// SPDX-License-Identifier: GPL-2.0

  /*

   * Copyright (c) 2000-2003,2005 Silicon Graphics, Inc.
   * All Rights Reserved.

   */
  #ifndef	__XFS_LOG_PRIV_H__
  #define __XFS_LOG_PRIV_H__
  
  struct xfs_buf;

  struct xlog;

  struct xlog_ticket;

  struct xfs_mount;
  
  /*

   * Flags for log structure

   */

  #define XLOG_ACTIVE_RECOVERY	0x2	/* in the middle of recovery */
  #define	XLOG_RECOVERY_NEEDED	0x4	/* log was recovered */
  #define XLOG_IO_ERROR		0x8	/* log hit an I/O error, and being
  					   shutdown */
  #define XLOG_TAIL_WARN		0x10	/* log tail verify warning issued */

  
  /*
   * get client id from packed copy.
   *
   * this hack is here because the xlog_pack code copies four bytes
   * of xlog_op_header containing the fields oh_clientid, oh_flags
   * and oh_res2 into the packed copy.
   *
   * later on this four byte chunk is treated as an int and the
   * client id is pulled out.
   *
   * this has endian issues, of course.
   */

  static inline uint xlog_get_client_id(__be32 i)

  {

  	return be32_to_cpu(i) >> 24;

  }

  /*
   * In core log state
   */

  enum xlog_iclog_state {
  	XLOG_STATE_ACTIVE,	/* Current IC log being written to */
  	XLOG_STATE_WANT_SYNC,	/* Want to sync this iclog; no more writes */
  	XLOG_STATE_SYNCING,	/* This IC log is syncing */
  	XLOG_STATE_DONE_SYNC,	/* Done syncing to disk */

  	XLOG_STATE_CALLBACK,	/* Callback functions now */
  	XLOG_STATE_DIRTY,	/* Dirty IC log, not ready for ACTIVE status */
  	XLOG_STATE_IOERROR,	/* IO error happened in sync'ing log */
  };

  
  /*

   * Log ticket flags

   */

  #define XLOG_TIC_PERM_RESERV	0x1	/* permanent reservation */

  
  #define XLOG_TIC_FLAGS \

  	{ XLOG_TIC_PERM_RESERV,	"XLOG_TIC_PERM_RESERV" }

  /*
   * Below are states for covering allocation transactions.
   * By covering, we mean changing the h_tail_lsn in the last on-disk
   * log write such that no allocation transactions will be re-done during
   * recovery after a system crash. Recovery starts at the last on-disk
   * log write.
   *
   * These states are used to insert dummy log entries to cover
   * space allocation transactions which can undo non-transactional changes
   * after a crash. Writes to a file with space
   * already allocated do not result in any transactions. Allocations
   * might include space beyond the EOF. So if we just push the EOF a
   * little, the last transaction for the file could contain the wrong
   * size. If there is no file system activity, after an allocation
   * transaction, and the system crashes, the allocation transaction
   * will get replayed and the file will be truncated. This could
   * be hours/days/... after the allocation occurred.
   *
   * The fix for this is to do two dummy transactions when the
   * system is idle. We need two dummy transaction because the h_tail_lsn
   * in the log record header needs to point beyond the last possible
   * non-dummy transaction. The first dummy changes the h_tail_lsn to
   * the first transaction before the dummy. The second dummy causes
   * h_tail_lsn to point to the first dummy. Recovery starts at h_tail_lsn.
   *
   * These dummy transactions get committed when everything
   * is idle (after there has been some activity).
   *
   * There are 5 states used to control this.
   *
   *  IDLE -- no logging has been done on the file system or
   *		we are done covering previous transactions.
   *  NEED -- logging has occurred and we need a dummy transaction
   *		when the log becomes idle.
   *  DONE -- we were in the NEED state and have committed a dummy
   *		transaction.
   *  NEED2 -- we detected that a dummy transaction has gone to the
   *		on disk log with no other transactions.
   *  DONE2 -- we committed a dummy transaction when in the NEED2 state.
   *
   * There are two places where we switch states:
   *
   * 1.) In xfs_sync, when we detect an idle log and are in NEED or NEED2.
   *	We commit the dummy transaction and switch to DONE or DONE2,
   *	respectively. In all other states, we don't do anything.
   *
   * 2.) When we finish writing the on-disk log (xlog_state_clean_log).
   *
   *	No matter what state we are in, if this isn't the dummy
   *	transaction going out, the next state is NEED.
   *	So, if we aren't in the DONE or DONE2 states, the next state
   *	is NEED. We can't be finishing a write of the dummy record
   *	unless it was committed and the state switched to DONE or DONE2.
   *
   *	If we are in the DONE state and this was a write of the
   *		dummy transaction, we move to NEED2.
   *
   *	If we are in the DONE2 state and this was a write of the
   *		dummy transaction, we move to IDLE.
   *
   *
   * Writing only one dummy transaction can get appended to
   * one file space allocation. When this happens, the log recovery
   * code replays the space allocation and a file could be truncated.
   * This is why we have the NEED2 and DONE2 states before going idle.
   */
  
