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Try to identify the places in WAL code where thread-safety depends on the

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underlying architecture supporting atomic load and store of aligned 32-bit
values.

FossilOrigin-Name: 47d44be4a68d377d0049a12b2587dbbcc0870b469473e1098f7c0358fe8c7532
This commit is contained in:
drh
2018-08-30 20:28:18 +00:00
parent ed9272159e
commit 876c7ea3fa
3 changed files with 23 additions and 11 deletions
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20
src/wal.c
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@@ -258,6 +258,18 @@ int sqlite3WalTrace = 0;
# define WALTRACE(X)
#endif
/*
** WAL mode depends on atomic aligned 32-bit loads and stores in a few
** places. The following macros try to make this explicit.
*/
#if GCC_VESRION>=5004000
# define AtomicLoad(PTR) __atomic_load_n((PTR),__ATOMIC_RELAXED)
# define AtomicStore(PTR,VAL) __atomic_store_n((PTR),(VAL),__ATOMIC_RELAXED)
#else
# define AtomicLoad(PTR) (*(PTR))
# define AtomicStore(PTR,VAL) (*(PTR) = (VAL))
#endif
/*
** The maximum (and only) versions of the wal and wal-index formats
** that may be interpreted by this version of SQLite.
@@ -2555,7 +2567,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
}
#endif
for(i=1; i<WAL_NREADER; i++){
u32 thisMark = pInfo->aReadMark[i];
u32 thisMark = AtomicLoad(pInfo->aReadMark+i);
if( mxReadMark<=thisMark && thisMark<=mxFrame ){
assert( thisMark!=READMARK_NOT_USED );
mxReadMark = thisMark;
@@ -2568,7 +2580,7 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
for(i=1; i<WAL_NREADER; i++){
rc = walLockExclusive(pWal, WAL_READ_LOCK(i), 1);
if( rc==SQLITE_OK ){
mxReadMark = pInfo->aReadMark[i] = mxFrame;
mxReadMark = AtomicStore(pInfo->aReadMark+i,mxFrame);
mxI = i;
walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1);
break;
@@ -2620,9 +2632,9 @@ static int walTryBeginRead(Wal *pWal, int *pChanged, int useWal, int cnt){
** we can guarantee that the checkpointer that set nBackfill could not
** see any pages past pWal->hdr.mxFrame, this problem does not come up.
*/
pWal->minFrame = pInfo->nBackfill+1;
pWal->minFrame = AtomicLoad(&pInfo->nBackfill)+1;
walShmBarrier(pWal);
if( pInfo->aReadMark[mxI]!=mxReadMark
if( AtomicLoad(pInfo->aReadMark+mxI)!=mxReadMark
|| memcmp((void *)walIndexHdr(pWal), &pWal->hdr, sizeof(WalIndexHdr))
){
walUnlockShared(pWal, WAL_READ_LOCK(mxI));