{-# LANGUAGE ForeignFunctionInterface #-} {-# LANGUAGE MagicHash #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE UnboxedTuples #-} {-# LANGUAGE UnliftedFFITypes#-} {-# LANGUAGE GHCForeignImportPrim #-} -- | -- This module exposes an interface for capturing the state of a thread's -- execution stack for diagnostics purposes: 'cloneMyStack', -- 'cloneThreadStack'. -- -- Such a "cloned" stack can be decoded with 'decode' to a stack trace, given -- that the @-finfo-table-map@ is enabled. -- -- @since 4.17.0.0 module GHC.Stack.CloneStack ( StackSnapshot(..), StackEntry(..), cloneMyStack, cloneThreadStack, decode ) where import Control.Concurrent.MVar import Data.Maybe (catMaybes) import Foreign import GHC.Conc.Sync import GHC.Exts (Int (I#), RealWorld, StackSnapshot#, ThreadId#, Array#, sizeofArray#, indexArray#, State#, StablePtr#) import GHC.IO (IO (..)) import GHC.InfoProv (InfoProv (..), InfoProvEnt, ipLoc, ipeProv, peekInfoProv) import GHC.Stable -- | A frozen snapshot of the state of an execution stack. -- -- @since 4.17.0.0 data StackSnapshot = StackSnapshot !StackSnapshot# foreign import prim "stg_decodeStackzh" decodeStack# :: StackSnapshot# -> State# RealWorld -> (# State# RealWorld, Array# (Ptr InfoProvEnt) #) foreign import prim "stg_cloneMyStackzh" cloneMyStack# :: State# RealWorld -> (# State# RealWorld, StackSnapshot# #) foreign import prim "stg_sendCloneStackMessagezh" sendCloneStackMessage# :: ThreadId# -> StablePtr# PrimMVar -> State# RealWorld -> (# State# RealWorld, (# #) #) {- Note [Stack Cloning] ~~~~~~~~~~~~~~~~~~~~ "Cloning" a stack means that it's `StgStack` closure is copied including the stack memory (`stack[]`). Closures referenced by stack closures are not copied, i.e. pointer payloads are still referred to by the same pointer. In other words: Only those parts that are affected by stack evaluation are "cloned". The stack pointer (sp) of the clone is adjusted to be valid, i.e. to point into the cloned stack. The clone is "offline"/"cold", i.e. it won't be evaluated any further. This is useful for further analyses like stack unwinding or traversal because all pointers stay valid. StackSnapshot# -------------- A cloned stack is represented in Haskell by `StackSnapshot !StackSnapshot#`. `StackSnapshot#` is a primitive type, it's value is a pointer to the stack in RTS (`StgStack*`). To take advantage of the garbage collector, the representation cannot be `Ptr` or `StablePtr`: - Closures referenced by a `Ptr` may be garbage collected at any time (without checking if it's still in use). - `StablePtr` has to be freed explictly, which would introduce nasty state handling. By using a primitive type, the stack closure (and its transitive closures) is kept and managed by the garbage collector as long as it's in use and automatically freed later. As closures referred to by stack closures (e.g. payloads) may be used by other closures that are not related to stack cloning, the memory has to be managed by the garbage collector; i.e. one cannot simply call free() in the RTS C code because it's hard to figure out what to free while the garbage collector is built to do this job. RTS interface ------------- There are two different ways to clone a stack: 1. `cloneMyStack#` - A primop for cloning the active thread's stack. 2. `sendCloneStackMessage#` - A primop for cloning another thread's stack. Sends a RTS message (Messages.c) with a MVar to that thread. The cloned stack is received by taking it out of this MVar. `cloneMyStack#` has to be a primop, because new primitive types (`StackSnapshot#`) cannot be marshalled by FFI. Using a `Ptr StackSnapshot` as FFI return type would not save the snapshot from being garbage collected, as discussed in the section above. C API ------------- `cloneStack` is the function that really clones a given stack and returns the clone: `StgStack* cloneStack(Capability* capability, const StgStack* stack)` It's called directly by `stg_cloneMyStackzh` (`PrimOps.cmm`), the `cloneMyStack#` primop. To clone another thread's stack, there's a message passing mechanism such that the receiver's capability clones its. So, there's no need to stop/pause the other thread as it's capability will fulfill the cloning request when it's ready to do so. The message is defined in `Closures.h`: ``` typedef struct MessageCloneStack_ { StgHeader header; Message *link; StgMVar *result; StgTSO *tso; } MessageCloneStack; ``` The fields are: - `header`: It's a closure and thus subject to garbage collection (no manual memory management needed) - `link`: Messages form a singly linked list in `Capability`, referred to by `capability->inbox`. - `result`: An `MVar`. When the message is sent it's empty, after cloning the `StackSnapshot` is put into it. - `tso`: `tso->stackobj` is the stack to clone. The asynchronous flow can be split into sending this message and