path: root/kern/gsync.c

/* Copyright (C) 2016 Free Software Foundation, Inc.
   Contributed by Agustina Arzille <avarzille@riseup.net>, 2016.

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License
   as published by the Free Software Foundation; either
   version 2 of the license, or (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public
   License along with this program; if not, see
   <http://www.gnu.org/licenses/>.
*/

#include <kern/gsync.h>
#include <kern/kmutex.h>
#include <kern/sched_prim.h>
#include <kern/thread.h>
#include <kern/list.h>
#include <vm/vm_map.h>
#include <vm/vm_kern.h>

/* An entry in the global hash table. */
struct gsync_hbucket
{
  struct list entries;
  struct kmutex lock;
};

/* A key used to uniquely identify an address that a thread is
 * waiting on. Its members' values depend on whether said
 * address is shared or task-local. Note that different types of keys
 * should never compare equal, since a task map should never have
 * the same address as a VM object. */
union gsync_key
{
  struct
    {
      vm_map_t map;
      vm_offset_t addr;
    } local;

  struct
    {
      vm_object_t obj;
      vm_offset_t off;
    } shared;

  struct
    {
      unsigned long u;
      unsigned long v;
    } any;
};

/* A thread that is blocked on an address with 'gsync_wait'. */
struct gsync_waiter
{
  struct list link;
  union gsync_key key;
  thread_t waiter;
};

/* Needed data for temporary mappings. */
struct vm_args
{
  vm_object_t obj;
  vm_offset_t off;
};

#define GSYNC_NBUCKETS   512
static struct gsync_hbucket gsync_buckets[GSYNC_NBUCKETS];

void gsync_setup (void)
{
  int i;
  for (i = 0; i < GSYNC_NBUCKETS; ++i)
    {
      list_init (&gsync_buckets[i].entries);
      kmutex_init (&gsync_buckets[i].lock);
    }
}

/* Convenience comparison functions for gsync_key's. */

static inline int
gsync_key_eq (const union gsync_key *lp,
  const union gsync_key *rp)
{
  return (lp->any.u == rp->any.u && lp->any.v == rp->any.v);
}

static inline int
gsync_key_lt (const union gsync_key *lp,
  const union gsync_key *rp)
{
  return (lp->any.u < rp->any.u ||
    (lp->any.u == rp->any.u && lp->any.v < rp->any.v));
}

#define MIX2_LL(x, y)   ((((x) << 5) | ((x) >> 27)) ^ (y))

static inline unsigned int
gsync_key_hash (const union gsync_key *keyp)
{
  unsigned int ret = sizeof (void *);
#ifndef __LP64__
  ret = MIX2_LL (ret, keyp->any.u);
  ret = MIX2_LL (ret, keyp->any.v);
#else
  ret = MIX2_LL (ret, keyp->any.u & ~0U);
  ret = MIX2_LL (ret, keyp->any.u >> 32);
  ret = MIX2_LL (ret, keyp->any.v & ~0U);
  ret = MIX2_LL (ret, keyp->any.v >> 32);
#endif
  return (ret);
}

/* Perform a VM lookup for the address in the map. The FLAGS
 * parameter is used to specify some attributes for the address,
 * such as protection. Place the corresponding VM object/offset pair
 * in VAP. Returns 0 if successful, -1 otherwise. */
static int
probe_address (vm_map_t map, vm_offset_t addr,
  int flags, struct vm_args *vap)
{
  vm_prot_t prot = VM_PROT_READ |
    ((flags & GSYNC_MUTATE) ? VM_PROT_WRITE : 0);
  vm_map_version_t ver;
  vm_prot_t rprot;
  boolean_t wired_p;

  if (vm_map_lookup (&map, addr, prot, &ver,
      &vap->obj, &vap->off, &rprot, &wired_p) != KERN_SUCCESS)
    return (-1);
  else if ((rprot & prot) != prot)
    {
      vm_object_unlock (vap->obj);
      return (-1);
    }

