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Move cv_destroy() outside zp->z_range_lock() 

With the recent SPL change (d599e4fa) that forces cv_destroy()
to block until all waiters have been woken.  It is now unsafe
to call cv_destroy() under the zp->z_range_lock() because it
is used as the condition variable mutex.  If there are waiters
cv_destroy() will block until they wake up and aquire the mutex.
However, they will never aquire the mutex because cv_destroy()
will not return allowing it's caller to drop the lock.  Deadlock.

To avoid this cv_destroy() is now run asynchronously in a taskq.
This solves two problems:

1) It is no longer run under the zp->z_range_lock so no deadlock.
2) Since cv_destroy() may now block we don't want this slowing
   down zfs_range_unlock() and throttling the system.

This was not as much of an issue under OpenSolaris because their
cv_destroy() implementation does not do anything.  They do however
risk a bad paging request if cv_destroy() returns, the memory holding
the condition variable is free'd, and then the waiters wake up and
try to reference it.  It's a very small unlikely race, but it is
possible.
This commit is contained in:
Brian Behlendorf 2011-02-04 14:38:11 -08:00
parent c0d35759c5
commit 8926ab7a50
1 changed files with 36 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

59
module/zfs/zfs_rlock.c
View File

@ -453,6 +453,20 @@ zfs_range_lock(znode_t *zp, uint64_t off, uint64_t len, rl_type_t type)
return (new);
}
static void
zfs_range_free(void *arg)
{
rl_t *rl = arg;
if (rl->r_write_wanted)
cv_destroy(&rl->r_wr_cv);
if (rl->r_read_wanted)
cv_destroy(&rl->r_rd_cv);
kmem_free(rl, sizeof (rl_t));
}
/*
* Unlock a reader lock
*/
@ -472,14 +486,14 @@ zfs_range_unlock_reader(znode_t *zp, rl_t *remove)
*/
if (remove->r_cnt == 1) {
avl_remove(tree, remove);
if (remove->r_write_wanted) {
mutex_exit(&zp->z_range_lock);
if (remove->r_write_wanted)
cv_broadcast(&remove->r_wr_cv);
cv_destroy(&remove->r_wr_cv);
}
if (remove->r_read_wanted) {
if (remove->r_read_wanted)
cv_broadcast(&remove->r_rd_cv);
cv_destroy(&remove->r_rd_cv);
}
taskq_dispatch(system_taskq, zfs_range_free, remove, 0);
} else {
ASSERT3U(remove->r_cnt, ==, 0);
ASSERT3U(remove->r_write_wanted, ==, 0);
@ -505,19 +519,21 @@ zfs_range_unlock_reader(znode_t *zp, rl_t *remove)
rl->r_cnt--;
if (rl->r_cnt == 0) {
avl_remove(tree, rl);
if (rl->r_write_wanted) {
if (rl->r_write_wanted)
cv_broadcast(&rl->r_wr_cv);
cv_destroy(&rl->r_wr_cv);
}
if (rl->r_read_wanted) {
if (rl->r_read_wanted)
cv_broadcast(&rl->r_rd_cv);
cv_destroy(&rl->r_rd_cv);
}
kmem_free(rl, sizeof (rl_t));
taskq_dispatch(system_taskq,
zfs_range_free, rl, 0);
}
}
mutex_exit(&zp->z_range_lock);
kmem_free(remove, sizeof (rl_t));
}
kmem_free(remove, sizeof (rl_t));
}
/*
@ -537,22 +553,19 @@ zfs_range_unlock(rl_t *rl)
/* writer locks can't be shared or split */
avl_remove(&zp->z_range_avl, rl);
mutex_exit(&zp->z_range_lock);
if (rl->r_write_wanted) {
if (rl->r_write_wanted)
cv_broadcast(&rl->r_wr_cv);
cv_destroy(&rl->r_wr_cv);
}
if (rl->r_read_wanted) {
if (rl->r_read_wanted)
cv_broadcast(&rl->r_rd_cv);
cv_destroy(&rl->r_rd_cv);
}
kmem_free(rl, sizeof (rl_t));
taskq_dispatch(system_taskq, zfs_range_free, rl, 0);
} else {
/*
* lock may be shared, let zfs_range_unlock_reader()
* release the lock and free the rl_t
* release the zp->z_range_lock lock and free the rl_t
*/
zfs_range_unlock_reader(zp, rl);
mutex_exit(&zp->z_range_lock);
}
}