minix3/servers/vfs/tll.c

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2020-02-21 00:59:27 +05:30
/* This file contains the implementation of the three-level-lock. */
#include "fs.h"
#include "glo.h"
#include "tll.h"
#include "threads.h"
#include <assert.h>
static int tll_append(tll_t *tllp, tll_access_t locktype);
static int tll_append(tll_t *tllp, tll_access_t locktype)
{
struct worker_thread *queue;
assert(self != NULL);
assert(tllp != NULL);
assert(locktype != TLL_NONE);
/* Read-only and write-only requests go to the write queue. Read-serialized
* requests go to the serial queue. Then we wait for an event to signal it's
* our turn to go. */
queue = NULL;
if (locktype == TLL_READ || locktype == TLL_WRITE) {
if (tllp->t_write == NULL)
tllp->t_write = self;
else
queue = tllp->t_write;
} else {
if (tllp->t_serial == NULL)
tllp->t_serial = self;
else
queue = tllp->t_serial;
}
if (queue != NULL) { /* Traverse to end of queue */
while (queue->w_next != NULL) queue = queue->w_next;
queue->w_next = self;
}
self->w_next = NULL; /* End of queue */
/* Now wait for the event it's our turn */
worker_wait();
tllp->t_current = locktype;
tllp->t_status &= ~TLL_PEND;
tllp->t_owner = self;
if (tllp->t_current == TLL_READ) {
tllp->t_readonly++;
tllp->t_owner = NULL;
} else if (tllp->t_current == TLL_WRITE)
assert(tllp->t_readonly == 0);
/* Due to the way upgrading and downgrading works, read-only requests are
* scheduled to run after a downgraded lock is released (because they are
* queued on the write-only queue which has priority). This results from the
* fact that the downgrade operation cannot know whether the next locktype on
* the write-only queue is really write-only or actually read-only. However,
* that means that read-serialized requests stay queued, while they could run
* simultaneously with read-only requests. See if there are any and grant
* the head request access */
if (tllp->t_current == TLL_READ && tllp->t_serial != NULL) {
tllp->t_owner = tllp->t_serial;
tllp->t_serial = tllp->t_serial->w_next;
tllp->t_owner->w_next = NULL;
assert(!(tllp->t_status & TLL_PEND));
tllp->t_status |= TLL_PEND;
worker_signal(tllp->t_owner);
}
return(OK);
}
void tll_downgrade(tll_t *tllp)
{
/* Downgrade three-level-lock tll from write-only to read-serialized, or from
* read-serialized to read-only. Caveat: as we can't know whether the next
* lock type on the write queue is actually read-only or write-only, we can't
* grant access to that type. It will be granted access once we unlock. Also,
* because we apply write-bias, we can't grant access to read-serialized
* either, unless nothing is queued on the write-only stack. */
assert(self != NULL);
assert(tllp != NULL);
assert(tllp->t_owner == self);
switch(tllp->t_current) {
case TLL_WRITE: tllp->t_current = TLL_READSER; break;
case TLL_READSER:
/* If nothing is queued on write-only, but there is a pending lock
* requesting read-serialized, grant it and keep the lock type. */
if (tllp->t_write == NULL && tllp->t_serial != NULL) {
tllp->t_owner = tllp->t_serial;
tllp->t_serial = tllp->t_serial->w_next; /* Remove head */
tllp->t_owner->w_next = NULL;
assert(!(tllp->t_status & TLL_PEND));
tllp->t_status |= TLL_PEND;
worker_signal(tllp->t_owner);
} else {
tllp->t_current = TLL_READ;
tllp->t_owner = NULL;
}
tllp->t_readonly++; /* Either way, there's one more read-only lock */
break;
default: panic("VFS: Incorrect lock state");
}
if (tllp->t_current != TLL_WRITE && tllp->t_current != TLL_READSER)
assert(tllp->t_owner == NULL);
}
void tll_init(tll_t *tllp)
{
/* Initialize three-level-lock tll */
assert(tllp != NULL);
tllp->t_current = TLL_NONE;
tllp->t_readonly = 0;
tllp->t_status = TLL_DFLT;
tllp->t_write = NULL;
tllp->t_serial = NULL;
tllp->t_owner = NULL;
}
int tll_islocked(tll_t *tllp)
{
assert(tllp >= (tll_t *) PAGE_SIZE);
return(tllp->t_current != TLL_NONE);
}
int tll_locked_by_me(tll_t *tllp)
{
assert(tllp >= (tll_t *) PAGE_SIZE);
assert(self != NULL);
return(tllp->t_owner == self && !(tllp->t_status & TLL_PEND));
}
int tll_lock(tll_t *tllp, tll_access_t locktype)
{
/* Try to lock three-level-lock tll with type locktype */
assert(self != NULL);
assert(tllp >= (tll_t *) PAGE_SIZE);
assert(locktype != TLL_NONE);
self->w_next = NULL;
if (locktype != TLL_READ && locktype != TLL_READSER && locktype != TLL_WRITE)
panic("Invalid lock type %d\n", locktype);
/* If this locking has pending locks, we wait */
if (tllp->t_status & TLL_PEND)
return tll_append(tllp, locktype);
/* If we already own this lock don't lock it again and return immediately */
if (tllp->t_owner == self) {
assert(tllp->t_status == TLL_DFLT);
return(EBUSY);
}
/* If this lock is not accessed by anyone, locktype is granted off the bat */
if (tllp->t_current == TLL_NONE) {
