On Sun, Feb 04, 2024 at 04:25:58PM +0000, Al Viro wrote: > *IF* you can tell the symlink body without blocking (e.g. you > have some cached information from the last time you've asked > the server and have reasons to trust it to be still valid), > sure, you can return it without dropping out of RCU mode. > > It would be fairly useless for CIFS, since ->d_revalidate() of > CIFS dentries would reject RCU mode anyway. That's what normally > saves you from having ->get_link() called that way, but it's not > guaranteed - there are convoluted setups that avoid having > ->d_revalidate() called first. > > See the description of RCU mode filesystem exposure in the > last posting in this thread for more details. PS: if you decide to go for handling RCU pathwalk mode in CIFS, you definitely want to read the first half of https://lore.kernel.org/linux-fsdevel/20240204022743.GI2087318@ZenIV/ or whatever it eventually turns into. It obviously needs quite a bit of massage before it starts to resemble proper docs - currently it's just the summary I'd put together while going through the audit: which methods are exposed, how can they tell, what is and what is not guaranteed for them, etc., with a bit of "why does VFS bother with something that unpleasant" thrown into the mix as an explanation. Any assistance with turning that into a coherent text would be very welcome - I think that description of RCU pathwalk regarding its impact on the filesystems would be useful.
