Allow caller to block signals.

Greetings,

We use libunwind just for stack traces (I suspect many others do as well).

The use pattern is:

GetStackTrace(void** result, int max_depth)
{
...
  unw_getcontext(&uc);
  unw_init_local(&cursor, &uc);

  while (n < max_depth) {
    if (unw_get_reg(&cursor, UNW_REG_IP, (unw_word_t *) &ip) < 0) {
      break;
    }
    result[n++] = ip;
    if (unw_step(&cursor) <= 0) {
      break;
    }
  }

Given this usage, it is quite convenient for us to block signals (or
prevent signal handlers from re-entering libunwind by other means) at the
"top level", which makes most of the sigprocmask calls performed by
libunwind itself unneccessary.

The second patch in this series adds a configure option which removes most
of the sigprocmask calls.

Attached patch is a preliminary for it -- consolidating all of the
"sigprocmask; mutex_lock;" sequences into lock_acquire and "mutex_unlock;
sigprocmask;" sequences into lock_release.

Thanks,
--
Paul Pluzhnikov

commit 402d15b123d54a7669db7cf17a76dd315094e472
Author: Paul Pluzhnikov <ppluzhnikov@google.com>
Date:   Mon Sep 21 10:18:28 2009 -0700

    Replace "sigprocmask + mutext_lock" with a single lock_acquire.
    Likewise, replace "mutext_unlock + sigprocmask" with lock_release.

src/arm/Gglobal.c

@@ -44,8 +44,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&arm_lock);
+  lock_acquire (&arm_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -61,6 +60,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&arm_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&arm_lock, saved_mask);
 }

src/dwarf/Gparser.c

@@ -489,11 +489,10 @@ get_rs_cache (unw_addr_space_t as, intrmask_t *saved_maskp)
   if (AO_test_and_set (&cache->busy) == AO_TS_SET)
     return NULL;
 # else
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, saved_maskp);
   if (likely (caching == UNW_CACHE_GLOBAL))
     {
       Debug (16, "%s: acquiring lock\n", __FUNCTION__);
-      mutex_lock (&cache->lock);
+      lock_acquire (&cache->lock, *saved_maskp);
     }
 # endif
 #endif
@@ -521,8 +520,7 @@ put_rs_cache (unw_addr_space_t as, struct dwarf_rs_cache *cache,
   AO_CLEAR (&cache->busy);
 # else
   if (likely (as->caching_policy == UNW_CACHE_GLOBAL))
-    mutex_unlock (&cache->lock);
-  sigprocmask (SIG_SETMASK, saved_maskp, NULL);
+    lock_release (&cache->lock, *saved_maskp);
 # endif
 #endif
 }

src/hppa/Gglobal.c

@@ -35,8 +35,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&hppa_lock);
+  lock_acquire (&hppa_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -52,6 +51,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&hppa_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&hppa_lock, saved_mask);
 }

src/ia64/Gglobal.c

@@ -71,8 +71,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&unw.lock);
+  lock_acquire (&unw.lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -120,6 +119,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&unw.lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&unw.lock, saved_mask);
 }

src/ia64/Gscript.c

@@ -113,11 +113,10 @@ get_script_cache (unw_addr_space_t as, intrmask_t *saved_maskp)
   if (AO_test_and_set (&cache->busy) == AO_TS_SET)
     return NULL;
 # else
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, saved_maskp);
   if (likely (caching == UNW_CACHE_GLOBAL))
     {
       Debug (16, "%s: acquiring lock\n", __FUNCTION__);
-      mutex_lock (&cache->lock);
+      lock_acquire (&cache->lock, *saved_maskp);
     }
 # endif
 #endif
@@ -144,8 +143,7 @@ put_script_cache (unw_addr_space_t as, struct ia64_script_cache *cache,
   AO_CLEAR (&cache->busy);
 # else
   if (likely (as->caching_policy == UNW_CACHE_GLOBAL))
-    mutex_unlock (&cache->lock);
-  sigprocmask (SIG_SETMASK, saved_maskp, NULL);
+    lock_release (&cache->lock, *saved_maskp);
 # endif
 #endif
 }

src/mips/Gglobal.c

@@ -44,8 +44,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&mips_lock);
+  lock_acquire (&mips_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -61,6 +60,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&mips_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&mips_lock, saved_mask);
 }

src/ppc32/Gglobal.c

@@ -115,8 +115,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&ppc32_lock);
+  lock_acquire (&ppc32_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -132,6 +131,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&ppc32_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&ppc32_lock, saved_mask);
 }

src/ppc64/Gglobal.c

@@ -162,8 +162,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&ppc64_lock);
+  lock_acquire (&ppc64_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -179,6 +178,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&ppc64_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&ppc64_lock, saved_mask);
 }

src/x86/Gglobal.c

@@ -47,8 +47,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&x86_lock);
+  lock_acquire (&x86_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -64,6 +63,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&x86_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&x86_lock, saved_mask);
 }

src/x86_64/Gglobal.c

@@ -61,8 +61,7 @@ tdep_init (void)
 
   sigfillset (&unwi_full_mask);
 
-  sigprocmask (SIG_SETMASK, &unwi_full_mask, &saved_mask);
-  mutex_lock (&x86_64_lock);
+  lock_acquire (&x86_64_lock, saved_mask);
   {
     if (!tdep_needs_initialization)
       /* another thread else beat us to it... */
@@ -78,6 +77,5 @@ tdep_init (void)
     tdep_needs_initialization = 0;	/* signal that we're initialized... */
   }
  out:
-  mutex_unlock (&x86_64_lock);
-  sigprocmask (SIG_SETMASK, &saved_mask, NULL);
+  lock_release (&x86_64_lock, saved_mask);
 }