All non-Async-Signal-Safe functions (e.g. malloc and die) were removed
between 'fork' and 'exec' in start_command in order to avoid potential
deadlocking when forking while multiple threads are running. This
deadlocking is possible when a thread (other than the one forking) has
acquired a lock and didn't get around to releasing it before the fork.
This leaves the lock in a locked state in the resulting process with no
hope of it ever being released.
Add a note describing this potential pitfall before the call to 'fork()'
so people working in this section of the code know to only use
Async-Signal-Safe functions in the child process.
Signed-off-by: Brandon Williams <bmwill@xxxxxxxxxx>
---
run-command.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/run-command.c b/run-command.c
index 615b6e9c9..df1edd963 100644
--- a/run-command.c
+++ b/run-command.c
@@ -537,6 +537,15 @@ int start_command(struct child_process *cmd)
prepare_cmd(&argv, cmd);
childenv = prep_childenv(cmd->env);
+ /*
+ * NOTE: In order to prevent deadlocking when using threads special
+ * care should be taken with the function calls made in between the
+ * fork() and exec() calls. No calls should be made to functions which
+ * require acquiring a lock (e.g. malloc) as the lock could have been
+ * held by another thread at the time of forking, causing the lock to
+ * never be released in the child process. This means only
+ * Async-Signal-Safe functions are permitted in the child.
+ */
cmd->pid = fork();
failed_errno = errno;
if (!cmd->pid) {
--
2.12.2.816.g2cccc81164-goog