linux/include/linux/iocontext.h
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   1#ifndef IOCONTEXT_H
   2#define IOCONTEXT_H
   3
   4#include <linux/radix-tree.h>
   5#include <linux/rcupdate.h>
   6#include <linux/workqueue.h>
   7
   8enum {
   9        ICQ_EXITED              = 1 << 2,
  10};
  11
  12/*
  13 * An io_cq (icq) is association between an io_context (ioc) and a
  14 * request_queue (q).  This is used by elevators which need to track
  15 * information per ioc - q pair.
  16 *
  17 * Elevator can request use of icq by setting elevator_type->icq_size and
  18 * ->icq_align.  Both size and align must be larger than that of struct
  19 * io_cq and elevator can use the tail area for private information.  The
  20 * recommended way to do this is defining a struct which contains io_cq as
  21 * the first member followed by private members and using its size and
  22 * align.  For example,
  23 *
  24 *      struct snail_io_cq {
  25 *              struct io_cq    icq;
  26 *              int             poke_snail;
  27 *              int             feed_snail;
  28 *      };
  29 *
  30 *      struct elevator_type snail_elv_type {
  31 *              .ops =          { ... },
  32 *              .icq_size =     sizeof(struct snail_io_cq),
  33 *              .icq_align =    __alignof__(struct snail_io_cq),
  34 *              ...
  35 *      };
  36 *
  37 * If icq_size is set, block core will manage icq's.  All requests will
  38 * have its ->elv.icq field set before elevator_ops->elevator_set_req_fn()
  39 * is called and be holding a reference to the associated io_context.
  40 *
  41 * Whenever a new icq is created, elevator_ops->elevator_init_icq_fn() is
  42 * called and, on destruction, ->elevator_exit_icq_fn().  Both functions
  43 * are called with both the associated io_context and queue locks held.
  44 *
  45 * Elevator is allowed to lookup icq using ioc_lookup_icq() while holding
  46 * queue lock but the returned icq is valid only until the queue lock is
  47 * released.  Elevators can not and should not try to create or destroy
  48 * icq's.
  49 *
  50 * As icq's are linked from both ioc and q, the locking rules are a bit
  51 * complex.
  52 *
  53 * - ioc lock nests inside q lock.
  54 *
  55 * - ioc->icq_list and icq->ioc_node are protected by ioc lock.
  56 *   q->icq_list and icq->q_node by q lock.
  57 *
  58 * - ioc->icq_tree and ioc->icq_hint are protected by ioc lock, while icq
  59 *   itself is protected by q lock.  However, both the indexes and icq
  60 *   itself are also RCU managed and lookup can be performed holding only
  61 *   the q lock.
  62 *
  63 * - icq's are not reference counted.  They are destroyed when either the
  64 *   ioc or q goes away.  Each request with icq set holds an extra
  65 *   reference to ioc to ensure it stays until the request is completed.
  66 *
  67 * - Linking and unlinking icq's are performed while holding both ioc and q
  68 *   locks.  Due to the lock ordering, q exit is simple but ioc exit
  69 *   requires reverse-order double lock dance.
  70 */
  71struct io_cq {
  72        struct request_queue    *q;
  73        struct io_context       *ioc;
  74
  75        /*
  76         * q_node and ioc_node link io_cq through icq_list of q and ioc
  77         * respectively.  Both fields are unused once ioc_exit_icq() is
  78         * called and shared with __rcu_icq_cache and __rcu_head which are
  79         * used for RCU free of io_cq.
  80         */
  81        union {
  82                struct list_head        q_node;
  83                struct kmem_cache       *__rcu_icq_cache;
  84        };
  85        union {
  86                struct hlist_node       ioc_node;
  87                struct rcu_head         __rcu_head;
  88        };
  89
  90        unsigned int            flags;
  91};
  92
  93/*
  94 * I/O subsystem state of the associated processes.  It is refcounted
  95 * and kmalloc'ed. These could be shared between processes.
  96 */
  97struct io_context {
  98        atomic_long_t refcount;
  99        atomic_t active_ref;
 100        atomic_t nr_tasks;
 101
 102        /* all the fields below are protected by this lock */
 103        spinlock_t lock;
 104
 105        unsigned short ioprio;
 106
 107        /*
 108         * For request batching
 109         */
 110        int nr_batch_requests;     /* Number of requests left in the batch */
 111        unsigned long last_waited; /* Time last woken after wait for request */
 112
 113        struct radix_tree_root  icq_tree;
 114        struct io_cq __rcu      *icq_hint;
 115        struct hlist_head       icq_list;
 116
 117        struct work_struct release_work;
 118};
 119
 120/**
 121 * get_io_context_active - get active reference on ioc
 122 * @ioc: ioc of interest
 123 *
 124 * Only iocs with active reference can issue new IOs.  This function
 125 * acquires an active reference on @ioc.  The caller must already have an
 126 * active reference on @ioc.
 127 */
 128static inline void get_io_context_active(struct io_context *ioc)
 129{
 130        WARN_ON_ONCE(atomic_long_read(&ioc->refcount) <= 0);
 131        WARN_ON_ONCE(atomic_read(&ioc->active_ref) <= 0);
 132        atomic_long_inc(&ioc->refcount);
 133        atomic_inc(&ioc->active_ref);
 134}
 135
 136static inline void ioc_task_link(struct io_context *ioc)
 137{
 138        get_io_context_active(ioc);
 139
 140        WARN_ON_ONCE(atomic_read(&ioc->nr_tasks) <= 0);
 141        atomic_inc(&ioc->nr_tasks);
 142}
 143
 144struct task_struct;
 145#ifdef CONFIG_BLOCK
 146void put_io_context(struct io_context *ioc);
 147void put_io_context_active(struct io_context *ioc);
 148void exit_io_context(struct task_struct *task);
 149struct io_context *get_task_io_context(struct task_struct *task,
 150                                       gfp_t gfp_flags, int node);
 151#else
 152struct io_context;
 153static inline void put_io_context(struct io_context *ioc) { }
 154static inline void exit_io_context(struct task_struct *task) { }
 155#endif
 156
 157#endif
 158
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