Race condition in fio atexit code

[Erik Lattimore <elatt@xxxxxxxxxxxx>]

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--
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