  #define XLOG_STATE_COVER_IDLE	0
  #define XLOG_STATE_COVER_NEED	1
  #define XLOG_STATE_COVER_DONE	2
  #define XLOG_STATE_COVER_NEED2	3
  #define XLOG_STATE_COVER_DONE2	4
  
  #define XLOG_COVER_OPS		5

  /* Ticket reservation region accounting */ 

  #define XLOG_TIC_LEN_MAX	15

  
  /*
   * Reservation region
   * As would be stored in xfs_log_iovec but without the i_addr which
   * we don't care about.
   */
  typedef struct xlog_res {

  	uint	r_len;	/* region length		:4 */
  	uint	r_type;	/* region's transaction type	:4 */

  } xlog_res_t;

  typedef struct xlog_ticket {

  	struct list_head   t_queue;	 /* reserve/write queue */

  	struct task_struct *t_task;	 /* task that owns this ticket */

  	xlog_tid_t	   t_tid;	 /* transaction identifier	 : 4  */

  	atomic_t	   t_ref;	 /* ticket reference count       : 4  */

  	int		   t_curr_res;	 /* current reservation in bytes : 4  */
  	int		   t_unit_res;	 /* unit reservation in bytes    : 4  */
  	char		   t_ocnt;	 /* original count		 : 1  */
  	char		   t_cnt;	 /* current count		 : 1  */
  	char		   t_clientid;	 /* who does this belong to;	 : 1  */
  	char		   t_flags;	 /* properties of reservation	 : 1  */

          /* reservation array fields */
  	uint		   t_res_num;                    /* num in array : 4 */

  	uint		   t_res_num_ophdrs;		 /* num op hdrs  : 4 */
  	uint		   t_res_arr_sum;		 /* array sum    : 4 */
  	uint		   t_res_o_flow;		 /* sum overflow : 4 */

  	xlog_res_t	   t_res_arr[XLOG_TIC_LEN_MAX];  /* array of res : 8 * 15 */ 

  } xlog_ticket_t;

  /*
   * - A log record header is 512 bytes.  There is plenty of room to grow the
   *	xlog_rec_header_t into the reserved space.
   * - ic_data follows, so a write to disk can start at the beginning of
   *	the iclog.

   * - ic_forcewait is used to implement synchronous forcing of the iclog to disk.

   * - ic_next is the pointer to the next iclog in the ring.

   * - ic_log is a pointer back to the global log structure.

   * - ic_size is the full size of the log buffer, minus the cycle headers.

   * - ic_offset is the current number of bytes written to in this iclog.
   * - ic_refcnt is bumped when someone is writing to the log.
   * - ic_state is the state of the iclog.