  return (0);
}

/* Initialize the key with its needed members, depending on whether the
 * address is local or shared. Also stores the VM object and offset inside
 * the argument VAP for future use. */
static int
gsync_prepare_key (task_t task, vm_offset_t addr, int flags,
  union gsync_key *keyp, struct vm_args *vap)
{
  if (probe_address (task->map, addr, flags, vap) < 0)
    return (-1);
  else if (flags & GSYNC_SHARED)
    {
      /* For a shared address, we need the VM object
       * and offset as the keys. */
      keyp->shared.obj = vap->obj;
      keyp->shared.off = vap->off;
    }
  else
    {
      /* Task-local address. The keys are the task's map and
       * the virtual address itself. */
      keyp->local.map = task->map;
      keyp->local.addr = addr;
    }

  return ((int)(gsync_key_hash (keyp) % GSYNC_NBUCKETS));
}

static inline struct gsync_waiter*
node_to_waiter (struct list *nodep)
{
  return (list_entry (nodep, struct gsync_waiter, link));
}

static inline struct list*
gsync_find_key (const struct list *entries,
  const union gsync_key *keyp, int *exactp)
{
  /* Look for a key that matches. We take advantage of the fact
   * that the entries are sorted to break out of the loop as
   * early as possible. */
  struct list *runp;
  list_for_each (entries, runp)
    {
      struct gsync_waiter *p = node_to_waiter (runp);
      if (gsync_key_lt (keyp, &p->key))
        break;
      else if (gsync_key_eq (keyp, &p->key))
        {
          if (exactp != 0)
            *exactp = 1;
          break;
        }
    }

  return (runp);
}

/* Create a temporary mapping in the kernel.*/
static inline vm_offset_t
temp_mapping (struct vm_args *vap, vm_offset_t addr, vm_prot_t prot)
{
  vm_offset_t paddr = VM_MIN_KERNEL_ADDRESS;
  /* Adjust the offset for addresses that aren't page-aligned. */
  vm_offset_t off = vap->off - (addr - trunc_page (addr));

  if (vm_map_enter (kernel_map, &paddr, PAGE_SIZE,
      0, TRUE, vap->obj, off, FALSE, prot, VM_PROT_ALL,
      VM_INHERIT_DEFAULT) != KERN_SUCCESS)
    paddr = 0;

  return (paddr);
}

kern_return_t gsync_wait (task_t task, vm_offset_t addr,
  unsigned int lo, unsigned int hi, natural_t msec, int flags)
{
  if (task == 0)
    return (KERN_INVALID_TASK);
  else if (addr % sizeof (int) != 0)
    return (KERN_INVALID_ADDRESS);

  vm_map_lock_read (task->map);

  struct gsync_waiter w;
  struct vm_args va;
  boolean_t remote = task != current_task ();
  int bucket = gsync_prepare_key (task, addr, flags, &w.key, &va);

  if (bucket < 0)
    {
      vm_map_unlock_read (task->map);
      return (KERN_INVALID_ADDRESS);
    }
  else if (remote)
    /* The VM object is returned locked. However, we are about to acquire
     * a sleeping lock for a bucket, so we must not hold any simple
     * locks. To prevent this object from going away, we add a reference
     * to it when requested. */
    vm_object_reference_locked (va.obj);

  /* We no longer need the lock on the VM object. */
  vm_object_unlock (va.obj);

  struct gsync_hbucket *hbp = gsync_buckets + bucket;
  kmutex_lock (&hbp->lock, FALSE);

  /* Before doing any work, check that the expected value(s)
   * match the contents of the address. Otherwise, the waiting
   * thread could potentially miss a wakeup. */

  boolean_t equal;
  if (! remote)
    equal = ((unsigned int *)addr)[0] == lo &&
      ((flags & GSYNC_QUAD) == 0 ||
       ((unsigned int *)addr)[1] == hi);
  else
    {
      vm_offset_t paddr = temp_mapping (&va, addr, VM_PROT_READ);
      if (unlikely (paddr == 0))
        {
          kmutex_unlock (&hbp->lock);
          vm_map_unlock_read (task->map);
          /* Make sure to remove the reference we added. */
          vm_object_deallocate (va.obj);
          return (KERN_MEMORY_FAILURE);
        }

      vm_offset_t off = addr & (PAGE_SIZE - 1);
      paddr += off;

      equal = ((unsigned int *)paddr)[0] == lo &&
        ((flags & GSYNC_QUAD) == 0 ||
         ((unsigned int *)paddr)[1] == hi);

      paddr -= off;

