Re: smp_rendezvous_action: Are atomics correctly used ?

On Thu, Mar 09, 2017 at 02:52:09PM +0100, Alexandre Martins wrote:
> Le jeudi 9 mars 2017, 15:07:54 Konstantin Belousov a ?crit :
> > On Thu, Mar 09, 2017 at 10:59:27AM +0100, Alexandre Martins wrote:
> > > I have the save question for the cpu_ipi_pending here:
> > > 
> > > https://svnweb.freebsd.org/base/head/sys/x86/x86/mp_x86.c?view=annotate#l1
> > > 080> 
> > > Le jeudi 9 mars 2017, 10:43:14 Alexandre Martins a ?crit :
> > > > Hello,
> > > > 
> > > > I'm curently reading the code of the function smp_rendezvous_action, in
> > > > kern/subr_smp.c file. In that function, i see that the variable
> > > > smp_rv_waiters is read in some while() loop in a non-atomic way.
> > > > 
> > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=annotate#l
> > > > 412
> > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=annotate#l
> > > > 458
> > > > https://svnweb.freebsd.org/base/head/sys/kern/subr_smp.c?view=annotate#l
> > > > 472
> > > > 
> > > > I suspect one of my freeze to be due by that.
> > 
> > You should provide either evidence or, at least, some reasoning supporting
> > your claims.
> 
> I curently have a software watchdog that triger and does a coredump. In the 
> coredumps, I always see a CPU trying to write-lock a "rm lock". Every time, 
> that CPU is spinning into the smp_rendezvous_action, in the first while loop) 
> while the others are into the idle threads.
> 
> The fact is that freeze is not clear and I start to search "exotic" causes to 
> explain it.
This sounds as the 'usual' deadlock, where some other thread owns rmlock in
read mode.  I recommend you to follow the
https://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/kerneldebug-deadlocks.html