   *
   * Because of cacheline contention on large machines, we need to separate
   * various resources onto different cachelines. To start with, make the
   * structure cacheline aligned. The following fields can be contended on
   * by independent processes:
   *

   *	- ic_callbacks

   *	- ic_refcnt
   *	- fields protected by the global l_icloglock
   *
   * so we need to ensure that these fields are located in separate cachelines.
   * We'll put all the read-only and l_icloglock fields in the first cacheline,
   * and move everything else out to subsequent cachelines.

   */

  typedef struct xlog_in_core {

  	wait_queue_head_t	ic_force_wait;
  	wait_queue_head_t	ic_write_wait;

  	struct xlog_in_core	*ic_next;
  	struct xlog_in_core	*ic_prev;

  	struct xlog		*ic_log;

  	u32			ic_size;

  	u32			ic_offset;

  	enum xlog_iclog_state	ic_state;

  	char			*ic_datap;	/* pointer to iclog data */

  
  	/* Callback structures need their own cacheline */
  	spinlock_t		ic_callback_lock ____cacheline_aligned_in_smp;

  	struct list_head	ic_callbacks;

  
  	/* reference counts need their own cacheline */
  	atomic_t		ic_refcnt ____cacheline_aligned_in_smp;

  	xlog_in_core_2_t	*ic_data;
  #define ic_header	ic_data->hic_header

  #ifdef DEBUG
  	bool			ic_fail_crc : 1;
  #endif

  	struct semaphore	ic_sema;
  	struct work_struct	ic_end_io_work;
  	struct bio		ic_bio;
  	struct bio_vec		ic_bvec[];

  } xlog_in_core_t;
  
  /*

   * The CIL context is used to aggregate per-transaction details as well be
   * passed to the iclog for checkpoint post-commit processing.  After being
   * passed to the iclog, another context needs to be allocated for tracking the
   * next set of transactions to be aggregated into a checkpoint.
   */
  struct xfs_cil;
  
  struct xfs_cil_ctx {
  	struct xfs_cil		*cil;
  	xfs_lsn_t		sequence;	/* chkpt sequence # */
  	xfs_lsn_t		start_lsn;	/* first LSN of chkpt commit */
  	xfs_lsn_t		commit_lsn;	/* chkpt commit record lsn */
  	struct xlog_ticket	*ticket;	/* chkpt ticket */
  	int			nvecs;		/* number of regions */
  	int			space_used;	/* aggregate size of regions */
  	struct list_head	busy_extents;	/* busy extents in chkpt */
  	struct xfs_log_vec	*lv_chain;	/* logvecs being pushed */

  	struct list_head	iclog_entry;

  	struct list_head	committing;	/* ctx committing list */

  	struct work_struct	discard_endio_work;

  };
  
  /*
   * Committed Item List structure
   *
   * This structure is used to track log items that have been committed but not
   * yet written into the log. It is used only when the delayed logging mount
   * option is enabled.
   *
   * This structure tracks the list of committing checkpoint contexts so
   * we can avoid the problem of having to hold out new transactions during a
   * flush until we have a the commit record LSN of the checkpoint. We can
   * traverse the list of committing contexts in xlog_cil_push_lsn() to find a
   * sequence match and extract the commit LSN directly from there. If the
   * checkpoint is still in the process of committing, we can block waiting for
   * the commit LSN to be determined as well. This should make synchronous
   * operations almost as efficient as the old logging methods.
   */
  struct xfs_cil {

  	struct xlog		*xc_log;

  	struct list_head	xc_cil;
  	spinlock_t		xc_cil_lock;

  
  	struct rw_semaphore	xc_ctx_lock ____cacheline_aligned_in_smp;

  	struct xfs_cil_ctx	*xc_ctx;

  
  	spinlock_t		xc_push_lock ____cacheline_aligned_in_smp;
  	xfs_lsn_t		xc_push_seq;

  	struct list_head	xc_committing;

  	wait_queue_head_t	xc_commit_wait;

  	xfs_lsn_t		xc_current_sequence;

  	struct work_struct	xc_push_work;

  	wait_queue_head_t	xc_push_wait;	/* background push throttle */

  } ____cacheline_aligned_in_smp;

