Re: [patch] Fix for bz22161: ncsd: avoid dangling lock in netgroup cache timeout code

[Jonathan Nieder <jrnieder at gmail dot com>]

DJ Delorie wrote:

> Patch for https://sourceware.org/bugzilla/show_bug.cgi?id=22161
>
>  "From the bz: in nscd/netgroupcache.c in addinnetgrX() we call
>   mempool_alloc(..., 1) which takes a lock on the database.  If we
>   exit via the "bump timeout" clause, the lock is not released."
>
> This patch adds an unlock if mempool_alloc actually took a lock, in
> the case where we return early because the timeout hasn't timed out.
>
> Because the dangling lock is a read lock, queries to the database
> continue to work.  The cache prune thread eventually becomes
> deadlocked, and queries stop seeing updates (i.e. they return stale
> data forever).
>
> Original patch by Al Heisner via https://bugzilla.redhat.com/show_bug.cgi?id=1277672
>
> 2017-09-20  DJ Delorie  <dj@redhat.com>
>
> 	* nscd/netgroupcache.c (addinnetgrX): Release read lock after
> 	bumping timeout values.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>

Thank you.

> diff --git a/nscd/netgroupcache.c b/nscd/netgroupcache.c
> index cd0c3ea..3ca96f8 100644
> --- a/nscd/netgroupcache.c
> +++ b/nscd/netgroupcache.c
> @@ -584,6 +584,8 @@ addinnetgrX (struct database_dyn *db, int fd, request_header *req,
>        dh->timeout = timeout;
>        dh->ttl = dataset->head.ttl;
>        ++dh->nreloads;
> +      if (cacheable)
> +        pthread_rwlock_unlock (&db->lock);
>        return timeout;
>      }
>