[ruby-core:109174] [Ruby master Bug#18818] SEGV (Fiber scheduler?)

["nevans (Nicholas Evans)" <noreply@ruby-lang.org>]

Issue #18818 has been updated by nevans (Nicholas Evans).

File 0001-Mark-blocked-fibers-in-waitq-Mutex-Queue-etc.patch added

IMO, neither ruby nor C should *ever* be in a position where they are referencing freed memory.  And, although `sync_waiter` lists check `waiter->th->status != THREAD_KILLED` when they are being traversed, that would be dangerous if the waiter's thread or fiber (including the stack the `sync_waiter` lives on) has been freed, right?

At first I thought this `SEGV` could only happen while using FiberScheduler with non-blocking fibers.  But dead threads can be GCed too, right?  So I suspect it can also happen if threads have been killed abnormally (skipping the `delete_from_waitq` code).  Forking is handled (`rb_thread_atfork` for mutex and `fork_gen` for queues and condition variables), but are there other scenarios where a thread could end without a chance to run its cleanup (e.g. `delete_from_waitq`)?  I'm more concerned with scenarios that can be created from within ruby alone than with C extensions or FFI.

One alternative: maintain "weak ref" relationship between wait queues and their blocked fibers.  If we added a waiter(s) field to `rb_fiber_t` we could then remove it(them) in `fiber_free`.  But weak pointers are inherently strange and surprising, even from the wait list of a queue or mutex (etc).  Not only would it need to be documented, I believe most developers will simply *assume* (like I did) that fibers can be reached by the queue or mutex (etc) that they are waiting on.  Anything else is surprising, *especially* if you've spent time in go (i.e. named channels connecting unnamed processes).

And, as far as I can tell, fibers are marked by the other two places in places in standard lib that use them:
* `enumerator.c`: `enumerator_mark` marks both `fib` (the enumerator fiber) and `dst` (the resuming fiber)
* `ext/monitor/monitor.c`: `monitor_mark` and `VALUE owner`

--------

I did look at Haskell's GC to try and understand how they use it for deadlock detection.  The simple version, translated slightly for ruby:
1. mvars and stm (for ruby: mutexes, queues, and condition variables) will mark blocked threads (fibers) as reachable.
2. thread/fiber schedulers do *not* mark blocked threads/fibers.
3. GC needs to have a special case for blocking threads/fibers:
   * After marking everything reachable from the GC roots, but before running any finalizers:
   * All living but unreachable (i.e. blocked and deadlocked) threads/fibers are "resurrected"
   * Marking continues for everything that is reachable from the resurrected threads/fibers.
   * The appropriate deadlock exception is raised in all resurrected threads (which unblocks them).
   * GC can move on to finalizers and sweeping *only after the resurrected fibers have been marked.*

Ruby's common memory use and object reference patterns are *very* different from Haskell's, and I suspect this would be less likely to catch deadlocks for ruby than it is for Haskell.  But if ruby *did* add something like this to its GC, I suspect that libraries would shift their usage patterns to make it more useful!

At any rate, adding deadlock detection to GC would still require marking blocked fibers as reachable from the object they are waiting for.  :)

--------

Also, I updated my PR and attached the patch here.

----------------------------------------
Bug #18818: SEGV (Fiber scheduler?)
https://bugs.ruby-lang.org/issues/18818#change-98314

* Author: nevans (Nicholas Evans)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* ruby -v: 3.1.2, 3.0.4, master
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
The attached script (and/or others like it) can cause SEGV in 3.0, 3.1, and master.  It has always behaved as expected when I use `optflags=-O0`.