  
  /*

   * The amount of log space we allow the CIL to aggregate is difficult to size.
   * Whatever we choose, we have to make sure we can get a reservation for the
   * log space effectively, that it is large enough to capture sufficient
   * relogging to reduce log buffer IO significantly, but it is not too large for
   * the log or induces too much latency when writing out through the iclogs. We
   * track both space consumed and the number of vectors in the checkpoint
   * context, so we need to decide which to use for limiting.

   *
   * Every log buffer we write out during a push needs a header reserved, which
   * is at least one sector and more for v2 logs. Hence we need a reservation of
   * at least 512 bytes per 32k of log space just for the LR headers. That means
   * 16KB of reservation per megabyte of delayed logging space we will consume,
   * plus various headers.  The number of headers will vary based on the num of
   * io vectors, so limiting on a specific number of vectors is going to result
   * in transactions of varying size. IOWs, it is more consistent to track and
   * limit space consumed in the log rather than by the number of objects being
   * logged in order to prevent checkpoint ticket overruns.
   *
   * Further, use of static reservations through the log grant mechanism is
   * problematic. It introduces a lot of complexity (e.g. reserve grant vs write
   * grant) and a significant deadlock potential because regranting write space
   * can block on log pushes. Hence if we have to regrant log space during a log
   * push, we can deadlock.
   *
   * However, we can avoid this by use of a dynamic "reservation stealing"
   * technique during transaction commit whereby unused reservation space in the
   * transaction ticket is transferred to the CIL ctx commit ticket to cover the
   * space needed by the checkpoint transaction. This means that we never need to
   * specifically reserve space for the CIL checkpoint transaction, nor do we
   * need to regrant space once the checkpoint completes. This also means the
   * checkpoint transaction ticket is specific to the checkpoint context, rather
   * than the CIL itself.
   *

   * With dynamic reservations, we can effectively make up arbitrary limits for
   * the checkpoint size so long as they don't violate any other size rules.
   * Recovery imposes a rule that no transaction exceed half the log, so we are
   * limited by that.  Furthermore, the log transaction reservation subsystem
   * tries to keep 25% of the log free, so we need to keep below that limit or we
   * risk running out of free log space to start any new transactions.
   *

   * In order to keep background CIL push efficient, we only need to ensure the
   * CIL is large enough to maintain sufficient in-memory relogging to avoid
   * repeated physical writes of frequently modified metadata. If we allow the CIL
   * to grow to a substantial fraction of the log, then we may be pinning hundreds
   * of megabytes of metadata in memory until the CIL flushes. This can cause
   * issues when we are running low on memory - pinned memory cannot be reclaimed,
   * and the CIL consumes a lot of memory. Hence we need to set an upper physical
   * size limit for the CIL that limits the maximum amount of memory pinned by the
   * CIL but does not limit performance by reducing relogging efficiency
   * significantly.
   *
   * As such, the CIL push threshold ends up being the smaller of two thresholds:
   * - a threshold large enough that it allows CIL to be pushed and progress to be
   *   made without excessive blocking of incoming transaction commits. This is
   *   defined to be 12.5% of the log space - half the 25% push threshold of the
   *   AIL.
   * - small enough that it doesn't pin excessive amounts of memory but maintains
   *   close to peak relogging efficiency. This is defined to be 16x the iclog
   *   buffer window (32MB) as measurements have shown this to be roughly the
   *   point of diminishing performance increases under highly concurrent
   *   modification workloads.

