Locking for the per-process resource limits structure.

- struct plimit includes a mutex to protect a reference count.  The plimit
  structure is treated similarly to struct ucred in that is is always copy
  on write, so having a reference to a structure is sufficient to read from
  it without needing a further lock.
- The proc lock protects the p_limit pointer and must be held while reading
  limits from a process to keep the limit structure from changing out from
  under you while reading from it.
- Various global limits that are ints are not protected by a lock since
  int writes are atomic on all the archs we support and thus a lock
  wouldn't buy us anything.
- All accesses to individual resource limits from a process are abstracted
  behind a simple lim_rlimit(), lim_max(), and lim_cur() API that return
  either an rlimit, or the current or max individual limit of the specified
  resource from a process.
- dosetrlimit() was renamed to kern_setrlimit() to match existing style of
  other similar syscall helper functions.
- The alpha OSF/1 compat layer no longer calls getrlimit() and setrlimit()
  (it didn't used the stackgap when it should have) but uses lim_rlimit()
  and kern_setrlimit() instead.
- The svr4 compat no longer uses the stackgap for resource limits calls,
  but uses lim_rlimit() and kern_setrlimit() instead.
- The ibcs2 compat no longer uses the stackgap for resource limits.  It
  also no longer uses the stackgap for accessing sysctl's for the
  ibcs2_sysconf() syscall but uses kernel_sysctl() instead.  As a result,
  ibcs2_sysconf() no longer needs Giant.
- The p_rlimit macro no longer exists.

Submitted by:	mtm (mostly, I only did a few cleanups and catchups)
Tested on:	i386
Compiled on:	alpha, amd64

1 files changed, 7 insertions, 5 deletions

diff --git a/sys/fs/msdosfs/msdosfs_vnops.c b/sys/fs/msdosfs/msdosfs_vnops.c
index 0e1ea91..728fd10 100644
--- a/sys/fs/msdosfs/msdosfs_vnops.c
+++ b/sys/fs/msdosfs/msdosfs_vnops.c
@@ -646,13 +646,15 @@ msdosfs_write(ap)
 	/*
 	 * If they've exceeded their filesize limit, tell them about it.
 	 */
-	if (td &&
-	    ((uoff_t)uio->uio_offset + uio->uio_resid >
-	    td->td_proc->p_rlimit[RLIMIT_FSIZE].rlim_cur)) {
+	if (td != NULL) {
 		PROC_LOCK(td->td_proc);
-		psignal(td->td_proc, SIGXFSZ);
+		if ((uoff_t)uio->uio_offset + uio->uio_resid >
+		    lim_cur(td->td_proc, RLIMIT_FSIZE)) {
+			psignal(td->td_proc, SIGXFSZ);
+			PROC_UNLOCK(td->td_proc);
+			return (EFBIG);
+		}
 		PROC_UNLOCK(td->td_proc);
-		return (EFBIG);
 	}
 
 	if ((uoff_t)uio->uio_offset + uio->uio_resid > DOS_FILESIZE_MAX)