the wren device uses seek before performing read/write operation. as multiple
threads could issue read/write in paralel, a race can happen. it may even cause
filesystem corruption. removing the seek and replacing read/write with
pread/pwrite also saves a syscall.

Reference: /n/sources/patch/applied/cwfs-seek-race
Date: Fri Aug  7 03:47:26 CES 2009
Signed-off-by: cinap_lenrek@gmx.de

--- /sys/src/cmd/cwfs/wren.c	Fri Aug  7 03:44:45 2009
+++ /sys/src/cmd/cwfs/wren.c	Fri Aug  7 03:44:43 2009
@@ -91,8 +91,7 @@
 	if(b >= dr->max) {
 		print("wrenread: block out of range %Z(%lld)\n", d, (Wideoff)b);
 		r = 0x040;
-	} else if (seek(d->wren.fd, (vlong)b*RBUFSIZE, 0) < 0 ||
-	    read(d->wren.fd, c, RBUFSIZE) != RBUFSIZE) {
+	} else if (pread(d->wren.fd, c, RBUFSIZE, (vlong)b*RBUFSIZE) != RBUFSIZE) {
 		print("wrenread: error on %Z(%lld): %r\n", d, (Wideoff)b);
 		cons.nwrenre++;
 		r = 1;
@@ -112,11 +111,11 @@
 		print("wrenwrite: block out of range %Z(%lld)\n",
 			d, (Wideoff)b);
 		r = 0x040;
-	} else if (seek(d->wren.fd, (vlong)b*RBUFSIZE, 0) < 0 ||
-	    write(d->wren.fd, c, RBUFSIZE) != RBUFSIZE) {
+	} else if (pwrite(d->wren.fd, c, RBUFSIZE, (vlong)b*RBUFSIZE) != RBUFSIZE) {
 		print("wrenwrite: error on %Z(%lld): %r\n", d, (Wideoff)b);
 		cons.nwrenwe++;
 		r = 1;
 	}
+
 	return r;
 }