From e503cd6ed336d70d716e194ef6c5469330bea9da Mon Sep 17 00:00:00 2001 From: Brandon Williams Date: Wed, 19 Apr 2017 16:13:26 -0700 Subject: run-command: add note about forking and threading 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 Signed-off-by: Junio C Hamano --- run-command.c | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'run-command.c') diff --git a/run-command.c b/run-command.c index 615b6e9c9c..df1edd963f 100644 --- a/run-command.c +++ b/run-command.c @@ -537,6 +537,15 @@ fail_pipe: 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) { -- cgit 1.2.3-korg