Bug #18818


SEGV (Fiber scheduler?)

Added by nevans (Nicholas Evans) 21 days ago. Updated 21 days ago.

Target version:
ruby -v:
3.1.2, 3.0.4, master


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 
completed in 0.00031349004711955786
../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]
./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:

completed in 0.00031569297425448895
completed in 0.1176840600091964
completed in 0.19209402799606323
completed in 0.21404067997355014
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. :)


test.rb (5.6 KB) test.rb test script nevans (Nicholas Evans), 06/05/2022 09:01 PM
segv-master-5ce0be022f.txt (11.8 KB) segv-master-5ce0be022f.txt more VM dump output from make run nevans (Nicholas Evans), 06/05/2022 09:29 PM
0001-Mark-Fibers-in-Mutex-Queue-SizedQueue-wait-lists.patch (1.66 KB) 0001-Mark-Fibers-in-Mutex-Queue-SizedQueue-wait-lists.patch nevans (Nicholas Evans), 06/06/2022 06:30 PM

Updated by mame (Yusuke Endoh) 21 days ago

  • Assignee set to ioquatix (Samuel Williams)

Updated by ioquatix (Samuel Williams) 21 days ago

Unfortunately Mutex, Queue and probably other objects don't take a reference to the waiting fiber, so your fiber is being garbage collected after it's in the wait list. Then some other object is allocated and unblock is called with it (or not = SEGV).

You need to keep track of blocked fibers:

def block...
  @blocked[fiber] = true

def unblock

This prevents the fiber from going out of scope.

Updated by ioquatix (Samuel Williams) 21 days ago

@ko1 (Koichi Sasada) I saw this problem because fiber is not retained while waiting, because we have waiting threads but not waiting fibers at VM level IIRC. Probably we need to make mutex/queue mark the wait list correctly? Is there performance issue?

Updated by nevans (Nicholas Evans) 21 days ago

Aha! Thanks, that makes perfect sense. And it does indeed fix it. I knew this toy scheduler wasn't good, and my original version did retain references to the waiting fibers, but I was slowly golfing it down to the smallest readable version that could possibly work.

IMO, pure ruby code should never need to worry about SEGV nor should a ruby method ever be called with garbage collected or reallocated values. And the most obvious answer (to me) is that the mutex/queue wait lists should mark all waiting fibers. I had already assumed that they did.

Probably there are scenarios where it's useful to allow waiting fibers to be GCed? In that case, the fiber scheduler should still never be passed "fiber" arguments unless they really truly are fibers (and the correct fibers, in-case some other fiber is allocated). Perhaps the fiber scheduler should be given a callback, e.g. fiber_will_gc(fiber): not only would this give the scheduler an opportunity to do something before they are simply abandoned, it would make explicit to fiber scheduler users and implementers the expectation that they can be GCed and abandoned. But that sounds a lot more complicated than simply adding it to the wait list, and I can't recall the scenarios where it would be useful.

Updated by nevans (Nicholas Evans) 21 days ago

There might be a good reason we don't want to do this (or want to do it differently). But I can confirm that marking the fibers in the wait queues makes the SEGV go away! :D

Patch also at


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