[struct buf] Unlocked access to b_vflags?

[Alexander Lochmann]

Thx!
Do you mind adding "Found by LockDoc" to the Commit-Msg?

Cheers,
Alex

On 13.04.21 12:25, Konstantin Belousov wrote:
> 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 at FreeBSD.org>
> Date:   Tue Apr 13 13:22:56 2021 +0300
> 
>      b_vflags update requries bufobj lock
>      
>      Reported by:    Alexander Lochmann <alexander.lochmann at 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) {
> 

-- 
Technische Universität Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116
http://ess.cs.tu-dortmund.de/Staff/al

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