ref: a6e9ab0ced8523fb8099afa4547dc27793e27462
dir: /writer.c/
#include "all.h"
/* Only 1 writer to maintain the sequence of writes.
Else, the directory entry could get written before the directory
content by another write process.
This creates a mess to recover on a crash.
*/
/* below is from nemo's Pg 252 */
typedef struct Dirties Dirties;
typedef struct Wbuf Wbuf;
struct Dirties
{
QLock lck; /* controls access to this queue */
Wbuf *head, *tail; /* linked list of dirty blocks yet to be written to the disk */
s32 n; /* number of dirty blocks in this linked list */
Rendez isfull; /* write throttling */
Rendez isempty; /* writer does not have to keep polling to find work */
} drts = {0};
struct Wbuf
{
u64 blkno; /* block number on the disk, primary key */
u16 len;
Wbuf *prev, *next; /* writer queue */
Iobuf *iobuf; /* pointer to the used Iobuf in the buffer cache */
union{
u8 *payload; /* "real" contents */
Content *io; /* cast'able to contents */
};
};
u64 npendingwrites; /* write throttling */
u8 stopwrites = 0;
static void stats(void);
static Wbuf *
pluck(Wbuf *b)
{
if(b == nil)
return nil;
else if(b->prev == nil && b->next == nil){
/* only one */
drts.head = drts.tail = nil;
goto Done;
}else if(b->prev == nil){
/* first in the linked list */
drts.head = b->next;
b->next = nil;
drts.head->prev = nil;
goto Done;
}else if(b->prev != nil && b->next != nil){
/* somewhere in the middle */
b->next->prev = b->prev;
b->prev->next = b->next;
b->prev = b->next = nil;
goto Done;
}else if(b->next == nil){
/* last in the linked list */
drts.tail = b->prev;
b->prev->next = nil;
b->prev = nil;
goto Done;
}
panic("pluck should not be here\n");
return nil; // too late, b was written already
Done:
drts.n--;
if(drts.n < 0)
panic("drts.n < 0\n");
return b;
}
/*
the Iobuf should be wlock()'ed at entry and until it
is placed in the writer queue.
It is unlocked after it is placed in writer queue to
avoid another process putting it in the writer queue
before us. This ensures that the write order
is maintained and a newer write being overwritten
by older write.
*/
void
putwrite(Iobuf *b)
{
Wbuf *w;
if(chatty9p > 4){
dprint("putwrite start p->blkno %llud\n", b->blkno);
stats();
}
w = emalloc9p(sizeof(Wbuf));
w->blkno = b->blkno;
w->len = b->len;
w->payload = allocmemunits(b->len);
memcpy(w->payload, b->xiobuf, b->len*Unit);
incref(&b->dirties);
w->iobuf = b;
qlock(&drts.lck);
if(drts.n == npendingwrites)
rsleep(&drts.isfull);
if(chkwunlock(b) == 0){
showbuf(b);
panic("putwrite chkwunlock(p) == 0 called by %#p\n", getcallerpc(&b));
}
if(drts.head == nil){
drts.head = drts.tail = w;
}else{
drts.tail->next = w;
w->prev = drts.tail;
drts.tail = w;
}
drts.n++;
if(drts.n == 1)
rwakeup(&drts.isempty);
qunlock(&drts.lck);
if(chatty9p > 4 && b!=nil){
dprint("putwrite done b->blkno %llud\n", b->blkno);
stats();
}
}
/*
dirties is decremented without a wlock() on the buffer in dowrite().
Using a wlock() in dowrite() deadlocks with putwrite().
getbuf() guarantees that even a free'ed block cannot be
stolen until the dirties == 0. This avoids dirty blocks
being stolen by other block numbers.
incref(dirties) only happens with a wlock() in putwrite().
*/
void
dowrite(void)
{
Wbuf *b;
u64 n;
qlock(&drts.lck);
if(drts.n == 0){
if(stopwrites){
qunlock(&drts.lck);
return;
}
rsleep(&drts.isempty);
if(drts.n == 0 && stopwrites){
qunlock(&drts.lck);
return;
}
}
// dprint("dowrite: drts.n %llud\n", drts.n);
if(drts.head == nil){
qunlock(&drts.lck);
return;
}
b = pluck(drts.head);
if(drts.n == npendingwrites-1)
rwakeup(&drts.isfull);
qunlock(&drts.lck);
if(chatty9p > 4 && b!=nil)
dprint("getwrite b->blkno %llud\n", b->blkno);
if((n = devwrite(b->blkno, b->payload, b->len)) != b->len*Unit){
dprint("%s\n", errstring[Esystem]);
panic("error writing block %llud: %llud bytes: %r\n",
b->blkno, n);
}
if(chatty9p > 4)
dprint("dowrite %llud wunlock()'ed\n", b->blkno);
decref(&b->iobuf->dirties);
freememunits(b->payload, b->len);
free(b);
if(chatty9p > 4 && b!=nil)
stats();
}
void
initwriter(void)
{
char name[Namelen];
// set the locks used by the Rendezes
drts.isempty.l = &drts.lck;
drts.isfull.l = &drts.lck;
switch(rfork(RFPROC|RFMEM)){
case -1:
panic("can't fork");
case 0:
if(chatty9p > 4)
dprint("writer started\n");
break;
default:
return;
}
snprint(name, Namelen, "%s writer", service);
procsetname(name);
while(stopwrites == 0 || drts.n > 0){
dowrite();
}
if(chatty9p > 4)
dprint("%s process exited\n", name);
exits(nil);
}
void
stopwriter(void)
{
u64 n;
stopwrites = 1;
do{
qlock(&drts.lck);
if(chatty9p > 4)
dprint("stopwriter drts.n %d\n", drts.n);
if(drts.n == 0)
rwakeup(&drts.isempty);
n = drts.n;
qunlock(&drts.lck);
if(n == 0)
return;
else
sleep(1000);
}while(n > 0);
}
static void
stats(void)
{
dprint("dirties nwrites %d hd %llud tl %llud\n",
drts.n,
drts.head == nil ? 0 : drts.head->blkno,
drts.tail == nil ? 0 : drts.tail->blkno);
}
u64
pendingwrites(void)
{
u64 n;
qlock(&drts.lck);
n = drts.n;
if(chatty9p>4)
stats();
qunlock(&drts.lck);
return n;
}
void
sync(void)
{
while(drts.n > 0)
sleep(1000);
}