   *
   * To prevent the CIL from overflowing upper commit size bounds, we introduce a
   * new threshold at which we block committing transactions until the background
   * CIL commit commences and switches to a new context. While this is not a hard
   * limit, it forces the process committing a transaction to the CIL to block and
   * yeild the CPU, giving the CIL push work a chance to be scheduled and start
   * work. This prevents a process running lots of transactions from overfilling
   * the CIL because it is not yielding the CPU. We set the blocking limit at
   * twice the background push space threshold so we keep in line with the AIL
   * push thresholds.
   *
   * Note: this is not a -hard- limit as blocking is applied after the transaction
   * is inserted into the CIL and the push has been triggered. It is largely a
   * throttling mechanism that allows the CIL push to be scheduled and run. A hard
   * limit will be difficult to implement without introducing global serialisation
   * in the CIL commit fast path, and it's not at all clear that we actually need
   * such hard limits given the ~7 years we've run without a hard limit before
   * finding the first situation where a checkpoint size overflow actually
   * occurred. Hence the simple throttle, and an ASSERT check to tell us that
   * we've overrun the max size.

   */

  #define XLOG_CIL_SPACE_LIMIT(log)	\
  	min_t(int, (log)->l_logsize >> 3, BBTOB(XLOG_TOTAL_REC_SHIFT(log)) << 4)

  #define XLOG_CIL_BLOCKING_SPACE_LIMIT(log)	\
  	(XLOG_CIL_SPACE_LIMIT(log) * 2)

  /*

   * ticket grant locks, queues and accounting have their own cachlines
   * as these are quite hot and can be operated on concurrently.
   */
  struct xlog_grant_head {
  	spinlock_t		lock ____cacheline_aligned_in_smp;
  	struct list_head	waiters;
  	atomic64_t		grant;
  };
  
  /*

   * The reservation head lsn is not made up of a cycle number and block number.
   * Instead, it uses a cycle number and byte number.  Logs don't expect to
   * overflow 31 bits worth of byte offset, so using a byte number will mean
   * that round off problems won't occur when releasing partial reservations.
   */

  struct xlog {

  	/* The following fields don't need locking */
  	struct xfs_mount	*l_mp;	        /* mount point */

  	struct xfs_ail		*l_ailp;	/* AIL log is working with */

  	struct xfs_cil		*l_cilp;	/* CIL log is working with */

  	struct xfs_buftarg	*l_targ;        /* buftarg of log */

  	struct workqueue_struct	*l_ioend_workqueue; /* for I/O completions */

  	struct delayed_work	l_work;		/* background flush work */

  	uint			l_flags;
  	uint			l_quotaoffs_flag; /* XFS_DQ_*, for QUOTAOFFs */

  	struct list_head	*l_buf_cancel_table;

  	int			l_iclog_hsize;  /* size of iclog header */
  	int			l_iclog_heads;  /* # of iclog header sectors */

  	uint			l_sectBBsize;   /* sector size in BBs (2^n) */

  	int			l_iclog_size;	/* size of log in bytes */

  	int			l_iclog_bufs;	/* number of iclog buffers */
  	xfs_daddr_t		l_logBBstart;   /* start block of log */
  	int			l_logsize;      /* size of log in bytes */
  	int			l_logBBsize;    /* size of log in BB chunks */

  	/* The following block of fields are changed while holding icloglock */

  	wait_queue_head_t	l_flush_wait ____cacheline_aligned_in_smp;

  						/* waiting for iclog flush */

  	int			l_covered_state;/* state of "covering disk
  						 * log entries" */

  	xlog_in_core_t		*l_iclog;       /* head log queue	*/

  	spinlock_t		l_icloglock;    /* grab to change iclog state */

  	int			l_curr_cycle;   /* Cycle number of log writes */
  	int			l_prev_cycle;   /* Cycle number before last
  						 * block increment */
  	int			l_curr_block;   /* current logical log block */
  	int			l_prev_block;   /* previous logical log block */

  	/*

  	 * l_last_sync_lsn and l_tail_lsn are atomics so they can be set and
  	 * read without needing to hold specific locks. To avoid operations
  	 * contending with other hot objects, place each of them on a separate
  	 * cacheline.

