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Re: [PATCH] #13594: Avoid race in nscd
On Monday, May 14, 2012 23:22:47 Roland McGrath wrote:
> > +/* Try acquiring lock for mapptr, return 1 if not possible. */
> > +extern int __nscd_acquire_maplock (struct locked_map_ptr *mapptr)
> > attribute_hidden;
>
> Line too long. Put attribute_hidden on the next line, indented two
> spaces.
>
> If its return value is only 0 or 1, make it bool.
>
> I had in mind an inline function so as not to perturb the performance
> characteristics of the hot case. Probably that level of
> micro-optimization does not really matter. (If it does, probably the
> best is to have the initial cmpxchg inlined and the cold case loop in
> an uninlined function.) But I'd like an opinion from someone who knows
> nscd better than I do.
I've made it an inline function now and used bool.
Here's the patch, tested on Linux/x86-64. Ok now?
Andreas
diff --git a/nscd/nscd-client.h b/nscd/nscd-client.h
index e57a23c..5003d39 100644
--- a/nscd/nscd-client.h
+++ b/nscd/nscd-client.h
@@ -1,5 +1,4 @@
-/* Copyright (c) 1998, 1999, 2000, 2003, 2004, 2005, 2006, 2007, 2009, 2011
- Free Software Foundation, Inc.
+/* Copyright (c) 1998-2012 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Thorsten Kukuk <kukuk@suse.de>, 1998.
@@ -322,6 +321,24 @@ struct locked_map_ptr
};
#define libc_locked_map_ptr(class, name) class struct locked_map_ptr name
+/* Try acquiring lock for mapptr, returns true if it succeeds, false
+ if not. */
+static inline bool __nscd_acquire_maplock (volatile struct locked_map_ptr *mapptr)
+{
+ int cnt = 0;
+ while (__builtin_expect (atomic_compare_and_exchange_val_acq (&mapptr->lock,
+ 1, 0) != 0, 0))
+ {
+ // XXX Best number of rounds?
+ if (__builtin_expect (++cnt > 5, 0))
+ return false;
+
+ atomic_delay ();
+ }
+
+ return true;
+}
+
/* Open socket connection to nscd server. */
extern int __nscd_open_socket (const char *key, size_t keylen,
diff --git a/nscd/nscd_gethst_r.c b/nscd/nscd_gethst_r.c
index c1661f8..d64ad2e 100644
--- a/nscd/nscd_gethst_r.c
+++ b/nscd/nscd_gethst_r.c
@@ -1,5 +1,4 @@
-/* Copyright (C) 1998-2005, 2006, 2007, 2008, 2009, 2011
- Free Software Foundation, Inc.
+/* Copyright (C) 1998-2012 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@cygnus.com>, 1998.
@@ -100,9 +99,18 @@ libc_freeres_fn (hst_map_free)
uint32_t
__nscd_get_nl_timestamp (void)
{
+ uint32_t retval;
if (__nss_not_use_nscd_hosts != 0)
return 0;
+ /* __nscd_get_mapping can change hst_map_handle.mapped to NO_MAPPING.
+ However, __nscd_get_mapping assumes the prior value was not NO_MAPPING.
+ Thus we have to acquire the lock to prevent this thread from changing
+ hst_map_handle.mapped to NO_MAPPING while another thread is inside
+ __nscd_get_mapping. */
+ if (!__nscd_acquire_maplock (&__hst_map_handle))
+ return 0;
+
struct mapped_database *map = __hst_map_handle.mapped;
if (map == NULL
@@ -112,9 +120,14 @@ __nscd_get_nl_timestamp (void)
map = __nscd_get_mapping (GETFDHST, "hosts", &__hst_map_handle.mapped);
if (map == NO_MAPPING)
- return 0;
+ retval = 0;
+ else
+ retval = map->head->extra_data[NSCD_HST_IDX_CONF_TIMESTAMP];
+
+ /* Release the lock. */
+ __hst_map_handle.lock = 0;
- return map->head->extra_data[NSCD_HST_IDX_CONF_TIMESTAMP];
+ return retval;
}
diff --git a/nscd/nscd_helper.c b/nscd/nscd_helper.c
index 92558b6..96fb93d 100644
--- a/nscd/nscd_helper.c
+++ b/nscd/nscd_helper.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1998-2007, 2008, 2009 Free Software Foundation, Inc.
+/* Copyright (C) 1998-2012 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@cygnus.com>, 1998.
@@ -419,7 +419,6 @@ __nscd_get_mapping (request_type type, const char *key,
return result;
}
-
struct mapped_database *
__nscd_get_map_ref (request_type type, const char *name,
volatile struct locked_map_ptr *mapptr, int *gc_cyclep)
@@ -428,16 +427,8 @@ __nscd_get_map_ref (request_type type, const char *name,
if (cur == NO_MAPPING)
return cur;
- int cnt = 0;
- while (__builtin_expect (atomic_compare_and_exchange_val_acq (&mapptr->lock,
- 1, 0) != 0, 0))
- {
- // XXX Best number of rounds?
- if (__builtin_expect (++cnt > 5, 0))
- return NO_MAPPING;
-
- atomic_delay ();
- }
+ if (!__nscd_acquire_maplock (mapptr))
+ return NO_MAPPING;
cur = mapptr->mapped;
--
Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
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