From 1cdc3ecccd087f5be2e3cdfa6827f7cee57a8206 Mon Sep 17 00:00:00 2001
From: Simon Marlow <marlowsd@gmail.com>
Date: Thu, 12 Jul 2018 10:13:47 -0400
Subject: [PATCH] Fix deadlock between STM and throwTo
There was a lock-order reversal between lockTSO() and the TVar lock,
see #15136 for the details.
It turns out we can fix this pretty easily by just deleting all the
locking code(!). The principle for unblocking a `BlockedOnSTM` thread
then becomes the same as for other kinds of blocking: if the TSO
belongs to this capability then we do it directly, otherwise we send a
message to the capability that owns the TSO. That is, a thread blocked
on STM is owned by its capability, as it should be.
The possible downside of this is that we might send multiple messages
to wake up a thread when the thread is on another capability. This is
safe, it's just not very efficient. I'll try to do some experiments
to see if this is a problem.
Test Plan: Test case from #15136 doesn't deadlock any more.
Reviewers: bgamari, osa1, erikd
Reviewed By: osa1
Subscribers: rwbarton, thomie, carter
GHC Trac Issues: #15136
Differential Revision: https://phabricator.haskell.org/D4956
(cherry picked from commit 7fc418df856d9b58034eeec48915646e67a7a562)
---
rts/RaiseAsync.c | 9 ---------
rts/SMPClosureOps.h | 9 ---------
rts/STM.c | 19 +------------------
rts/Threads.c | 5 ++++-
rts/sm/Sanity.c | 3 ++-
5 files changed, 7 insertions(+), 38 deletions(-)
diff --git a/rts/RaiseAsync.c b/rts/RaiseAsync.c
index f5e96a2c435..b08acc40782 100644
--- a/rts/RaiseAsync.c
+++ b/rts/RaiseAsync.c
@@ -416,21 +416,12 @@ check_target:
}
case BlockedOnSTM:
- lockTSO(target);
- // Unblocking BlockedOnSTM threads requires the TSO to be
- // locked; see STM.c:unpark_tso().
- if (target->why_blocked != BlockedOnSTM) {
- unlockTSO(target);
- goto retry;
- }
if ((target->flags & TSO_BLOCKEX) &&
((target->flags & TSO_INTERRUPTIBLE) == 0)) {
blockedThrowTo(cap,target,msg);
- unlockTSO(target);
return THROWTO_BLOCKED;
} else {
raiseAsync(cap, target, msg->exception, false, NULL);
- unlockTSO(target);
return THROWTO_SUCCESS;
}
diff --git a/rts/SMPClosureOps.h b/rts/SMPClosureOps.h
index 1d18e1b018f..c73821a7829 100644
--- a/rts/SMPClosureOps.h
+++ b/rts/SMPClosureOps.h
@@ -124,15 +124,6 @@ EXTERN_INLINE void unlockClosure(StgClosure *p, const StgInfoTable *info)
p->header.info = info;
}
-// Handy specialised versions of lockClosure()/unlockClosure()
-INLINE_HEADER void lockTSO(StgTSO *tso);
-INLINE_HEADER void lockTSO(StgTSO *tso)
-{ lockClosure((StgClosure *)tso); }
-
-INLINE_HEADER void unlockTSO(StgTSO *tso);
-INLINE_HEADER void unlockTSO(StgTSO *tso)
-{ unlockClosure((StgClosure*)tso, (const StgInfoTable *)&stg_TSO_info); }
-
#endif /* CMINUSMINUS */
#include "EndPrivate.h"
diff --git a/rts/STM.c b/rts/STM.c
index 058eec74091..abb44172dd0 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -332,24 +332,7 @@ static void unpark_tso(Capability *cap, StgTSO *tso) {
// queues: it's up to the thread itself to remove it from the wait queues
// if it decides to do so when it is scheduled.
- // Unblocking a TSO from BlockedOnSTM is done under the TSO lock,
- // to avoid multiple CPUs unblocking the same TSO, and also to
- // synchronise with throwTo(). The first time the TSO is unblocked
- // we mark this fact by setting block_info.closure == STM_AWOKEN.
- // This way we can avoid sending further wakeup messages in the
- // future.
- lockTSO(tso);
- if (tso->why_blocked == BlockedOnSTM &&
- tso->block_info.closure == &stg_STM_AWOKEN_closure) {
- TRACE("unpark_tso already woken up tso=%p", tso);
- } else if (tso -> why_blocked == BlockedOnSTM) {
- TRACE("unpark_tso on tso=%p", tso);
- tso->block_info.closure = &stg_STM_AWOKEN_closure;
- tryWakeupThread(cap,tso);
- } else {
- TRACE("spurious unpark_tso on tso=%p", tso);
- }
- unlockTSO(tso);
+ tryWakeupThread(cap,tso);
}
static void unpark_waiters_on(Capability *cap, StgTVar *s) {
diff --git a/rts/Threads.c b/rts/Threads.c
index be6962246d4..78c5b6cc843 100644
--- a/rts/Threads.c
+++ b/rts/Threads.c
@@ -297,8 +297,11 @@ tryWakeupThread (Capability *cap, StgTSO *tso)
goto unblock;
}
- case BlockedOnBlackHole:
case BlockedOnSTM:
+ tso->block_info.closure = &stg_STM_AWOKEN_closure;
+ goto unblock;
+
+ case BlockedOnBlackHole:
case ThreadMigrating:
goto unblock;
diff --git a/rts/sm/Sanity.c b/rts/sm/Sanity.c
index e5a22fdafe4..8d4171b1cd5 100644
--- a/rts/sm/Sanity.c
+++ b/rts/sm/Sanity.c
@@ -547,7 +547,8 @@ checkTSO(StgTSO *tso)
ASSERT(next == END_TSO_QUEUE ||
info == &stg_MVAR_TSO_QUEUE_info ||
info == &stg_TSO_info ||
- info == &stg_WHITEHOLE_info); // happens due to STM doing lockTSO()
+ info == &stg_WHITEHOLE_info); // used to happen due to STM doing
+ // lockTSO(), might not happen now
if ( tso->why_blocked == BlockedOnMVar
|| tso->why_blocked == BlockedOnMVarRead
--
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