Hi all, new newbie here.
I've been studying the sys_open() code and have run into a problem with
the synchronization using RCU. The code snippet I'm having problems with
is in get_unused_fd_flags():
FD_SET(fd, fdt->open_fds);
if (flags & O_CLOEXEC)
FD_SET(fd, fdt->close_on_exec);
else
FD_CLR(fd, fdt->close_on_exec);
RCU requires first creating a local copy, updating that copy, updating
the RCU protected pointer with the local copy and finally deallocating
the original version after a grace period. This snippet, however,
directly modifies the shared structure and therefore exposes
intermediate states to other readers.
I'm assuming it doesn't matter as the fd allocated won't be properly
accessible until after the syscall has finished and therefore those bits
won't be actually used until then. Still, I want to know if this really
violates the RCU mechanic or if I'm missing something.
Cheers,
Martin
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