Date: Mon, 11 Feb 2002 22:41:45 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Matteo <drummino@yahoo.com> Cc: <freebsd-stable@FreeBSD.ORG>, <freebsd-bugs@FreeBSD.ORG>, <aiuto@gufi.org>, <dillon@FreeBSD.ORG> Subject: Re: Crash System with MSDOS file-system driver! Message-ID: <20020211221141.R682-100000@gamplex.bde.org> In-Reply-To: <20020208163715.14085.qmail@web13205.mail.yahoo.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, 8 Feb 2002, Matteo wrote:
> Hi all,
>
> I've 4.5-RELEASE and 4.4-STABLE.
>
> Try to follow this steps:
>
> 1) Mount a msdos floppy write protected in /floppy
> 2) cd /floppy
> 3) rm somefile
> At this point kernel show this message:
>
> fd0c: hard error writing fsbn 56 of 56-63 (ST0
> 40<abnrml> ST1 2<write_prosend out a error messages...
> etc etc
>
> Next Step:
>
> 4) ls
>
> And system halt! I try with a UFS floppy and it's all
> ok.
%%%
The hang is caused by msdosfs_readdir() "handling" reads of 0 bytes by
spinning forever. msdosfs_read() seems to have the same bug.
ufs_readdir() works because it calls VOP_READ() == ffs_read() which
treats reads of 0 bytes as EOF instead of spinning forever.
Reads of 0 bytes and other short reads shouldn't happen, but they
happen because failed writes clobber b_resid and breadn() later returns
with the clobbered value. Filesystems use b_resid to determine the
extent of i/o in a few places (all (?) cloned from ufs_readwrite.c).
This is probably wrong and very incomplete -- most places, including
critical block allocation code, just assume that breadn() returns an
error if everything could not be read.
The patch mainly just resets b_resid in breadn(). Determining the
value after the i/o that filled the buffer doesn't seem to be easy,
but it should be 0.
Index: vfs_bio.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/vfs_bio.c,v
retrieving revision 1.296
diff -u -r1.296 vfs_bio.c
--- vfs_bio.c 31 Jan 2002 18:39:44 -0000 1.296
+++ vfs_bio.c 11 Feb 2002 11:09:22 -0000
@@ -614,6 +614,13 @@
vfs_busy_pages(bp, 0);
VOP_STRATEGY(vp, bp);
++readwait;
+ } else {
+ /*
+ * Recover from failed writes clobbering b_resid. b_resid is
+ * strictly only valid after i/o, but some filesystems expect
+ * it to give the part of `size' that was not readable here.
+ */
+ bp->b_resid = 0;
}
for (i = 0; i < cnt; i++, rablkno++, rabsize++) {
@@ -640,6 +647,16 @@
if (readwait) {
rv = bufwait(bp);
+ /*
+ * Convert short reads to i/o errors, since short reads aren't
+ * useful for the buffer cache and they are mostly not handled
+ * in callers.
+ * XXX we don't always get here for read-ahead blocks.
+ * b_resid for read-ahead blocks may be clobbered by failed
+ * attempts to write the blocks.
+ */
+ if (rv == 0 && bp->b_resid != 0)
+ rv = EIO;
}
return (rv);
}
%%%
Bruce
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-stable" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20020211221141.R682-100000>