svn commit: r329796 - head/sys/dev/vt

[Andriy Gapon]

On 22/02/2018 04:26, John Baldwin wrote:
> Author: jhb
> Date: Thu Feb 22 02:26:29 2018
> New Revision: 329796
> URL: https://svnweb.freebsd.org/changeset/base/329796
> 
> Log:
>   Avoid grabbing locks when grabbing the vt(4) console for DDB.
>   
>   Trying to grab locks during cngrab() when entering the debugger is
>   deadlock prone as all other CPUs are already halted (and thus unable
>   to release locks) when cngrab() is invoked.  One could instead use
>   try-locks.  However, the case that the try-lock fails still has to
>   be handled.  In addition, if the try-lock works it doesn't provide
>   any greater ordering guarantees than is already provided by entering
>   and exiting DDB.  It is simpler to define a simpler path for the
>   case that the try-lock would fail and always use that when entering
>   DDB.  Messing with timers, etc. when entering DDB is dubious even if
>   the try-lock succeeds.
>   
>   This patch attempts to use the smallest possible set of operations to
>   grab the vt(4) console when entering DDB without using any locks.
>   
>   Reviewed by:	emaste
>   Tested by:	Matthew Macy
>   MFC after:	1 week

There are some additional, harder problems in that path:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213334

-- 
Andriy Gapon