Re: [PATCH] eal: fix data race in multi-process support

[Thomas Monjalon <thomas@monjalon.net>]

17/12/2021 19:29, Stephen Hemminger:
> If DPDK is built with thread sanitizer it reports a race
> in setting of multiprocess file descriptor. The fix is to
> use atomic operations when updating mp_fd.

Please could explain more the condition of the race?
Is it between init and cleanup of the same file descriptor?
How atomic is helping here?


> 
> Simple example:
> $ dpdk-testpmd -l 1-3 --no-huge
> ...
> EAL: Error - exiting with code: 1
>   Cause: Creation of mbuf pool for socket 0 failed: Cannot allocate memory
> ==================
> WARNING: ThreadSanitizer: data race (pid=83054)
>   Write of size 4 at 0x55e3b7fce450 by main thread:
>     #0 rte_mp_channel_cleanup <null> (dpdk-testpmd+0x160d79c)
>     #1 rte_eal_cleanup <null> (dpdk-testpmd+0x1614fb5)
>     #2 rte_exit <null> (dpdk-testpmd+0x15ec97a)
>     #3 mbuf_pool_create.cold <null> (dpdk-testpmd+0x242e1a)
>     #4 main <null> (dpdk-testpmd+0x5ab05d)
> 
>   Previous read of size 4 at 0x55e3b7fce450 by thread T2:
>     #0 mp_handle <null> (dpdk-testpmd+0x160c979)
>     #1 ctrl_thread_init <null> (dpdk-testpmd+0x15ff76e)
> 
>   As if synchronized via sleep:
>     #0 nanosleep ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:362 (libtsan.so.0+0x5cd8e)
>     #1 get_tsc_freq <null> (dpdk-testpmd+0x1622889)
>     #2 set_tsc_freq <null> (dpdk-testpmd+0x15ffb9c)
>     #3 rte_eal_timer_init <null> (dpdk-testpmd+0x1622a34)
>     #4 rte_eal_init.cold <null> (dpdk-testpmd+0x26b314)
>     #5 main <null> (dpdk-testpmd+0x5aab45)
> 
>   Location is global 'mp_fd' of size 4 at 0x55e3b7fce450 (dpdk-testpmd+0x0000027c7450)
> 
>   Thread T2 'rte_mp_handle' (tid=83057, running) created by main thread at:
>     #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:962 (libtsan.so.0+0x58ba2)
>     #1 rte_ctrl_thread_create <null> (dpdk-testpmd+0x15ff870)
>     #2 rte_mp_channel_init.cold <null> (dpdk-testpmd+0x269986)
>     #3 rte_eal_init <null> (dpdk-testpmd+0x1615b28)
>     #4 main <null> (dpdk-testpmd+0x5aab45)