Ruby » Ruby master

Bug #20169

Updated by ko1 (Koichi Sasada) about 1 year ago

1. `GC.compact` introduces read barriers to detect read accesses to the pages. 
 2. I/O operations release GVL to pass the control while their execution, and another thread can call `GC.compact` (or auto compact feature I guess, but not checked yet). 
 3. Call `write(ptr)` can return `EFAULT` when `GC.compact` is running because `ptr` can point read-barrier protected pages (embed strings). pages. 

 Reproducible steps: 


 Apply the following patch to increase possibility: 

 ```patch 
 diff --git a/io.c b/io.c 
 index f6cd2c1a56..83d67ba2dc 100644 
 --- a/io.c 
 +++ b/io.c 
 @@ -1212,8 +1212,12 @@ internal_write_func(void *ptr) 
          } 
      } 

 +      int cnt = 0; 
    retry: 
 -      do_write_retry(write(iis->fd, iis->buf, iis->capa)); 
 +      for (; cnt < 1000; cnt++) { 
 +          do_write_retry(write(iis->fd, iis->buf, iis->capa)); 
 +          if (result <= 0) break; 
 +      } 

      if (result < 0 && !iis->nonblock) { 
          int e = errno; 
 ``` 

 Run the following code: 

 ```ruby 
 t1 = Thread.new{ 10_000.times.map{"#{_1}"}; GC.compact while true } 
 t2 = Thread.new{ 
   i=0 
   $stdout.write "<#{i+=1}>" while true 
 } 
 t2.join 
 ``` 

 and  

 ``` 
 $ make run 
 (snip) 
 4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4><4>#<Thread:0x00007fa61b4dd758 ../../src/trunk/test.rb:3 run> terminated with exception (report_on_exception is true): 
 ../../src/trunk/test.rb:5:in `write': Bad address @ io_write - <STDOUT> (Errno::EFAULT) 
         from ../../src/trunk/test.rb:5:in `block in <main>' 
 ../../src/trunk/test.rb:5:in `write': Bad address @ io_write - <STDOUT> (Errno::EFAULT) 
         from ../../src/trunk/test.rb:5:in `block in <main>' 
 make: *** [uncommon.mk:1383: run] Error 1 
 ``` 

 I think this is why we get `EFAULT` on CI. To increase possibilities running many busy processes (`ruby -e 'loop{}'` for example) will help (and on CI environment there are such busy processes accidentally).

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