[Libunwind-devel] unw_set_cache_size and per-thread caches

[Milian Wolff]

Hey Bert, others.

I notice that unw_set_cache_size is disabled / always fails when libunwind is 
compiled with --enable-per-thread-cache:

  /* Currently not supported for per-thread cache due to memory leak */
  /* A pthread-key destructor would work, but is not signal safe */
#if defined(HAVE___THREAD) && HAVE___THREAD
  return -1;
#endif

Can you, or someone else, give me some more information on the reasoning here?

I'm especially surprised, since I personally would never consider 
unw_set_cache_size to be called from a signal. I am probably misunderstanding 
something here?

Actually, I would even go as far as saying that the cache size can and should 
only be configured once before it was ever used. So flushing the caches 
shouldn't even be required... Could we maybe at least support this use-case in 
the HAVE___THREAD scenario? I.e. something like the following:

diff --git a/src/mi/Gset_cache_size.c b/src/mi/Gset_cache_size.c
index 07b282e2..f1f97eea 100644
--- a/src/mi/Gset_cache_size.c
+++ b/src/mi/Gset_cache_size.c
@@ -38,12 +38,6 @@ unw_set_cache_size (unw_addr_space_t as, size_t size, int 
flag)
   if (flag != 0)
     return -1;
 
-  /* Currently not supported for per-thread cache due to memory leak */
-  /* A pthread-key destructor would work, but is not signal safe */
-#if defined(HAVE___THREAD) && HAVE___THREAD
-  return -1;
-#endif
-
   /* Round up to next power of two, slowly but portably */
   while(power < size)
     {
@@ -58,6 +52,13 @@ unw_set_cache_size (unw_addr_space_t as, size_t size, int 
flag)
   if (log_size == as->global_cache.log_size)
     return 0;   /* no change */
 
+  /* Changing cache size after it was used once is not supported for per-
thread cache due to memory leak */
+  /* A pthread-key destructor would work, but is not signal safe */
+#if defined(HAVE___THREAD) && HAVE___THREAD
+  if (as->global_cache.log_size)
+    return -1;
+#endif
+
   as->global_cache.log_size = log_size;
 #endif
 
If that is acceptable, then I'll push this up for review on Github.

Cheers
-- 
Milian Wolff
address@hidden
http://milianw.de

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