      /* Note that the call to 'vm_map_remove' will unreference
       * the VM object, so we don't have to do it ourselves. */
      vm_map_remove (kernel_map, paddr, paddr + PAGE_SIZE);
    }

  /* Done with the task's map. */
  vm_map_unlock_read (task->map);

  if (! equal)
    {
      kmutex_unlock (&hbp->lock);
      return (KERN_INVALID_ARGUMENT);
    }

  /* Look for the first entry in the hash bucket that
   * compares strictly greater than this waiter. */
  struct list *runp;
  list_for_each (&hbp->entries, runp)
    if (gsync_key_lt (&w.key, &node_to_waiter(runp)->key))
      break;

  /* Finally, add ourselves to the list and go to sleep. */
  list_add (runp->prev, runp, &w.link);
  w.waiter = current_thread ();

  if (flags & GSYNC_TIMED)
    thread_will_wait_with_timeout (w.waiter, msec);
  else
    thread_will_wait (w.waiter);

  kmutex_unlock (&hbp->lock);
  thread_block (thread_no_continuation);

  /* We're back. */
  kern_return_t ret = KERN_SUCCESS;
  if (current_thread()->wait_result != THREAD_AWAKENED)
    {
      /* We were interrupted or timed out. */
      kmutex_lock (&hbp->lock, FALSE);
      if (!list_node_unlinked (&w.link))
        list_remove (&w.link);
      kmutex_unlock (&hbp->lock);

      /* Map the error code. */
      ret = current_thread()->wait_result == THREAD_INTERRUPTED ?
        KERN_INTERRUPTED : KERN_TIMEDOUT;
    }

  return (ret);
}

/* Remove a waiter from the queue, wake it up, and
 * return the next node. */
static inline struct list*
dequeue_waiter (struct list *nodep)
{
  struct list *nextp = list_next (nodep);
  list_remove (nodep);
  list_node_init (nodep);
  clear_wait (node_to_waiter(nodep)->waiter,
    THREAD_AWAKENED, FALSE);
  return (nextp);
}

kern_return_t gsync_wake (task_t task,
  vm_offset_t addr, unsigned int val, int flags)
{
  if (task == 0)
    return (KERN_INVALID_TASK);
  else if (addr % sizeof (int) != 0)
    return (KERN_INVALID_ADDRESS);

  vm_map_lock_read (task->map);

  union gsync_key key;
  struct vm_args va;
  int bucket = gsync_prepare_key (task, addr, flags, &key, &va);

  if (bucket < 0)
    {
      vm_map_unlock_read (task->map);
      return (KERN_INVALID_ADDRESS);
    }
  else if (current_task () != task && (flags & GSYNC_MUTATE) != 0)
    /* See above on why we do this. */
    vm_object_reference_locked (va.obj);

  /* Done with the VM object lock. */
  vm_object_unlock (va.obj);

  kern_return_t ret = KERN_INVALID_ARGUMENT;
  struct gsync_hbucket *hbp = gsync_buckets + bucket;

  kmutex_lock (&hbp->lock, FALSE);

  if (flags & GSYNC_MUTATE)
    {
      /* Set the contents of the address to the specified value,
       * even if we don't end up waking any threads. Note that
       * the buckets' simple locks give us atomicity. */

      if (task != current_task ())
        {
          vm_offset_t paddr = temp_mapping (&va, addr,
            VM_PROT_READ | VM_PROT_WRITE);

          if (paddr == 0)
            {
              kmutex_unlock (&hbp->lock);
              vm_map_unlock_read (task->map);
              vm_object_deallocate (va.obj);
              return (KERN_MEMORY_FAILURE);
            }