  	 */
  	/* lsn of last LR on disk */
  	atomic64_t		l_last_sync_lsn ____cacheline_aligned_in_smp;

  	/* lsn of 1st LR with unflushed * buffers */
  	atomic64_t		l_tail_lsn ____cacheline_aligned_in_smp;

  	struct xlog_grant_head	l_reserve_head;
  	struct xlog_grant_head	l_write_head;

  	struct xfs_kobj		l_kobj;

  	/* The following field are used for debugging; need to hold icloglock */
  #ifdef DEBUG

  	void			*l_iclog_bak[XLOG_MAX_ICLOGS];

  #endif

  	/* log recovery lsn tracking (for buffer submission */
  	xfs_lsn_t		l_recovery_lsn;

  };

  #define XLOG_BUF_CANCEL_BUCKET(log, blkno) \

  	((log)->l_buf_cancel_table + ((uint64_t)blkno % XLOG_BC_TABLE_SIZE))

  #define XLOG_FORCED_SHUTDOWN(log) \
  	(unlikely((log)->l_flags & XLOG_IO_ERROR))

  /* common routines */

  extern int
  xlog_recover(
  	struct xlog		*log);
  extern int
  xlog_recover_finish(
  	struct xlog		*log);

  extern void

  xlog_recover_cancel(struct xlog *);

  extern __le32	 xlog_cksum(struct xlog *log, struct xlog_rec_header *rhead,

  			    char *dp, int size);

  extern kmem_zone_t *xfs_log_ticket_zone;

  struct xlog_ticket *
  xlog_ticket_alloc(
  	struct xlog	*log,
  	int		unit_bytes,
  	int		count,
  	char		client,

  	bool		permanent);

  static inline void
  xlog_write_adv_cnt(void **ptr, int *len, int *off, size_t bytes)
  {
  	*ptr += bytes;
  	*len -= bytes;
  	*off += bytes;
  }

  void	xlog_print_tic_res(struct xfs_mount *mp, struct xlog_ticket *ticket);

  void	xlog_print_trans(struct xfs_trans *);

  int	xlog_write(struct xlog *log, struct xfs_log_vec *log_vector,
  		struct xlog_ticket *tic, xfs_lsn_t *start_lsn,
  		struct xlog_in_core **commit_iclog, uint flags,
  		bool need_start_rec);

  int	xlog_commit_record(struct xlog *log, struct xlog_ticket *ticket,

  		struct xlog_in_core **iclog, xfs_lsn_t *lsn);

  void	xfs_log_ticket_ungrant(struct xlog *log, struct xlog_ticket *ticket);
  void	xfs_log_ticket_regrant(struct xlog *log, struct xlog_ticket *ticket);

  
  /*

   * When we crack an atomic LSN, we sample it first so that the value will not
   * change while we are cracking it into the component values. This means we
   * will always get consistent component values to work from. This should always

   * be used to sample and crack LSNs that are stored and updated in atomic

   * variables.
   */
  static inline void
  xlog_crack_atomic_lsn(atomic64_t *lsn, uint *cycle, uint *block)
  {
  	xfs_lsn_t val = atomic64_read(lsn);
  
  	*cycle = CYCLE_LSN(val);
  	*block = BLOCK_LSN(val);
  }
  
  /*
   * Calculate and assign a value to an atomic LSN variable from component pieces.
   */
  static inline void
  xlog_assign_atomic_lsn(atomic64_t *lsn, uint cycle, uint block)
  {
  	atomic64_set(lsn, xlog_assign_lsn(cycle, block));
  }
  
  /*

   * When we crack the grant head, we sample it first so that the value will not

   * change while we are cracking it into the component values. This means we
   * will always get consistent component values to work from.
   */
  static inline void

  xlog_crack_grant_head_val(int64_t val, int *cycle, int *space)

