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Rework the SQLITE_MUTEXFREE_SHMLOCK code to reduce contention.

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FossilOrigin-Name: d9157dd176a2d18c6e02a2a0c7e16cef2da43bf44be9765e0363f34aebad23e9
This commit is contained in:
dan
2018-12-10 15:24:29 +00:00
parent 5b5cda2027
commit c967a11569
5 changed files with 128 additions and 78 deletions
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19
src/wal.c
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@@ -2777,19 +2777,14 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
assert( pWal->readLock>0 || pWal->hdr.mxFrame==0 );
assert( pInfo->aReadMark[pWal->readLock]<=pSnapshot->mxFrame );
/* It is possible that there is a checkpointer thread running
** concurrent with this code. If this is the case, it may be that the
** checkpointer has already determined that it will checkpoint
/* If it were possible for a checkpointer thread to run concurrent
** with this code, it would be a problem. In this case, it could be
** that the checkpointer has already determined that it will checkpoint
** snapshot X, where X is later in the wal file than pSnapshot, but
** has not yet set the pInfo->nBackfillAttempted variable to indicate
** its intent. To avoid the race condition this leads to, ensure that
** there is no checkpointer process by taking a shared CKPT lock
** before checking pInfo->nBackfillAttempted.
**
** TODO: Does the aReadMark[] lock prevent a checkpointer from doing
** this already?
*/
rc = walLockShared(pWal, WAL_CKPT_LOCK);
** its intent. Fortunately this is not possible, as the call to
** sqlite3WalSnapshotOpen() that sets pWal->pSnapshot also takes a
** SHARED lock on the checkpointer slot. */
if( rc==SQLITE_OK ){
/* Check that the wal file has not been wrapped. Assuming that it has
@@ -2808,8 +2803,6 @@ int sqlite3WalBeginReadTransaction(Wal *pWal, int *pChanged){
rc = SQLITE_ERROR_SNAPSHOT;
}
/* Release the shared CKPT lock obtained above. */
walUnlockShared(pWal, WAL_CKPT_LOCK);
pWal->minFrame = 1;
}