          addr = paddr + (addr & (PAGE_SIZE - 1));
        }

      *(unsigned int *)addr = val;
      if (task != current_task ())
        vm_map_remove (kernel_map, addr, addr + sizeof (int));
    }

  vm_map_unlock_read (task->map);

  int found = 0;
  struct list *runp = gsync_find_key (&hbp->entries, &key, &found);
  if (found)
    {
      do
        runp = dequeue_waiter (runp);
      while ((flags & GSYNC_BROADCAST) &&
        !list_end (&hbp->entries, runp) &&
        gsync_key_eq (&node_to_waiter(runp)->key, &key));

      ret = KERN_SUCCESS;
    }

  kmutex_unlock (&hbp->lock);
  return (ret);
}

kern_return_t gsync_requeue (task_t task, vm_offset_t src,
  vm_offset_t dst, boolean_t wake_one, int flags)
{
  if (task == 0)
    return (KERN_INVALID_TASK);
  else if (src % sizeof (int) != 0 || dst % sizeof (int) != 0)
    return (KERN_INVALID_ADDRESS);

  union gsync_key src_k, dst_k;
  struct vm_args va;

  int src_bkt = gsync_prepare_key (task, src, flags, &src_k, &va);
  if (src_bkt < 0)
    return (KERN_INVALID_ADDRESS);

  /* Unlock the VM object before the second lookup. */
  vm_object_unlock (va.obj);

  int dst_bkt = gsync_prepare_key (task, dst, flags, &dst_k, &va);
  if (dst_bkt < 0)
    return (KERN_INVALID_ADDRESS);

  /* We never create any temporary mappings in 'requeue', so we
   * can unlock the VM object right now. */
  vm_object_unlock (va.obj);

  /* If we're asked to unconditionally wake up a waiter, then
   * we need to remove a maximum of two threads from the queue. */
  unsigned int nw = 1 + wake_one;
  struct gsync_hbucket *bp1 = gsync_buckets + src_bkt;
  struct gsync_hbucket *bp2 = gsync_buckets + dst_bkt;

  /* Acquire the locks in order, to prevent any potential deadlock. */
  if (bp1 == bp2)
    kmutex_lock (&bp1->lock, FALSE);
  else if ((unsigned long)bp1 < (unsigned long)bp2)
    {
      kmutex_lock (&bp1->lock, FALSE);
      kmutex_lock (&bp2->lock, FALSE);
    }
  else
    {
      kmutex_lock (&bp2->lock, FALSE);
      kmutex_lock (&bp1->lock, FALSE);
    }

  kern_return_t ret = KERN_SUCCESS;
  int exact;
  struct list *inp = gsync_find_key (&bp1->entries, &src_k, &exact);

  if (! exact)
    /* There are no waiters in the source queue. */
    ret = KERN_INVALID_ARGUMENT;
  else
    {
      struct list *outp = gsync_find_key (&bp2->entries, &dst_k, 0);

      /* We're going to need a node that points one past the
       * end of the waiters in the source queue. */
      struct list *endp = inp;

      do
        {
          /* Modify the keys while iterating. */
          node_to_waiter(endp)->key = dst_k;
          endp = list_next (endp);
        }
      while (((flags & GSYNC_BROADCAST) || --nw != 0) &&
        !list_end (&bp1->entries, endp) &&
        gsync_key_eq (&node_to_waiter(endp)->key, &src_k));

      /* Splice the list by removing waiters from the source queue
       * and inserting them into the destination queue. */
      inp->prev->next = endp;
      endp->prev->next = outp->next;
      endp->prev = inp->prev;

      outp->next = inp;
      inp->prev = outp;

      if (wake_one)
        (void)dequeue_waiter (inp);
    }

  /* Release the locks and we're done.*/
  kmutex_unlock (&bp1->lock);
  if (bp1 != bp2)
    kmutex_unlock (&bp2->lock);

  return (ret);
}