[PATCH v2 dwarves 2/2] dwarf_loader: use libdw__lock for

[Alan Maguire <alan.maguire@oracle.com>]

Eduard noticed [1] intermittent segmentation faults triggered by caching
done internally in dwarf_getlocation(s).  A binary tree of location
information is cached in the CU, and if multiple threads access it
concurrently we can get segmentation faults.  It is possible to
compile elfutils with experimental thread-safe support, but safer for
now to add locking to pahole.

No additional overhead in pahole encoding was observed
as a result of these changes.

Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Suggested-by: Jiri Olsa <olsajiri@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

[1] https://lore.kernel.org/dwarves/080794545d8eb3df3d6eba90ac621111ab7171f5.camel@gmail.com/
---
 dwarf_loader.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index bc862b5..9299c1f 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -450,7 +450,14 @@ static bool attr_type(Dwarf_Die *die, uint32_t attr_name, Dwarf_Off *offset)
 static int attr_location(Dwarf_Die *die, Dwarf_Op **expr, size_t *exprlen)
 {
 	Dwarf_Attribute attr;
+	int ret = 1;
+
 	if (dwarf_attr(die, DW_AT_location, &attr) != NULL) {
+		/* use libdw__lock as dwarf_getlocation(s) has concurrency
+		 * issues when libdw is not compiled with experimental
+		 * --enable-thread-safety
+		 */
+		pthread_mutex_lock(&libdw__lock);
 		if (dwarf_getlocation(&attr, expr, exprlen) == 0) {
 			/* DW_OP_addrx needs additional lookup for real addr. */
 			if (*exprlen != 0 && expr[0]->atom == DW_OP_addrx) {
@@ -462,11 +469,12 @@ static int attr_location(Dwarf_Die *die, Dwarf_Op **expr, size_t *exprlen)
 
 				expr[0]->number = address;
 			}
-			return 0;
+			ret = 0;
 		}
+		pthread_mutex_unlock(&libdw__lock);
 	}
 
-	return 1;
+	return ret;
 }
 
 /* The struct dwarf_tag has a fixed size while the 'struct tag' is just the base
@@ -1193,6 +1201,10 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 	int loc_num = -1;
 	int ret = -1;
 
+	/* use libdw__lock as dwarf_getlocation(s) has concurrency issues
+	 * when libdw is not compiled with experimental --enable-thread-safety
+	 */
+	pthread_mutex_lock(&libdw__lock);
 	while ((offset = __dwarf_getlocations(attr, offset, &base, &start, &end, &expr, &exprlen)) > 0) {
 		loc_num++;
 
@@ -1228,6 +1240,7 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 		}
 	}
 out:
+	pthread_mutex_unlock(&libdw__lock);
 	return ret;
 }
 
-- 
2.31.1