Date: Sat, 21 Jul 2007 23:52:04 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Kostik Belousov <kostikbel@gmail.com> Cc: bugs@FreeBSD.org, fs@FreeBSD.org Subject: Re: msdosfs not MPSAFE Message-ID: <20070721233613.Q3366@besplex.bde.org> In-Reply-To: <20070721063434.GI2200@deviant.kiev.zoral.com.ua> References: <20070710233455.O2101@besplex.bde.org> <20070712084115.GA2200@deviant.kiev.zoral.com.ua> <20070712225324.F9515@besplex.bde.org> <20070712142127.GD2200@deviant.kiev.zoral.com.ua> <20070716195556.P12807@besplex.bde.org> <20070721063434.GI2200@deviant.kiev.zoral.com.ua>
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On Sat, 21 Jul 2007, Kostik Belousov wrote:
> On Mon, Jul 16, 2007 at 08:18:14PM +1000, Bruce Evans wrote:
>> sx xlocking works, but is not quite right:
>> % /*
>> % + * XXX msdosfs_lookup() is split up because unlocking before all the
>> returns
>> % + * in the original function would be too churning.
>> % + */
>> % +int
>> % +msdosfs_lookup(ap)
>> % + struct vop_cachedlookup_args *ap;
>> % +{
>> % + int error;
>> % +
>> % + sx_xlock(&mbnambuf_lock);
>> % + error = msdosfs_lookup_locked(ap);
>> % + sx_xunlock(&mbnambuf_lock);
>> % + return (error);
>> % +}
>> % +
>> % +/*
>
> Assume that a directory A is participating in lookup() from two threads:
> thread 1 lookup the A itself;
> thread 2 lookup some entry in the A.
> Then, thread 1 would have mbnambuf_lock locked, and may wait for A'
> vnode lock;
> thread 2 shall own vnode lock for A, then locking mbnambuf_lock.
>
> I do not see what may prevent this LOR scenario from realizing, or what
> make it harmless.
Nothing I can see either. The wrapper is too global.
Next try: move locking into the inner loop in msdosfs_lookup(). Unlocking
is not as ugly as I feared. The following has only been tested at compile
time:
% Index: msdosfs_lookup.c
% ===================================================================
% RCS file: /home/ncvs/src/sys/fs/msdosfs/msdosfs_lookup.c,v
% retrieving revision 1.40
% diff -u -2 -r1.40 msdosfs_lookup.c
% --- msdosfs_lookup.c 26 Dec 2003 17:24:37 -0000 1.40
% +++ msdosfs_lookup.c 21 Jul 2007 13:27:37 -0000
% @@ -54,4 +54,5 @@
% #include <sys/bio.h>
% #include <sys/buf.h>
% +#include <sys/sx.h>
% #include <sys/vnode.h>
% #include <sys/mount.h>
% @@ -63,4 +64,6 @@
% #include <fs/msdosfs/fat.h>
%
% +extern struct sx mbnambuf_lock;
% +
% /*
% * When we search a directory the blocks containing directory entries are
% @@ -192,4 +195,5 @@
% */
% tdp = NULL;
% + sx_xlock(&mbnambuf_lock);
% mbnambuf_init();
% /*
% @@ -206,4 +210,5 @@
% if (error == E2BIG)
% break;
% + sx_xunlock(&mbnambuf_lock);
% return (error);
% }
% @@ -211,4 +216,5 @@
% if (error) {
% brelse(bp);
% + sx_xunlock(&mbnambuf_lock);
% return (error);
% }
% @@ -240,4 +246,5 @@
% if (dep->deName[0] == SLOT_EMPTY) {
% brelse(bp);
% + sx_xunlock(&mbnambuf_lock);
% goto notfound;
% }
% @@ -301,4 +308,5 @@
% dp->de_fndcnt = wincnt - 1;
%
% + sx_xunlock(&mbnambuf_lock);
% goto found;
% }
% @@ -310,4 +318,5 @@
% brelse(bp);
% } /* for (frcn = 0; ; frcn++) */
% + sx_xunlock(&mbnambuf_lock);
%
% notfound:
After moving the locking into msdosfs_conv.c and adding assertions there,
this should be a good enough fix until the mbnambuf interface is changed.
This bug is in all versions since 5.2-RELEASE.
Bruce
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