk_pager_bucket;
 /* A ports bucket to hold file pager ports.  */
 struct port_bucket *file_pager_bucket;
 
+/* Stores a reference to the requests instance used by the file pager so its
+   worker threads can be inhibited and resumed.  */
+struct pager_requests *file_pager_requests;
+
 pthread_spinlock_t node_to_page_lock = PTHREAD_SPINLOCK_INITIALIZER;
 
 
@@ -1217,11 +1221,38 @@ create_disk_pager (void)
   file_pager_bucket = ports_create_bucket ();
 
   /* Start libpagers worker threads.  */
-  err = pager_start_workers (file_pager_bucket);
+  err = pager_start_workers (file_pager_bucket, &file_pager_requests);
   if (err)
     ext2_panic ("can't create libpager worker threads: %s", strerror (err));
 }
 
+error_t
+inhibit_ext2_pager (void)
+{
+  error_t err;
+
+  /* The file pager can rely on the disk pager, so inhibit the file
+     pager first.  */
+
+  err = pager_inhibit_workers (file_pager_requests);
+  if (err)
+    return err;
+
+  err = pager_inhibit_workers (diskfs_disk_pager_requests);
+  /* We don't want only one pager disabled.  */
+  if (err)
+    pager_resume_workers (file_pager_requests);
+
+  return err;
+}
+
+void
+resume_ext2_pager (void)
+{
+  pager_resume_workers (diskfs_disk_pager_requests);
+  pager_resume_workers (file_pager_requests);
+}
+
 /* Call this to create a FILE_DATA pager and return a send right.
    NODE must be locked.  */
 mach_port_t