sss_client: Fix race condition in memory cache

Thread safe initialisation was fixed in ticket #2380, but there is
still race condition in reinitialisation.

If caches is invalidated with command sss_cache -U (-G or -E) then
client code will need to reinitialize fast memory cache.
Let say we have two threads. The 1st thread find out that memory cache
should be reinitialized; therefore the fast memory cached is unmapped
and context destroyed. In the same time, 2nd thread tried to check
header of memory cache whether it is initialized and valid. As a result
of previously unmapped memory the 2nd thread access
out of bound memory (SEGFAULT).

The destroying of fast memory cache cannot be done any time. We need
to be sure that there isn't any other thread which uses mmaped memory.
The new counter of active threads was added for this purpose. The state
of fast memory cache was converted from boolean to three value state
(UNINITIALIZED, INITIALIZED, RECYCLED)
UNINITIALIZED
    - the fast memory cache need to be initialized.
    - if there is a problem with initialisation the state will not change
    - after successful initialisation, the state will change to INITIALIZED
INITIALIZED
    - if the cahe was invalidated or there is any other problem was
      detected in memory cache header the state will change to RECYCLED
      and memory cache IS NOT destroyed.
RECYCLED
    - nothing will be done is there are any active threads which may use
      the data from mmaped memory
    - if there aren't active threads the fast memory cahe is destroyed and
      state is changed to UNINITIALIZED.

https://fedorahosted.org/sssd/ticket/2445

Reviewed-by: Michal Židek <mzidek@redhat.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/sss_client/nss_mc.h b/src/sss_client/nss_mc.h
index 685cc41c0..050bd4100 100644
--- a/src/sss_client/nss_mc.h
+++ b/src/sss_client/nss_mc.h
@@ -33,9 +33,15 @@
 typedef int errno_t;
 #endif
 
+enum sss_mc_state {
+    UNINITIALIZED = 0,
+    INITIALIZED,
+    RECYCLED,
+};
+
 /* common stuff */
 struct sss_cli_mc_ctx {
-    bool initialized;
+    enum sss_mc_state initialized;
     int fd;
 
     uint32_t seed;          /* seed from the tables header */
@@ -48,6 +54,8 @@ struct sss_cli_mc_ctx {
 
     uint32_t *hash_table;   /* hash table address (in mmap) */
     uint32_t ht_size;       /* size of hash table */
+
+    uint32_t active_threads; /* count of threads which use memory cache */
 };
 
 errno_t sss_nss_mc_get_ctx(const char *name, struct sss_cli_mc_ctx *ctx);