When I use it with `make run` on `master`:
```
./miniruby -I../lib -I. -I.ext/common  -r./x86_64-linux-fake  ../test.rb 
========================================================================
fiber_queue
completed in 0.00031349004711955786
========================================================================
fiber_sized_queue
../test.rb:62: [BUG] Segmentation fault at 0x0000000000000000
ruby 3.2.0dev (2022-06-05T06:18:26Z master 5ce0be022f) [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0005 p:---- s:0023 e:000022 CFUNC  :%
c:0004 p:0031 s:0018 e:000015 METHOD ../test.rb:62 [FINISH]
c:0003 p:---- s:0010 e:000009 CFUNC  :pop
c:0002 p:0009 s:0006 e:000005 BLOCK  ../test.rb:154 [FINISH]
c:0001 p:---- s:0003 e:000002 (none) [FINISH]

-- Ruby level backtrace information ----------------------------------------
../test.rb:154:in `block (2 levels) in <main>'
../test.rb:154:in `pop'
../test.rb:62:in `unblock'
../test.rb:62:in `%'

-- Machine register context ------------------------------------------------
 RIP: 0x000055eae9ffa417 RBP: 0x00007f80aba855d8 RSP: 0x00007f80a9789598
 RAX: 0x000000000000009b RBX: 0x00007f80a9789628 RCX: 0x00007f80ab9c37a0
 RDX: 0x00007f80a97895c0 RDI: 0x0000000000000000 RSI: 0x000000000000009b
  R8: 0x0000000000000000  R9: 0x00007f80a97895c0 R10: 0x0000000055550083
 R11: 0x00007f80ac32ace0 R12: 0x00007f80aba855d8 R13: 0x00007f80ab9c3780
 R14: 0x00007f80a97895c0 R15: 0x000000000000009b EFL: 0x0000000000010202

-- C level backtrace information -------------------------------------------
./miniruby(rb_vm_bugreport+0x5cf) [0x55eaea06b0ef]
./miniruby(rb_bug_for_fatal_signal+0xec) [0x55eae9e4fc2c]
./miniruby(sigsegv+0x4d) [0x55eae9fba30d]
[0x7f80ac153520]
./miniruby(rb_id_table_lookup+0x7) [0x55eae9ffa417]
./miniruby(callable_method_entry+0x103) [0x55eaea046bd3]
./miniruby(vm_respond_to+0x3f) [0x55eaea056c1f]
./miniruby(rb_check_funcall_default_kw+0x19c) [0x55eaea05788c]
./miniruby(rb_check_convert_type_with_id+0x8e) [0x55eae9f1b85e]
./miniruby(rb_str_format_m+0x1a) [0x55eae9fce82a]
./miniruby(vm_call_cfunc_with_frame+0x127) [0x55eaea041ac7]
./miniruby(vm_exec_core+0x114) [0x55eaea05d684]
./miniruby(rb_vm_exec+0x187) [0x55eaea04e747]
./miniruby(rb_funcallv_scope+0x1b0) [0x55eaea05a770]
./miniruby(rb_fiber_scheduler_unblock+0x3e) [0x55eae9fb979e]
./miniruby(sync_wakeup+0x10d) [0x55eae9ffd45d]
./miniruby(rb_szqueue_pop+0xf5) [0x55eae9ffefd5]
./miniruby(vm_call_cfunc_with_frame+0x127) [0x55eaea041ac7]
./miniruby(vm_exec_core+0x114) [0x55eaea05d684]
./miniruby(rb_vm_exec+0x187) [0x55eaea04e747]
./miniruby(rb_vm_invoke_proc+0x5f) [0x55eaea05584f]
./miniruby(rb_fiber_start+0x1da) [0x55eae9e1e24a]
./miniruby(fiber_entry+0x0) [0x55eae9e1e550]

```

I've attached the rest of the VM dump.  `make runruby` gives a nearly identical dump.  I can post a core dump or `rr` recording, if needed.
_
I'm sorry I didn't simplify the script more; small, seemingly irrelevant changes can change the failure or allow it to pass.  Sometimes it raises a bizarre exception instead of SEGV, most commonly a NoMethodError which seemingly indicates that the local vars have been shifted or scrambled.  For example, this particular SEGV was caused by a guard clause checking that `unblock(blocker, fiber)` was given a Fiber object.  Here, that object is invalid, but I've seen it be a string or some other object from elsewhere in the process.

For comparison, this is what the script output should look like:
```
========================================================================
fiber_queue
completed in 0.00031569297425448895
========================================================================
fiber_sized_queue
completed in 0.1176840600091964
========================================================================
fiber_sized_queue2
completed in 0.19209402799606323
========================================================================
fiber_sized_queue3
completed in 0.21404067997355014
========================================================================
fiber_sized_queue4
completed in 0.30277197097893804
```

I was attempting to create some simple benchmarks for `Queue` and `SizedQueue` with fibers, to mimic `benchmark/vm_thread_*queue*.rb`.  I never completed the benchmarks because of this SEGV.  :)

---Files--------------------------------
test.rb (5.6 KB)
segv-master-5ce0be022f.txt (11.8 KB)
0001-Mark-Fibers-in-Mutex-Queue-SizedQueue-wait-lists.patch (1.66 KB)
0001-Mark-blocked-fibers-in-waitq-Mutex-Queue-etc.patch (5.9 KB)


-- 
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