Date: Tue, 13 Apr 2021 13:25:00 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Alexander Lochmann <alexander.lochmann@tu-dortmund.de> Cc: freebsd-fs@freebsd.org Subject: Re: [struct buf] Unlocked access to b_vflags? Message-ID: <YHVxfMrU9lmw3sG9@kib.kiev.ua> In-Reply-To: <792c8a3d-8ea6-073f-3fda-b3eb793ef2b9@tu-dortmund.de>
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On Mon, Apr 12, 2021 at 11:19:05PM +0200, Alexander Lochmann wrote:
> Hi folks,
>
> I'm was digging through our data set when I encountered a strange situation:
> According to the code in trunc_dependencies() in sys/ufs/ffs/ffs_softdep.c,
> the bo_lock should be held. At least that's how I read the code.
> However, we see several thousands of accesses to b_vflags without the
> bo_lock held.
> At least the own b_lock is acquired.
> The access happens in line 7549: bp->b_vflags |= BV_SCANNED; [1]
> Can you please shed some light on this situation?
> Is the b_lock sufficeint, and somehow overrules the bo_lock?
> Am I missing something?
I think you found a valid race. There is one more place where BV_SCANNED
was manipulated without owning bufobj lock. Patch below should fix both.
commit a678470b1307542c5a46b930c119b2358863e0d2
Author: Konstantin Belousov <kib@FreeBSD.org>
Date: Tue Apr 13 13:22:56 2021 +0300
b_vflags update requries bufobj lock
Reported by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de> (trunc_dependencies())
diff --git a/sys/ufs/ffs/ffs_softdep.c b/sys/ufs/ffs/ffs_softdep.c
index 0091b5dcd3b8..23c0cf6e128b 100644
--- a/sys/ufs/ffs/ffs_softdep.c
+++ b/sys/ufs/ffs/ffs_softdep.c
@@ -7546,7 +7546,9 @@ trunc_dependencies(ip, freeblks, lastlbn, lastoff, flags)
BO_LOCK(bo);
goto cleanrestart;
}
+ BO_LOCK(bo);
bp->b_vflags |= BV_SCANNED;
+ BO_UNLOCK(bo);
bremfree(bp);
if (blkoff != 0) {
allocbuf(bp, blkoff);
diff --git a/sys/ufs/ffs/ffs_vnops.c b/sys/ufs/ffs/ffs_vnops.c
index dc638595eb7b..05eb19c0ee13 100644
--- a/sys/ufs/ffs/ffs_vnops.c
+++ b/sys/ufs/ffs/ffs_vnops.c
@@ -321,8 +321,9 @@ ffs_syncvnode(struct vnode *vp, int waitfor, int flags)
if (BUF_LOCK(bp,
LK_EXCLUSIVE | LK_SLEEPFAIL | LK_INTERLOCK,
BO_LOCKPTR(bo)) != 0) {
+ BO_LOCK(bo);
bp->b_vflags &= ~BV_SCANNED;
- goto next;
+ goto next_locked;
}
} else
continue;
@@ -385,6 +386,7 @@ ffs_syncvnode(struct vnode *vp, int waitfor, int flags)
* to start from a known point.
*/
BO_LOCK(bo);
+next_locked:
nbp = TAILQ_FIRST(&bo->bo_dirty.bv_hd);
}
if (waitfor != MNT_WAIT) {
help
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