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authorRobert Morris <[email protected]>2014-08-27 17:15:30 -0400
committerRobert Morris <[email protected]>2014-08-27 17:15:30 -0400
commit71453f72f285a17ccf0520b9dbdafdc701ff2f4a (patch)
tree978d30844e643e67a807a73db20ccc5d2823f8f2 /log.c
parent2c56547272e43b483d560a61692f1e24926a82fb (diff)
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a start at concurrent FS system calls
Diffstat (limited to 'log.c')
-rw-r--r--log.c82
1 files changed, 58 insertions, 24 deletions
diff --git a/log.c b/log.c
index 95cc4d5..159df98 100644
--- a/log.c
+++ b/log.c
@@ -5,18 +5,22 @@
#include "fs.h"
#include "buf.h"
-// Simple logging. Each file system system call
-// should be surrounded with begin_trans() and commit_trans() calls.
+// Simple logging that allows concurrent FS system calls.
//
-// The log holds at most one transaction at a time. Commit forces
-// the log (with commit record) to disk, then installs the affected
-// blocks to disk, then erases the log. begin_trans() ensures that
-// only one system call can be in a transaction; others must wait.
-//
-// Allowing only one transaction at a time means that the file
-// system code doesn't have to worry about the possibility of
-// one transaction reading a block that another one has modified,
-// for example an i-node block.
+// A log transaction contains the updates of *multiple* FS system
+// calls. The logging systems only commits when there are
+// no FS system calls active. Thus there is never
+// any reasoning required about whether a commit might
+// write an uncommitted system call's updates to disk.
+//
+// A system call should call begin_op()/end_op() to mark
+// its start and end. Usually begin_op() just increments
+// the count of in-progress FS system calls and returns.
+// But if it thinks the log is close to running out, it
+// blocks this system call, and causes the system to wait
+// until end_op() indicates there are no executing FS
+// system calls, at which point the last end_op() commits
+// all the system calls' writes.
//
// The log is a physical re-do log containing disk blocks.
// The on-disk log format:
@@ -38,13 +42,15 @@ struct log {
struct spinlock lock;
int start;
int size;
- int busy; // a transaction is active
+ int outstanding; // how many FS sys calls are executing.
+ int committing; // in commit(), please wait.
int dev;
struct logheader lh;
};
struct log log;
static void recover_from_log(void);
+static void commit();
void
initlog(void)
@@ -117,19 +123,52 @@ recover_from_log(void)
write_head(); // clear the log
}
+// an FS system call should call begin_op() when it starts.
void
-begin_trans(void)
+begin_op(void)
{
acquire(&log.lock);
- while (log.busy) {
- sleep(&log, &log.lock);
+ while(1){
+ if(log.committing){
+ sleep(&log, &log.lock);
+ } else {
+ // XXX wait (for a commit) if log is longish.
+ // need to reserve to avoid over-commit of log space.
+ log.outstanding += 1;
+ release(&log.lock);
+ break;
+ }
}
- log.busy = 1;
- release(&log.lock);
}
+// an FS system call should call end_op() after it finishes.
+// can't write the disk &c while holding locks, thus do_commit.
void
-commit_trans(void)
+end_op(void)
+{
+ int do_commit = 0;
+
+ acquire(&log.lock);
+ log.outstanding -= 1;
+ if(log.committing)
+ panic("log.committing");
+ if(log.outstanding == 0){
+ do_commit = 1;
+ log.committing = 1;
+ }
+ release(&log.lock);
+
+ if(do_commit){
+ commit();
+ acquire(&log.lock);
+ log.committing = 0;
+ wakeup(&log);
+ release(&log.lock);
+ }
+}
+
+static void
+commit()
{
if (log.lh.n > 0) {
write_head(); // Write header to disk -- the real commit
@@ -137,11 +176,6 @@ commit_trans(void)
log.lh.n = 0;
write_head(); // Erase the transaction from the log
}
-
- acquire(&log.lock);
- log.busy = 0;
- wakeup(&log);
- release(&log.lock);
}
// Caller has modified b->data and is done with the buffer.
@@ -159,7 +193,7 @@ log_write(struct buf *b)
if (log.lh.n >= LOGSIZE || log.lh.n >= log.size - 1)
panic("too big a transaction");
- if (!log.busy)
+ if (log.outstanding < 1)
panic("write outside of trans");
for (i = 0; i < log.lh.n; i++) {