Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: 4d798b17 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Eliminate STM_AWOKEN It was used as nullary closure for the block_info.closure in the case of a thread being awoken after an STM transaction. However, while it was written, it was never read, so contributed nothing to the behaviour. Furthermore, in the only place it was set (in tryWakeupThread) the why_blocked was immediately overwritten by the NotBlocked status, and the block_info was updated accordingly (by appendToRunQueue). So it didn't even serve a purpose of clarifying an intermediate state, there really was no such intermediate state. Cleaning this up will allow the BlockedOnSTM case to follow the same pattern as the other why_blocked cases that do not use the block_info, and in turn this reduces the number of different categories. - - - - - e1cece79 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Document that eventlog thread stop code ThreadBlocked is no longer used It has not been used since GHC 7.0.x (2011). In 7.2 all the BlockedOn* codes were added, and these were and are used instead of ThreadBlocked. - - - - - 795db115 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Add a proper mapping to eventlog external thread stop status That is the mapping from rts-internal codes, to the coes used in the status field in the eventlog EVENT_STOP_THREAD event. See issue #9003 for what goes wrong when we mess this up. In that ticket, people note that we should really not require the internal tso->why_blocked codes to leak into the external eventlog thread stop codes. The same principle applies to the StgThreadReturnCode. This change properly separates them, and explicitly maps between them using a pair of (compact, constant) tables. These tables are pretty small (with no alignment constraints) and will soon shrink so it seems a sensible tradeoff. We also introduce and use proper EVENT_STOP_THREAD constants in the event log format header. Previously there was not specification in the code for these (only in the docs): the values were encoded into the conversion code. This will allow us to renumber the internal why_blockd codes without breaking the eventlog output. - - - - - 6f1c8efa by Duncan Coutts at 2026-07-23T17:26:18-04:00 Remove unused tso->block_info.wakeup member Presumably it was used once, but not now. - - - - - 740b88a9 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Document StgTSOBlockInfo to say what cases use what members In principle, tso->why_blocked is the tag for the StgTSOBlockInfo union, so we should be able to say for each union member the why_blocked cases that use that member. - - - - - 5b92eae2 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Add a tso->block_info.mvar member and use it in preference to the generic block_info.closure union member, with casts. The plan is that when we know what case we're in (via tso->why_blocked) then we can always access the correct union member, and so we will only need to access block_info.closure for generic cases where we don't know or don't care. - - - - - d931715f by Duncan Coutts at 2026-07-23T17:26:18-04:00 Add a tso->block_info.unused member and use it in preference to the generic block_info.closure union member, with casts. The plan is that when we know what case we're in (via tso->why_blocked) then we can always access the correct union member, and so we will only need to access block_info.closure for generic cases where we don't know or don't care. - - - - - 47e28ebb by Duncan Coutts at 2026-07-23T17:26:18-04:00 Avoid storing to tso->block_info.closure In one case we can use a specific union member (.prev) instead. In several cases the stores were in fact redundant because of subsequent overwrites. In scavengeTSO we replace setting tso->block_info.closure to a valid closure, with an assertion that the block_info.unused is already set to END_TSO_QUEUE which is a valid (static) closure. - - - - - 96e4749d by Duncan Coutts at 2026-07-23T17:26:18-04:00 Renumber the tso->why_blocked constants We can do this now because we have separated the internal values from the external ones used in the eventlog. This lets us put them back into a deliberate order and consolodate some gaps. More importantly, it is a prepation for a slightly more sophisticated encoding. - - - - - 8f62661c by Duncan Coutts at 2026-07-23T17:26:18-04:00 Define constants for the existing stg_threadStatuszh return codes The stg_threadStatuszh reuses the internal tso->why_blocked codes but also extends them with a couple previously magic values. This is awkward since we need to know what those magic values are so we don't accidentally use those values to mean something else. By pulling a definition up to where the why_blocked codes are defined we will be able to avoid mistakenly assining those codes some meaning (or just changing the BlockedThreadComplete, BlockedThreadKilled code