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msys2-runtime/0042-cygthread-suspend-thread-before-terminating.patch
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,74 @@ | ||
| From e09c64ef65cf51011122e55b26abdbda9e70f7e6 Mon Sep 17 00:00:00 2001 | ||
| From: Jeremy Drake <[email protected]> | ||
| Date: Mon, 11 Nov 2024 20:09:49 -0800 | ||
| Subject: [PATCH 42/N] cygthread: suspend thread before terminating. | ||
|
|
||
| This addresses an extremely difficult to debug deadlock when running | ||
| under emulation on ARM64. | ||
|
|
||
| A relatively easy way to trigger this bug is to call `fork()`, then within the | ||
| child process immediately call another `fork()` and then `exit()` the | ||
| intermediate process. | ||
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| It would seem that there is a "code emulation" lock on the wait thread at | ||
| this stage, and if the thread is terminated too early, that lock still exists | ||
| albeit without a thread, and nothing moves forward. | ||
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| It seems that a `SuspendThread()` combined with a `GetThreadContext()` | ||
| (to force the thread to _actually_ be suspended, for more details see | ||
| https://devblogs.microsoft.com/oldnewthing/20150205-00/?p=44743) | ||
| makes sure the thread is "booted" from emulation before it is suspended. | ||
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| Hopefully this means it won't be holding any locks or otherwise leave | ||
| emulation in a bad state when the thread is terminated. | ||
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| Also, attempt to use `CancelSynchonousIo()` (as seen in `flock.cc`) to avoid | ||
| the need for `TerminateThread()` altogether. This doesn't always work, | ||
| however, so was not a complete fix for the deadlock issue. | ||
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||
| Addresses: https://cygwin.com/pipermail/cygwin-developers/2024-May/012694.html | ||
| Signed-off-by: Jeremy Drake <[email protected]> | ||
| --- | ||
| winsup/cygwin/cygthread.cc | 14 ++++++++++++++ | ||
| winsup/cygwin/sigproc.cc | 3 ++- | ||
| 2 files changed, 16 insertions(+), 1 deletion(-) | ||
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| diff --git a/winsup/cygwin/cygthread.cc b/winsup/cygwin/cygthread.cc | ||
| index 54918e7..4f16097 100644 | ||
| --- a/winsup/cygwin/cygthread.cc | ||
| +++ b/winsup/cygwin/cygthread.cc | ||
| @@ -302,6 +302,20 @@ cygthread::terminate_thread () | ||
| if (!inuse) | ||
| goto force_notterminated; | ||
|
|
||
| + if (_my_tls._ctinfo != this) | ||
| + { | ||
| + CONTEXT context; | ||
| + context.ContextFlags = CONTEXT_CONTROL; | ||
| + /* SuspendThread makes sure a thread is "booted" from emulation before | ||
| + it is suspended. As such, the emulator hopefully won't be in a bad | ||
| + state (aka, holding any locks) when the thread is terminated. */ | ||
| + SuspendThread (h); | ||
| + /* We need to call GetThreadContext, even though we don't care about the | ||
| + context, because SuspendThread is asynchronous and GetThreadContext | ||
| + will make sure the thread is *really* suspended before returning */ | ||
| + GetThreadContext (h, &context); | ||
| + } | ||
| + | ||
| TerminateThread (h, 0); | ||
| WaitForSingleObject (h, INFINITE); | ||
| CloseHandle (h); | ||
| diff --git a/winsup/cygwin/sigproc.cc b/winsup/cygwin/sigproc.cc | ||
| index a89f09d..0089626 100644 | ||
| --- a/winsup/cygwin/sigproc.cc | ||
| +++ b/winsup/cygwin/sigproc.cc | ||
| @@ -410,7 +410,8 @@ proc_terminate () | ||
| if (!have_execed || !have_execed_cygwin) | ||
| chld_procs[i]->ppid = 1; | ||
| if (chld_procs[i].wait_thread) | ||
| - chld_procs[i].wait_thread->terminate_thread (); | ||
| + if (!CancelSynchronousIo (chld_procs[i].wait_thread->thread_handle ())) | ||
| + chld_procs[i].wait_thread->terminate_thread (); | ||
| /* Release memory associated with this process unless it is 'myself'. | ||
| 'myself' is only in the chld_procs table when we've execed. We | ||
| reach here when the next process has finished initializing but we |
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