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 <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Brandon Williams committed
Apr 19, 2017 at 16:13 UTC
e503cd6ed336d70d716e194ef6c5469330bea9da
1 file changed
+9
run-command.c
+9
@@ -537,6 +537,15 @@ fail_pipe:
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prepare_cmd(&argv, cmd);
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childenv = prep_childenv(cmd->env);
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+ /*
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+ * NOTE: In order to prevent deadlocking when using threads special
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+ * care should be taken with the function calls made in between the
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+ * fork() and exec() calls. No calls should be made to functions which
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+ * require acquiring a lock (e.g. malloc) as the lock could have been
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+ * held by another thread at the time of forking, causing the lock to
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+ * never be released in the child process. This means only
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+ * Async-Signal-Safe functions are permitted in the child.
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+ */
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cmd->pid = fork();
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failed_errno = errno;
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if (!cmd->pid) {