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Overview
Comment: | Add a comment to wal.c to explain why a race condition is safe. |
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Downloads: | Tarball | ZIP archive |
Timelines: | family | ancestors | descendants | both | trunk |
Files: | files | file ages | folders |
SHA1: |
bc33af866403c23d548dd47056753158 |
User & Date: | dan 2015-04-13 17:43:43.335 |
Context
2015-04-13
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19:14 | Remove the out2-prerelease VDBE opcode property and its associated code, for a 0.5% performance improvement. (check-in: e29c7f2c91 user: drh tags: trunk) | |
17:43 | Add a comment to wal.c to explain why a race condition is safe. (check-in: bc33af8664 user: dan tags: trunk) | |
14:44 | Remove a faulty assert() from the btree balancing logic. (check-in: d06669d968 user: drh tags: trunk) | |
Changes
Changes to src/wal.c.
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1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 | ** safe to write into the database. Frames beyond mxSafeFrame might ** overwrite database pages that are in use by active readers and thus ** cannot be backfilled from the WAL. */ mxSafeFrame = pWal->hdr.mxFrame; mxPage = pWal->hdr.nPage; for(i=1; i<WAL_NREADER; i++){ u32 y = pInfo->aReadMark[i]; if( mxSafeFrame>y ){ assert( y<=pWal->hdr.mxFrame ); rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1); if( rc==SQLITE_OK ){ pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED); walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1); | > > > > > > > > | 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 | ** safe to write into the database. Frames beyond mxSafeFrame might ** overwrite database pages that are in use by active readers and thus ** cannot be backfilled from the WAL. */ mxSafeFrame = pWal->hdr.mxFrame; mxPage = pWal->hdr.nPage; for(i=1; i<WAL_NREADER; i++){ /* Thread-sanitizer reports that the following is an unsafe read, ** as some other thread may be in the process of updating the value ** of the aReadMark[] slot. The assumption here is that if that is ** happening, the other client may only be increasing the value, ** not decreasing it. So assuming either that either the "old" or ** "new" version of the value is read, and not some arbitrary value ** that would never be written by a real client, things are still ** safe. */ u32 y = pInfo->aReadMark[i]; if( mxSafeFrame>y ){ assert( y<=pWal->hdr.mxFrame ); rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(i), 1); if( rc==SQLITE_OK ){ pInfo->aReadMark[i] = (i==1 ? mxSafeFrame : READMARK_NOT_USED); walUnlockExclusive(pWal, WAL_READ_LOCK(i), 1); |
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