Re: Race condition in fio atexit code

[Jens Axboe <axboe@kernel.dk>]

On 2012-06-20 01:05, Erik Lattimore wrote:
> Lately it seems like we've been hitting this more frequently, so I figured I'd file a bug.  Fio starts up a thread running the function disk_thread_main, which periodically calls update_io_ticks, which calls update_io_tick_disk on each entry in a circular linked list. The function disk_thread_main returns when the global variable "threads" is set to null, but it's only checked a couple of times in the loop.
> 
> The main thread runs the test and exits, and has registered an atexit handler free_shm. This routine sets "threads" to null and frees up storage, including the storage where the linked list used by update_io_ticks is stored.
> 
> Occasionally, somehow, update_io_tick_disk winds up getting called with a null pointer and crashing. The problem may be exacerbated when memory is tight. Here's the backtrace of the core dump:
> 
> Program terminated with signal 11, Segmentation fault.
> #0  update_io_tick_disk (du=<optimized out>) at diskutil.c:80
> 80              if (!du->users)
> (gdb) t apply all bt
> 
> Thread 2 (Thread 0x7faab680b700 (LWP 23148)):
> #0  0x00007faab58df377 in shmdt () from /lib64/libc.so.6
> #1  0x000000000040b98d in free_shm () at init.c:231
> #2  0x00007faab583b7f5 in __run_exit_handlers () from /lib64/libc.so.6
> #3  0x00007faab583b845 in exit () from /lib64/libc.so.6
> #4  0x00007faab5824c3d in __libc_start_main () from /lib64/libc.so.6
> #5  0x0000000000408ed9 in _start ()
> 
> Thread 1 (Thread 0x7faab32dd700 (LWP 23149)):
> #0  update_io_tick_disk (du=<optimized out>) at diskutil.c:80
> #1  update_io_ticks () at diskutil.c:114
> #2  0x000000000043b303 in disk_thread_main (data=<optimized out>) at backend.c:1589
> #3  0x00007faab61907b6 in start_thread () from /lib64/libpthread.so.0
> #4  0x00007faab58dd9cd in clone () from /lib64/libc.so.6
> #5  0x0000000000000000 in ?? ()
> (gdb) q--

This is clearly a race in how the disk util thread is shut down and the
structures freed. I'll take a look at a fix. It would be useful if you
told me how you are hitting this most easily, as I don't recall seeing
it. Would make me more confident in a fix.

Also, are you sure it's threads == NULL, and not the du's themselves
being freed? They are in separate storage. It might be a good idea to
have diskutil.c:free_disk_util() signal and wait for the disk util
thread to shutdown before going further.

-- 
Jens Axboe