[Bug 220033] [if_ixgb] sys/dev/ixgb/if_ixgb.c: a sleep-under-mutex bug in ixge_get_buf

[Jia-Ju Bai]

The ixgb driver may sleep under a mutex, and the function call path in file
"sys/dev/ixgb/if_ixgb.c" in FreeBSD 11.0 is:
ixgb_init [line 750: acquire the mutex]
   ixgb_init_locked [line 751]
     ixgb_setup_receive_structures [line 699]
       ixgb_allocate_receive_structures [line 1913]
         ixgb_get_buf [line 1885]
           bus_dmamap_load(BUS_DMA_WAITOK) [line 1812] --> may sleep

The possible fix of these bugs is to set the last parameter in bus_dmamap_load
to "BUS_DMA_NOWAIT".

This bug is found by a static analysis tool written by myself, and it is
checked by my review of the FreeBSD code.

By the way, I am a freshman in developing FreeBSD drivers, and I am willing to
submit a patch. But I do not know how to write and submit a patch, and where to
submit, so I am looking forward to useful advice :)

Jia-Ju Bai