  {

  	*cycle = val >> 32;
  	*space = val & 0xffffffff;
  }
  
  static inline void

  xlog_crack_grant_head(atomic64_t *head, int *cycle, int *space)
  {
  	xlog_crack_grant_head_val(atomic64_read(head), cycle, space);
  }
  
  static inline int64_t
  xlog_assign_grant_head_val(int cycle, int space)
  {
  	return ((int64_t)cycle << 32) | space;
  }
  
  static inline void

  xlog_assign_grant_head(atomic64_t *head, int cycle, int space)

  {

  	atomic64_set(head, xlog_assign_grant_head_val(cycle, space));

  }
  
  /*

   * Committed Item List interfaces
   */

  int	xlog_cil_init(struct xlog *log);
  void	xlog_cil_init_post_recovery(struct xlog *log);
  void	xlog_cil_destroy(struct xlog *log);
  bool	xlog_cil_empty(struct xlog *log);

  /*
   * CIL force routines
   */

  xfs_lsn_t
  xlog_cil_force_lsn(
  	struct xlog *log,
  	xfs_lsn_t sequence);

  
  static inline void

  xlog_cil_force(struct xlog *log)

  {
  	xlog_cil_force_lsn(log, log->l_cilp->xc_current_sequence);
  }

  /*

   * Wrapper function for waiting on a wait queue serialised against wakeups
   * by a spinlock. This matches the semantics of all the wait queues used in the
   * log code.
   */

  static inline void
  xlog_wait(
  	struct wait_queue_head	*wq,
  	struct spinlock		*lock)
  		__releases(lock)

  {
  	DECLARE_WAITQUEUE(wait, current);
  
  	add_wait_queue_exclusive(wq, &wait);
  	__set_current_state(TASK_UNINTERRUPTIBLE);
  	spin_unlock(lock);
  	schedule();
  	remove_wait_queue(wq, &wait);
  }

  /*
   * The LSN is valid so long as it is behind the current LSN. If it isn't, this
   * means that the next log record that includes this metadata could have a
   * smaller LSN. In turn, this means that the modification in the log would not
   * replay.
   */
  static inline bool
  xlog_valid_lsn(
  	struct xlog	*log,
  	xfs_lsn_t	lsn)
  {
  	int		cur_cycle;
  	int		cur_block;
  	bool		valid = true;
  
  	/*
  	 * First, sample the current lsn without locking to avoid added
  	 * contention from metadata I/O. The current cycle and block are updated
  	 * (in xlog_state_switch_iclogs()) and read here in a particular order
  	 * to avoid false negatives (e.g., thinking the metadata LSN is valid
  	 * when it is not).
  	 *
  	 * The current block is always rewound before the cycle is bumped in
  	 * xlog_state_switch_iclogs() to ensure the current LSN is never seen in
  	 * a transiently forward state. Instead, we can see the LSN in a
  	 * transiently behind state if we happen to race with a cycle wrap.
  	 */

  	cur_cycle = READ_ONCE(log->l_curr_cycle);

  	smp_rmb();

  	cur_block = READ_ONCE(log->l_curr_block);

  
  	if ((CYCLE_LSN(lsn) > cur_cycle) ||
  	    (CYCLE_LSN(lsn) == cur_cycle && BLOCK_LSN(lsn) > cur_block)) {
  		/*
  		 * If the metadata LSN appears invalid, it's possible the check
  		 * above raced with a wrap to the next log cycle. Grab the lock
  		 * to check for sure.
  		 */
  		spin_lock(&log->l_icloglock);
  		cur_cycle = log->l_curr_cycle;
  		cur_block = log->l_curr_block;
  		spin_unlock(&log->l_icloglock);
  
  		if ((CYCLE_LSN(lsn) > cur_cycle) ||
  		    (CYCLE_LSN(lsn) == cur_cycle && BLOCK_LSN(lsn) > cur_block))
  			valid = false;
  	}
  
  	return valid;
  }

  #endif	/* __XFS_LOG_PRIV_H__ */