Re: rfc: fix double locking in the pool cache subroutines

[Andrew Doran <ad%netbsd.org@localhost>]

On Fri, Jul 11, 2008 at 11:13:04AM +0300, Andy Shevchenko wrote:
> Hello.
> 
> I'm using the NetBSD-current (yesterday snapshot) on OMAP/H4.
> When I try to start one of the libMicro (0.4.0) test called close_tcp
> I've got the locking panic (I've switched on the LOCKDEBUG option).
> 
> # tcc -p -e $PWD close_tcp
> tcc: journal file is /root/micro/results/0010e/journal
> 07:50:48  Execute /ts/close_tcp/close_tcp-tc
> Mutex error: lockdebug_wantlock: locking against myself
> 
> lock address : 0x0000000081ca9a00 type     :     sleep/adaptive
> initialized  : 0x0000000080410264
> shared holds :                  0 exclusive:                  1
> shares wanted:                  0 exclusive:                  1
> current cpu  :                  0 last held:                  0
> current lwp  : 0x0000000081cad4c0 last held: 0x0000000081cad4c0
> last locked  : 0x000000008040f660 unlocked : 0x000000008040f704
> owner field  : 0x0000000081cad4c0 wait/spin:                0/0

That does not make sense. What do the 'last locked', 'last unlocked'
and 'initialized' addresses resolve to? Can you post the backtrace
from the panic?

Andrew



> Turnstile chain at 0x80550f20.
> => No active turnstile for this lock.
> 
> panic: LOCKDEBUG
> Stopped in pid 0.5 (system) at  netbsd:cpu_Debugger+0x4:        bx      r14
> 
> 
> Some investigations lead to following quick fix:
> 
> diff --git a/sys/kern/subr_pool.c b/sys/kern/subr_pool.c
> index 32547e7..6ad1a09 100644
> --- a/sys/kern/subr_pool.c
> +++ b/sys/kern/subr_pool.c
> @@ -2541,7 +2541,9 @@ pool_cache_put_slow(pool_cache_cpu_t *cc, int s,
> void *object)
>       /* If there are no empty groups in the cache then allocate one. */
>       if (__predict_false((pcg = pc->pc_emptygroups) == NULL)) {
>               if (__predict_true(!pool_cache_disable)) {
> +                     mutex_exit(&pc->pc_lock);
>                       pcg = pool_get(pc->pc_pcgpool, PR_NOWAIT);
> +                     mutex_enter(&pc->pc_lock);
>               }
>               if (__predict_true(pcg != NULL)) {
>                       pcg->pcg_avail = 0;
> 
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko