On Mon, May 07, 2018 at 05:24:05PM +0200, Duy Nguyen wrote: > >>>> - static struct lock_file lock; > >>>> + struct lock_file lock = LOCK_INIT; > >>> > >>> Is it really safe to do this? I vaguely remember something about > >>> (global) linked list and signal handling which could trigger any time > >>> and probably at atexit() time too (i.e. die()). You don't want to > >>> depend on stack-based variables in that case. > >> > >> So I dug in a bit more about this. The original implementation does > >> not allow stack-based lock files at all in 415e96c8b7 ([PATCH] > >> Implement git-checkout-cache -u to update stat information in the > >> cache. - 2005-05-15). The situation has changed since 422a21c6a0 > >> (tempfile: remove deactivated list entries - 2017-09-05). At the end > >> of that second commit, Jeff mentioned "We can clean them up > >> individually" which I guess is what these patches do. Though I do not > >> know if we need to make sure to call "release" function or something/ > >> Either way you need more explanation and assurance than just "we can > >> drop their staticness" in the commit mesage. > > > > Thank you Duy for your comments. How about I write the commit message > > like so: > > +Jeff. Since he made it possible to remove lock file from the global > linked list, he probably knows well what to check when switching from > a static lock file to a stack-local one.
It should be totally safe. If you look at "struct lock_file", it is now simply a pointer to a tempfile allocated on the heap (in fact, I thought about getting rid of lock_file entirely, but the diff is noisy and it actually has some value as an abstraction over a pure tempfile). If you fail to call a release function, it will just hang around until program exit, which is more or less what the static version would do. The big difference is that if we re-enter the function while still holding the lock, then the static version would BUG() on trying to use the already-active lockfile. Whereas after this series, we'd try to create a new lockfile and say "woah, somebody else is holding the lock". > > After 076aa2cbd (tempfile: auto-allocate tempfiles on heap, 2017-09-05), > > we can have lockfiles on the stack. These `struct lock_file`s are local > > to their respective functions and we can drop their staticness. > > > > Each of these users either commits or rolls back the lock in every > > codepath, with these possible exceptions: > > > > * We bail using a call to `die()` or `exit()`. The lock will be > > cleaned up automatically. > > > > * We return early from a function `cmd_foo()` in builtin/, i.e., we > > are just about to exit. The lock will be cleaned up automatically. > > There are also signals which can be caught and run on its own stack (I > think) so whatever variable on the current stack should be safe, I > guess. Yes, the stack variables should all be intact during an exit or a signal. > > If I have missed some codepath where we do not exit, yet leave a locked > > lock around, that was so also before this patch. If we would later > > re-enter the same function, then before this patch, we would be retaking > > a lock for the very same `struct lock_file`, which feels awkward, but to > > the best of my reading has well-defined behavior. Whereas after this > > patch, we would attempt to take the lock with a completely fresh `struct > > lock_file`. In both cases, the result would simply be that the lock can > > not be taken, which is a situation we already handle. > > There is a difference here, if the lock is not released properly, > previously the lockfile is still untouched. If it's on stack, it may > be overwritten which can corrupt the linked list to get to the next > lock file. (and this is about calling the function in question just > _once_ not the second time). The only bits on the stack are just a pointer to the list item. So the linked list is fine if it goes out of scope while the tempfile is still active. That was the point of 076aa2cbd. -Peff
