[src/pgoyette-localcount]: src/sys/kern More locking fixes from riastradh@

[pgoyette <pgoyette%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/18a967b20f27
branches:  pgoyette-localcount
changeset: 852799:18a967b20f27
user:      pgoyette <pgoyette%NetBSD.org@localhost>
date:      Sun Jul 17 02:37:54 2016 +0000

description:
More locking fixes from riastradh@

diffstat:

 sys/kern/subr_devsw.c |  25 ++++++++++++++-----------
 1 files changed, 14 insertions(+), 11 deletions(-)

diffs (88 lines):

diff -r a5d1c9a9d404 -r 18a967b20f27 sys/kern/subr_devsw.c
--- a/sys/kern/subr_devsw.c     Sat Jul 16 23:42:35 2016 +0000
+++ b/sys/kern/subr_devsw.c     Sun Jul 17 02:37:54 2016 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: subr_devsw.c,v 1.34.2.2 2016/07/16 22:35:34 pgoyette Exp $     */
+/*     $NetBSD: subr_devsw.c,v 1.34.2.3 2016/07/17 02:37:54 pgoyette Exp $     */
 
 /*-
  * Copyright (c) 2001, 2002, 2007, 2008 The NetBSD Foundation, Inc.
@@ -69,7 +69,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_devsw.c,v 1.34.2.2 2016/07/16 22:35:34 pgoyette Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_devsw.c,v 1.34.2.3 2016/07/17 02:37:54 pgoyette Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_dtrace.h"
@@ -113,6 +113,7 @@
 
 kmutex_t       device_lock;
 kcondvar_t     device_cv;
+pserialize_t   device_psz;
 
 void (*biodone_vfs)(buf_t *) = (void *)nullop;
 
@@ -124,6 +125,7 @@
        KASSERT(sys_cdevsws < MAXDEVSW - 1);
        mutex_init(&device_lock, MUTEX_DEFAULT, IPL_NONE);
        cv_init(&device_cv, "devsw");
+       device_psz = pserialize_init();
 }
 
 int
@@ -359,7 +361,7 @@
 static void
 devsw_detach_locked(const struct bdevsw *bdev, const struct cdevsw *cdev)
 {
-       int i, j, s;
+       int i, j;
 
        KASSERT(mutex_owned(&device_lock));
 
@@ -390,7 +392,15 @@
        if (j < max_cdevsws )
                cdevsw[j] = NULL;
 
-       s = pserialize_read_enter();
+       /* We need to wait for all current readers to finish. */
+       pserialize_perform(device_psz);
+
+       /*
+        * Here, no new readers can reach the bdev and cdev via the
+        * {b,c}devsw[] arrays.  Wait for existing references to
+        * drain, and then destroy.
+        */
+
        if (i < max_bdevsws && bdev->d_localcount != NULL) {
                localcount_drain(bdev->d_localcount, &device_cv, &device_lock);
                localcount_fini(bdev->d_localcount);
@@ -399,7 +409,6 @@
                localcount_drain(cdev->d_localcount, &device_cv, &device_lock);
                localcount_fini(cdev->d_localcount);
        }
-       pserialize_read_exit(s);
 }
 
 int
@@ -444,9 +453,6 @@
        if (bmajor < 0 || bmajor >= max_bdevsws)
                return (NULL);
 
-       /* Prevent any concurrent attempts to detach the device */
-       mutex_enter(&device_lock);
-
        /* Start a read transaction to block localcount_drain() */
        s = pserialize_read_enter();
 
@@ -463,9 +469,6 @@
                localcount_acquire(bdevsw[bmajor]->d_localcount);
 
 out:   pserialize_read_exit(s);
-       mutex_exit(&device_lock);
-
-       return bdev;
 }
 
 void