Based on discussions on the svn-src mailing list, rework r218195:

 - entirely eliminate some calls to uio_yeild() as being unnecessary,
   such as in a sysctl handler.

 - move should_yield() and maybe_yield() to kern_synch.c and move the
   prototypes from sys/uio.h to sys/proc.h

 - add a slightly more generic kern_yield() that can replace the
   functionality of uio_yield().

 - replace source uses of uio_yield() with the functional equivalent,
   or in some cases do not change the thread priority when switching.

 - fix a logic inversion bug in vlrureclaim(), pointed out by bde@.

 - instead of using the per-cpu last switched ticks, use a per thread
   variable for should_yield().  With PREEMPTION, the only reasonable
   use of this is to determine if a lock has been held a long time and
   relinquish it.  Without PREEMPTION, this is essentially the same as
   the per-cpu variable.

1 files changed, 2 insertions, 2 deletions

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 42abf6e..7b5cad1 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -444,7 +444,7 @@ vn_rdwr(rw, vp, base, len, offset, segflg, ioflg, active_cred, file_cred,
  * Package up an I/O request on a vnode into a uio and do it.  The I/O
  * request is split up into smaller chunks and we try to avoid saturating
  * the buffer cache while potentially holding a vnode locked, so we 
- * check bwillwrite() before calling vn_rdwr().  We also call uio_yield()
+ * check bwillwrite() before calling vn_rdwr().  We also call kern_yield()
  * to give other processes a chance to lock the vnode (either other processes
  * core'ing the same binary, or unrelated processes scanning the directory).
  */
@@ -491,7 +491,7 @@ vn_rdwr_inchunks(rw, vp, base, len, offset, segflg, ioflg, active_cred,
 			break;
 		offset += chunk;
 		base = (char *)base + chunk;
-		uio_yield();
+		kern_yield(curthread->td_user_pri);
 	} while (len);
 	if (aresid)
 		*aresid = len + iaresid;