putting the cloned stack into the MVar (expecting the sender to get it from there). Sending: The public C function to send is `void sendCloneStackMessage(StgTSO *tso, HsStablePtr mvar)`. It prepares the message for the thread to clone (identified by it's `tso`) and sets the `result` MVar (pointed to by `mvar`). Then it sends the message by calling `sendMessage` which puts it into the Capabilities `inbox`. Receiving: Inbox processing is part of the big work finding loop in `schedule`. The function that dispatches messages is `executeMessage`. From there `void handleCloneStackMessage(MessageCloneStack *msg)` is called. `handleCloneStackMessage` clones the stack, lifts the result to `StackSnapshot` (MVar needs a lifted value, no primitive) and puts it into the MVar (`msg->mvar`). -} {- Note [Stack Decoding] ~~~~~~~~~~~~~~~~~~~~~ A cloned stack is decoded (unwound) by looking up the Info Table Provenance Entries (IPE) for every stack frame with `lookupIPE` in the RTS. The IPEs contain source locations and are pulled from the RTS/C world into Haskell. RTS interface ------------- The primop decodeStack# returns an array of IPE pointers that are later unmarshalled with HSC. If there is no IPE for a return frame (which can easily happen when a library wasn't compiled with `-finfo-table-map`), it's represented by a null pointer. Caveats: - decodeStack# has to be a primop (not a simple C FFI function), because there always has to be at least one active `TSO`. Otherwise, allocating memory with the garbage collector for the returned value fails. - decodeStack# has to be defined outside of `primops.txt.pp` because its return type `Array# (Ptr InfoProvEnt)` cannot be defined there: `InfoProvEnt` and `Ptr` would have to be imported which seems to be too specific for this file. Notes ----- The relevant notes are: - Note [Mapping Info Tables to Source Positions] - Note [Stacktraces from Info Table Provenance Entries (IPE based stack unwinding)] -} -- | Clone the stack of the executing thread -- -- @since 4.17.0.0 cloneMyStack :: IO StackSnapshot cloneMyStack = IO $ \s -> case (cloneMyStack# s) of (# s1, stack #) -> (# s1, StackSnapshot stack #) -- | Clone the stack of a thread identified by its 'ThreadId' -- -- @since 4.17.0.0 cloneThreadStack :: ThreadId -> IO StackSnapshot cloneThreadStack (ThreadId tid#) = do resultVar <- newEmptyMVar @StackSnapshot boxedPtr@(StablePtr ptr) <- newStablePtrPrimMVar resultVar -- Use the RTS's "message" mechanism to request that -- the thread captures its stack, saving the result -- into resultVar. IO $ \s -> case sendCloneStackMessage# tid# ptr s of (# s', (# #) #) -> (# s', () #) freeStablePtr boxedPtr takeMVar resultVar -- | Representation for the source location where a return frame was pushed on the stack. -- This happens every time when a @case ... of@ scrutinee is evaluated. data StackEntry = StackEntry { functionName :: String, moduleName :: String, srcLoc :: String, closureType :: Word } deriving (Show, Eq) -- | Decode a 'StackSnapshot' to a stacktrace (a list of 'StackEntry'). -- The stack trace is created from return frames with according 'InfoProvEnt' -- entries. To generate them, use the GHC flag @-finfo-table-map@. If there are -- no 'InfoProvEnt' entries, an empty list is returned. -- -- Please note: -- -- * To gather 'StackEntry' from libraries, these have to be -- compiled with @-finfo-table-map@, too. -- * Due to optimizations by GHC (e.g. inlining) the stacktrace may change -- with different GHC parameters and versions. -- * The stack trace is empty (by design) if there are no return frames on -- the stack. (These are pushed every time when a @case ... of@ scrutinee -- is evaluated.) -- -- @since 4.17.0.0 decode :: StackSnapshot -> IO [StackEntry] decode stackSnapshot = do stackEntries <- getDecodedStackArray stackSnapshot ipes <- mapM unmarshal stackEntries return $ catMaybes ipes where unmarshal :: Ptr InfoProvEnt -> IO (Maybe StackEntry) unmarshal ipe = if ipe == nullPtr then pure Nothing else do infoProv <- (peekInfoProv . ipeProv) ipe pure $ Just (toStackEntry infoProv) toStackEntry :: InfoProv -> StackEntry toStackEntry infoProv = StackEntry { functionName = ipLabel infoProv, moduleName = ipMod infoProv, srcLoc = ipLoc infoProv, -- read looks dangerous, be we can trust that the closure type is always there. closureType = read . ipDesc $ infoProv } getDecodedStackArray :: StackSnapshot -> IO [Ptr InfoProvEnt] getDecodedStackArray (StackSnapshot s) = IO $ \s0 -> case decodeStack# s s0 of (# s1, a #) -> (# s1, (go a ((I# (sizeofArray# a)) - 1)) #) where go :: Array# (Ptr InfoProvEnt) -> Int -> [Ptr InfoProvEnt] go stack 0 = [stackEntryAt stack 0] go stack i = (stackEntryAt stack i) : go stack (i - 1) stackEntryAt :: Array# (Ptr InfoProvEnt) -> Int -> Ptr InfoProvEnt stackEntryAt stack (I# i) = case indexArray# stack i of (# se #) -> se