Re: [9fans] Race condition in /sys/src/9/pc/trap.c?

[Richard Miller <9fans@hamnavoe.com>]

> ... procflushseg finds all processes that share s, sets them all up
> to flush their mmu states, and also sets any processor running such a process
> to flush its state (that's picked up by a clock interrupt).
>
> procflushseg will not proceed until all processes and processors that
> might need to flush state have done so. (s remains locked throughout.)

Coincidentally, I spent last Sunday debugging a deadlock in precisely
this spot.  I had absentmindedly tried to use VESA vga on a multiprocessor.
The aux/vga -l apparently succeeded and the screen looked great, but
as a stealthy side-effect the CPU which had done the VESA call stopped
responding to interrupts -- including the local APIC clock interrupt
required for the mmu flush as described above.

So, some time later when another process (upas/fs as it happens) on
the other CPU wanted to adjust a segment size, procflushseg was called
and never returned.

Debugging can be challenging when cause and effect are minutes or
hours apart ...