if necessary). - - - - - 42c69ae2 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Extend the tso->why_blocked encoding to indicate block_info closures We use some bit tricks to cheaply and generically test if a tso->why_blocked tag implies that the corresponding tso->block_info will contain a non-trivial valid closure (i.e. not just block_info.unused set to END_TSO_QUEUE). In particular we arrange for most why_blocked values to naturally have a distinguishing bit, but for the BlockedOn{Read,Write,Delay} cases, they can come in either non-closure or closure forms. We allow an additional bit to distinguish these cases. The non-closure forms are only from legacy I/O managers: select and win32-legacy. So this extra bit mechanism will be able to be retired once the legacy I/O managers are themselves retired. This means in a few places we need to untag the why_blocked value before inspecting it, but in most places we do not. - - - - - 7c64632b by Duncan Coutts at 2026-07-23T17:26:18-04:00 Use BlockInfoForceNonClosure in the select I/O manager - - - - - 8fd7104a by Duncan Coutts at 2026-07-23T17:26:18-04:00 Use BlockInfoForceNonClosure in the win32-legacy I/O manager for the BlockedOn{Read,Write} since these use the non-heap allocated StgAsyncIOResult. - - - - - e0da603b by Duncan Coutts at 2026-07-23T17:26:18-04:00 Enforce the why_blocked and block_info rules in checkTSO We now check the cases wher IsBlockInfoClosure should hold, the cases that are supposed to use block_info.unused == END_TSO_QUEUE, and which cases are allowed to use BlockInfoForceNonClosure. This partially enforces the use of why_blocked as a tag for the block_info union. We could be stricter and check for the correct expected info table for the closure cases. - - - - - 1dd0f381 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Use IsBlockInfoClosure to simplify several tests In GC and generic traversal we need to know if we should look at the block_info.closure or not. Now we can do just that using a cheap bit test on the why_blocked tag. This fixes issue 26717, where the problem was that some GC modes did not know when to look at block_info.closure, because the poll I/O manager uses a closure for BlockedOn{Read,Write} while the select I/O manager uses a non-closure. Now this information is in the why_blocked tag itself. - - - - - 7a00ffbc by Duncan Coutts at 2026-07-23T17:26:18-04:00 Remove the now-unused scavengeTSOIOManager The GC no longer has to delegate to the I/O manager, since it can use IsBlockInfoClosure to decide things itself. - - - - - 522a481f by Duncan Coutts at 2026-07-23T17:26:18-04:00 Remove duplicate assertion - - - - - 0874d965 by Duncan Coutts at 2026-07-23T17:26:18-04:00 Follow atomic access rules more consistently for tso->why_blocked The rule is this: store block_info *before* why_blocked store why_blocked using store release load why_blocked using load acquire load block_info *after* why_blocked This is a an atomic store release / load acquire pair and (if the reads are in a separate thread to the writes, and the read receives the value stored) then this guarantees a full "happens before" relationship of these stores and loads. In some cases, we do not need a full load acquire, because we don't read the block_info at all and so do not need any ordering. In this case we just need an atomic relaxed load. This was being followed in most places, but not all. If there's good reason in any case that we don't need atomic access, then we should document that in a comment. In the absence of that I think it's easier to follow the rule everywhere. - - - - - 8f0bdbe1 by Duncan Coutts at 2026-07-23T17:26:19-04:00 Add a changelog entry - - - - - 31 changed files: - + changelog.d/T26716 - docs/users_guide/eventlog-formats.rst - libraries/ghc-heap/GHC/Exts/Heap/FFIClosures_ProfilingDisabled.hsc - libraries/ghc-heap/GHC/Exts/Heap/FFIClosures_ProfilingEnabled.hsc - libraries/ghc-internal/src/GHC/Internal/Conc/Sync.hs - rts/IOManager.c - rts/IOManager.h - rts/Messages.c - rts/PrimOps.cmm - rts/RaiseAsync.c - rts/RaiseAsync.h - rts/STM.c - rts/Schedule.c - rts/StgMiscClosures.cmm - rts/Threads.c - rts/Trace.c - rts/Trace.h - rts/TraverseHeap.c - rts/include/Cmm.h - rts/include/rts/Constants.h - rts/include/rts/EventLogFormat.h - rts/include/rts/storage/TSO.h - rts/include/stg/MiscClosures.h - rts/posix/Poll.c - rts/posix/Select.c - rts/posix/Timeout.c - rts/sm/Compact.c - rts/sm/NonMovingMark.c - rts/sm/Sanity.c - rts/sm/Scav.c - rts/win32/AsyncMIO.c The diff was not included because it is too large. View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/26a44fb0d59492d7654ad7115059903... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/26a44fb0d59492d7654ad7115059903... You're receiving this email because of your account on gitlab.haskell.org. Manage all notifications: https://gitlab.haskell.org/-/profile/notifications | Help: https://gitlab.haskell.org/help