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This update further fine tunes the locking of snapshot vnodes in

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the ffs_copyonwrite routine to avoid a deadlock between the syncer
daemon trying to sync out a snapshot vnode and the bufdaemon
trying to write out a buffer containing the snapshot inode.
With any luck this will be the last snapshot race condition.

Sponsored by:	DARPA & NAI Labs.
This commit is contained in:
Kirk McKusick 2002-10-22 01:23:00 +00:00
parent 127ab960d5
commit 0152387ade
1 changed files with 21 additions and 12 deletions
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33
sys/ufs/ffs/ffs_snapshot.c
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@ -1759,9 +1759,9 @@ ffs_copyonwrite(devvp, bp)
struct thread *td = curthread;
struct fs *fs;
struct inode *ip;
struct vnode *vp;
struct vnode *vp = 0;
ufs2_daddr_t lbn, blkno;
int lower, upper, mid, indiroff, error = 0;
int lower, upper, mid, indiroff, snapshot_locked = 0, error = 0;
if (td->td_proc->p_flag & P_COWINPROGRESS)
panic("ffs_copyonwrite: recursive call");
@ -1769,8 +1769,6 @@ ffs_copyonwrite(devvp, bp)
ip = TAILQ_FIRST(snaphead);
fs = ip->i_fs;
lbn = fragstoblks(fs, dbtofsb(fs, bp->b_blkno));
vp = ITOV(ip);
vn_lock(vp, LK_EXCLUSIVE | LK_RETRY, td);
TAILQ_FOREACH(ip, snaphead, i_nextsnap) {
vp = ITOV(ip);
/*
@ -1781,6 +1779,7 @@ ffs_copyonwrite(devvp, bp)
*/
if (bp->b_vp == vp)
continue;
retry:
/*
* First check to see if it is in the preallocated list.
* By doing this check we avoid several potential deadlocks.
@ -1799,10 +1798,10 @@ ffs_copyonwrite(devvp, bp)
if (lower <= upper)
continue;
/*
* Check to see if block needs to be copied. Because
* all snapshots on a filesystem share a single lock,
* we ensure that we will never be in competition with
* another process to allocate a block.
* Check to see if block needs to be copied. We do not have
* to hold the snapshot lock while doing this lookup as it
* will never require any additional allocations for the
* snapshot inode.
*/
if (lbn < NDADDR) {
blkno = DIP(ip, i_db[lbn]);
@ -1827,10 +1826,19 @@ ffs_copyonwrite(devvp, bp)
if (blkno != 0)
continue;
/*
* Allocate the block into which to do the copy. Note that this
* allocation will never require any additional allocations for
* the snapshot inode.
* Allocate the block into which to do the copy. Since
* multiple processes may all try to copy the same block,
* we have to recheck our need to do a copy if we sleep
* waiting for the lock.
*
* Because all snapshots on a filesystem share a single
* lock, we ensure that we will never be in competition
* with another process to allocate a block.
*/
if (snapshot_locked == 0 &&
vn_lock(vp, LK_EXCLUSIVE | LK_SLEEPFAIL, td) != 0)
goto retry;
snapshot_locked = 1;
td->td_proc->p_flag |= P_COWINPROGRESS;
error = UFS_BALLOC(vp, lblktosize(fs, (off_t)lbn),
fs->fs_bsize, KERNCRED, 0, &cbp);
@ -1886,7 +1894,8 @@ ffs_copyonwrite(devvp, bp)
if (dopersistence && VTOI(vp)->i_effnlink > 0)
(void) VOP_FSYNC(vp, KERNCRED, MNT_WAIT, td);
}
VOP_UNLOCK(vp, 0, td);
if (snapshot_locked)
VOP_UNLOCK(vp, 0, td);
return (error);
}