GNU Linux-libre 5.4.257-gnu1

[releases.git] / fs / xfs / xfs_log_priv.h

// 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
 */
#define XLOG_STATE_ACTIVE    0x0001 /* Current IC log being written to */
#define XLOG_STATE_WANT_SYNC 0x0002 /* Want to sync this iclog; no more writes */
#define XLOG_STATE_SYNCING   0x0004 /* This IC log is syncing */
#define XLOG_STATE_DONE_SYNC 0x0008 /* Done syncing to disk */
#define XLOG_STATE_DO_CALLBACK \
                             0x0010 /* Process callback functions */
#define XLOG_STATE_CALLBACK  0x0020 /* Callback functions now */
#define XLOG_STATE_DIRTY     0x0040 /* Dirty IC log, not ready for ACTIVE status*/
#define XLOG_STATE_IOERROR   0x0080 /* IO error happened in sync'ing log */
#define XLOG_STATE_ALL       0x7FFF /* All possible valid flags */
#define XLOG_STATE_NOTUSED   0x8000 /* This IC log not being used */

/*
 * Flags to log ticket
 */
#define XLOG_TIC_INITED         0x1     /* has been initialized */
#define XLOG_TIC_PERM_RESERV    0x2     /* permanent reservation */

#define XLOG_TIC_FLAGS \
        { XLOG_TIC_INITED,      "XLOG_TIC_INITED" }, \
        { 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_io_size is the size of the currently pending log buffer write, which
 *      might be smaller than ic_size
 * - 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_io_size;
        u32                     ic_offset;
        unsigned short          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];
        /* log record crc error injection factor */
        uint32_t                l_badcrc_factor;
#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)       ((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,
        xfs_km_flags_t  alloc_flags);


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);

/*
 * 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);
}

/*
 * Unmount record type is used as a pseudo transaction type for the ticket.
 * It's value must be outside the range of XFS_TRANS_* values.
 */
#define XLOG_UNMOUNT_REC_TYPE   (-1U)

/*
 * 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(wait_queue_head_t *wq, spinlock_t *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__ */