Andreas Klebinger pushed to branch wip/andreask/fix-prof-segv at Glasgow Haskell Compiler / GHC Commits: eca76105 by Andreas Klebinger at 2026-06-19T09:11:46+00:00 Add test and changelog for #27123 fix. - - - - - 3 changed files: - + changelog.d/T27123.md - + testsuite/tests/rts/T27123.hs - testsuite/tests/rts/all.T Changes: ===================================== changelog.d/T27123.md ===================================== @@ -0,0 +1,7 @@ +section: compiler +synopsis: Fix two crashes that could happen in a multithreaded setting when profiling. +description: There where two bugs that could cause occasional segfaults or crashes with +an `PAP object entered` error when profiling. They only happened when two threads +where racing to evaluate the same thunk, and specific GC timings. +mrs: !16214 +issues: #27123 ===================================== testsuite/tests/rts/T27123.hs ===================================== @@ -0,0 +1,65 @@ +{-# OPTIONS_GHC -fno-full-laziness -fno-worker-wrapper #-} +{-# LANGUAGE MagicHash, UnboxedTuples #-} + +-- This test checks that the auto-apply code (stg_ap_0_fast, stg_ap_p) is robust +-- against another thread or the GC evaluating a closure at the same time. + +module Main + -- (main) +where + +import Control.Monad +import Control.Concurrent +import System.IO +import GHC.Data.SmallArray +import GHC.Exts +import GHC.IO + +type Arr = SmallMutableArray RealWorld (Int->Int) + +io :: (State# RealWorld -> (# State# RealWorld, a #)) -> IO a +io f = IO f + +io_ :: (State# RealWorld -> State# RealWorld ) -> IO () +io_ f = IO (\s -> case f s of s2 -> (# s2, () #)) + +{-# NOINLINE readSmallArray #-} +readSmallArray (SmallMutableArray arr) (I# idx) = IO $ \s -> case readSmallArray# arr idx s of + (# s2, r #) -> (# s2, r #) + +-- Continually overwrites the array with unevaluated thunks that will evaluated to +-- a PAP under profiling. +{-# NOINLINE mkThunks #-} +mkThunks :: Arr -> IO () +mkThunks arr = do + forever $ do + yield + forM_ [0..100] $ \_j -> do + forM_ [0..5 :: Int] $ \i -> do + -- With profiling results in a thunk that will evaluate to a PAP capturing the SCC + let g = {-# SCC g #-} succ + io_ (writeSmallArray arr i g) + +-- Evaluate the array repeatedly in the given order. +{-# NOINLINE evaluateThunks #-} +evaluateThunks :: Arr -> [Int] -> IO () +evaluateThunks arr idxs = do + forever $ do + yield + -- putStr "." >> hFlush stdout + forM [0..5000::Int] $ \j -> do + forM_ idxs $ \i -> do + !g <- readSmallArray arr i + seq (g i) (pure ()) + +main :: IO () +main = do + -- We spawn three threads. Two are evaluating the thunks in the array in opposite directions + -- One thread is + arr <- io (newSmallArray 6 (id)) + _ <- forkIO $ do + evaluateThunks arr [0..5] + _ <- forkIO $ do + evaluateThunks arr [5,4..0] + forkIO $ mkThunks arr + threadDelay 30_000_000 ===================================== testsuite/tests/rts/all.T ===================================== @@ -687,3 +687,5 @@ test('ClosureTable', ['-debug -O0 ClosureTable_c.c -I{top}/../rts -I{top}/../rts/include']) test('resizeMutableByteArrayInPlace', [req_cmm, extra_ways(['optasm', 'sanity']), only_ways(['optasm', 'sanity'])], compile_and_run, ['']) + +test('T27123', [extra_ways(['optasm']), when(have_profiling(), extra_ways['prof'])], compile_and_run, ['']) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/eca761054db8f82d1b280bf40e54d7f1... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/eca761054db8f82d1b280bf40e54d7f1... You're receiving this email because of your account on gitlab.haskell.org.