Fix zfs_fsync deadlock

There are three locks at play, when handling ZIL records. First is a
zl_issuer_lock, that is supposed to be held, whenever we're issuing
a new record to ZIL. Then there is zl_lock to actually protect the in
the zilog struct. And then there's a last lock, zcw_lock, which is
used to protect the lists of ZIL call-back waiters.

The order of locking is supposed to be:

zl_issuer_lock -> zcw_lock -> zl_lock; as implied their usage
in zil_alloc_lwb and zil_commit_waiter_timeout functions.

Function zil_commit_waiter_link_lwb goes against this, it is expecting
to be entered with zl_lock already held and only then it is taking
the zcw_lock;

This patch straightens the locking in zil_commit_waiter_link_lwb to
take the zl_lock on its own, correcting the locking order.

Following is an attached messsage from the Linux lockdep mechanism
describing the potential deadlock - in our production, it was a matter
of a few hours to hit this one...

(redacted to fit 72 cols)

systemd-journal/5561 is trying to acquire lock:
(&zilog->zl_lock){+.+.}-{4:4}, at: zil_alloc_lwb+0x1df/0x3e0

but task is already holding lock:
(&zcw->zcw_lock){+.+.}-{4:4}, at: zil_commit_impl+0x52b/0x1850

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&zcw->zcw_lock){+.+.}-{4:4}:
       __mutex_lock+0xac/0x9e0
       mutex_lock_nested+0x1b/0x20
       zil_commit_waiter_link_lwb+0x51/0x1a0 [zfs]
       zil_commit_impl+0x12b0/0x1850 [zfs]
       zil_commit+0x43/0x60 [zfs]
       zpl_writepages+0xf8/0x1a0 [zfs]
       do_writepages+0x43/0xf0
       __filemap_fdatawrite_range+0xd5/0x110
       filemap_write_and_wait_range+0x4b/0xb0
       zpl_fsync+0x4d/0xb0 [zfs]
       vfs_fsync_range+0x49/0x80
       do_fsync+0x3d/0x70
       __x64_sys_fsync+0x14/0x20
       do_syscall_64+0x38/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xa9

-> #0 (&zilog->zl_lock){+.+.}-{4:4}:
       __lock_acquire+0x12b6/0x2480
       lock_acquire+0xab/0x380
       __mutex_lock+0xac/0x9e0
       mutex_lock_nested+0x1b/0x20
       zil_alloc_lwb+0x1df/0x3e0 [zfs]
       zil_lwb_write_issue+0x265/0x3f0 [zfs]
       zil_commit_impl+0x577/0x1850 [zfs]
       zil_commit+0x43/0x60 [zfs]
       zpl_writepages+0xf8/0x1a0 [zfs]
       do_writepages+0x43/0xf0
       __filemap_fdatawrite_range+0xd5/0x110
       filemap_write_and_wait_range+0x4b/0xb0
       zpl_fsync+0x4d/0xb0 [zfs]
       vfs_fsync_range+0x49/0x80
       do_fsync+0x3d/0x70
       __x64_sys_fsync+0x14/0x20
       do_syscall_64+0x38/0x90
       entry_SYSCALL_64_after_hwframe+0x44/0xa9

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&zcw->zcw_lock);
                               lock(&zilog->zl_lock);
                               lock(&zcw->zcw_lock);
  lock(&zilog->zl_lock);

 *** DEADLOCK ***

2 locks held by systemd-journal/5561:
 #0: (&zilog->zl_issuer_lock){+.+.}-{4:4}, at: zil_commit_impl+0x4dc...
 #1: (&zcw->zcw_lock){+.+.}-{4:4}, at: zil_commit_impl+0x52b/0x1850

Fixes openzfs#7038
Fixes openzfs#10440

Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>

module/zfs/zil.c

@@ -998,11 +998,8 @@ zil_commit_waiter_skip(zil_commit_waiter_t *zcw)
 static void
 zil_commit_waiter_link_lwb(zil_commit_waiter_t *zcw, lwb_t *lwb)
 {
-	/*
-	 * The lwb_waiters field of the lwb is protected by the zilog's
-	 * zl_lock, thus it must be held when calling this function.
-	 */
-	ASSERT(MUTEX_HELD(&lwb->lwb_zilog->zl_lock));
+	ASSERT(MUTEX_HELD(&lwb->lwb_zilog->zl_issuer_lock));
+	ASSERT(!MUTEX_HELD(&lwb->lwb_zilog->zl_lock));
 
 	mutex_enter(&zcw->zcw_lock);
 	ASSERT(!list_link_active(&zcw->zcw_node));
@@ -1012,8 +1009,10 @@ zil_commit_waiter_link_lwb(zil_commit_waiter_t *zcw, lwb_t *lwb)
 	    lwb->lwb_state == LWB_STATE_ISSUED ||
 	    lwb->lwb_state == LWB_STATE_WRITE_DONE);
 
+	mutex_enter(&lwb->lwb_zilog->zl_lock);
 	list_insert_tail(&lwb->lwb_waiters, zcw);
 	zcw->zcw_lwb = lwb;
+	mutex_exit(&lwb->lwb_zilog->zl_lock);
 	mutex_exit(&zcw->zcw_lock);
 }
 
@@ -1635,10 +1634,8 @@ zil_lwb_commit(zilog_t *zilog, itx_t *itx, lwb_t *lwb)
 	 * For more details, see the comment above zil_commit().
 	 */
 	if (lrc->lrc_txtype == TX_COMMIT) {
-		mutex_enter(&zilog->zl_lock);
 		zil_commit_waiter_link_lwb(itx->itx_private, lwb);
 		itx->itx_private = NULL;
-		mutex_exit(&zilog->zl_lock);
 		return (lwb);
 	}