tllp->t_current = locktype;
if (tllp->t_current == TLL_READ)
tllp->t_readonly = 1;
else { /* Record owner if locktype is read-serialized or write-only */
tllp->t_owner = self;
}
if (tllp->t_current == TLL_WRITE)
assert(tllp->t_readonly == 0);
return(OK);
}
/* If the current lock is write-only, we have to wait for that lock to be
* released (regardless of the value of locktype). */
if (tllp->t_current == TLL_WRITE)
return tll_append(tllp, locktype);
/* However, if it's not and we're requesting a write-only lock, we have to
* wait until the last read access is released (additional read requests
* after this write-only requests are to be queued) */
if (locktype == TLL_WRITE)
return tll_append(tllp, locktype);
/* We have to queue read and read-serialized requests if we have a write-only
* request queued ("write bias") or when a read-serialized lock is trying to
* upgrade to write-only. The current lock for this tll is either read or
* read-serialized. */
if (tllp->t_write != NULL || (tllp->t_status & TLL_UPGR)) {
assert(!(tllp->t_status & TLL_PEND));
return tll_append(tllp, locktype);
}
/* If this lock is in read-serialized mode, we can allow read requests and
* queue read-serialized requests */
if (tllp->t_current == TLL_READSER) {
if (locktype == TLL_READ && !(tllp->t_status & TLL_UPGR)) {
tllp->t_readonly++;
return(OK);
} else
return tll_append(tllp, locktype);
}
/* Finally, if the current lock is read-only, we can change it to
* read-serialized if necessary without a problem. */
tllp->t_current = locktype; /* Either read-only or read-serialized */
if (tllp->t_current == TLL_READ) { /* We now have an additional reader */
tllp->t_readonly++;
tllp->t_owner = NULL;
} else {
assert(tllp->t_current != TLL_WRITE);
tllp->t_owner = self; /* We now have a new owner */
self->w_next = NULL;
}
return(OK);
}
int tll_haspendinglock(tll_t *tllp)
{
/* Is someone trying to obtain a lock? */
assert(tllp != NULL);
/* Someone is trying to obtain a lock if either the write/read-only queue or
* the read-serialized queue is not empty. */
return(tllp->t_write != NULL || tllp->t_serial != NULL);
}
int tll_unlock(tll_t *tllp)
{
/* Unlock a previously locked three-level-lock tll */
int signal_owner = 0;
assert(self != NULL);
assert(tllp != NULL);
if (tllp->t_owner == NULL || tllp->t_owner != self) {
/* This unlock must have been done by a read-only lock */
tllp->t_readonly--;
assert(tllp->t_readonly >= 0);
assert(tllp->t_current == TLL_READ || tllp->t_current == TLL_READSER);
/* If a read-serialized lock is trying to upgrade and there are no more
* read-only locks, the lock can now be upgraded to write-only */
if ((tllp->t_status & TLL_UPGR) && tllp->t_readonly == 0)
signal_owner = 1;
}
if (tllp->t_owner == self && tllp->t_current == TLL_WRITE)
assert(tllp->t_readonly == 0);
if(tllp->t_owner == self || (tllp->t_owner == NULL && tllp->t_readonly == 0)){
/* Let another read-serialized or write-only request obtain access.
* Write-only has priority, but only after the last read-only access
* has left. Read-serialized access will only be granted if there is
* no pending write-only access request. */
struct worker_thread *new_owner;
new_owner = NULL;
tllp->t_owner = NULL; /* Remove owner of lock */
if (tllp->t_write != NULL) {
if (tllp->t_readonly == 0) {
new_owner = tllp->t_write;
tllp->t_write = tllp->t_write->w_next;
}
} else if (tllp->t_serial != NULL) {
new_owner = tllp->t_serial;
tllp->t_serial = tllp->t_serial->w_next;
}
/* New owner is head of queue or NULL if no proc is available */
if (new_owner != NULL) {
tllp->t_owner = new_owner;
tllp->t_owner->w_next = NULL;
assert(tllp->t_owner != self);
signal_owner = 1;
}
}
/* If no one is using this lock, mark it as not in use */
if (tllp->t_owner == NULL) {
if (tllp->t_readonly == 0)
tllp->t_current = TLL_NONE;
else
tllp->t_current = TLL_READ;
}
if (tllp->t_current == TLL_NONE || tllp->t_current == TLL_READ) {
if (!signal_owner) {
tllp->t_owner = NULL;
}
}
/* If we have a new owner or the current owner managed to upgrade its lock,
* tell it to start/continue running */
if (signal_owner) {
assert(!(tllp->t_status & TLL_PEND));
tllp->t_status |= TLL_PEND;
worker_signal(tllp->t_owner);
}
return(OK);
}
void tll_upgrade(tll_t *tllp)
{
/* Upgrade three-level-lock tll from read-serialized to write-only */
assert(self != NULL);
assert(tllp != NULL);
assert(tllp->t_owner == self);
assert(tllp->t_current != TLL_READ); /* i.e., read-serialized or write-only*/
if (tllp->t_current == TLL_WRITE) return; /* Nothing to do */
if (tllp->t_readonly != 0) { /* Wait for readers to leave */
assert(!(tllp->t_status & TLL_UPGR));
tllp->t_status |= TLL_UPGR;
worker_wait();
tllp->t_status &= ~TLL_UPGR;
tllp->t_status &= ~TLL_PEND;
assert(tllp->t_readonly == 0);
}
tllp->t_current = TLL_WRITE;
}