5467a68cbf
For lockless accesses to dentries we don't have pinned we rely (among other things) upon having an RCU delay between dropping the last reference and actually freeing the memory. On the other hand, for things like pipes and sockets we neither do that kind of lockless access, nor want to deal with the overhead of an RCU delay every time a socket gets closed. So delay was made optional - setting DCACHE_RCUACCESS in ->d_flags made sure it would happen. We tried to avoid setting it unless we knew we need it. Unfortunately, that had led to recurring class of bugs, in which we missed the need to set it. We only really need it for dentries that are created by d_alloc_pseudo(), so let's not bother with trying to be smart - just make having an RCU delay the default. The ones that do *not* get it set the replacement flag (DCACHE_NORCU) and we'd better use that sparingly. d_alloc_pseudo() is the only such user right now. FWIW, the race that finally prompted that switch had been between __lock_parent() of immediate subdirectory of what's currently the root of a disconnected tree (e.g. from open-by-handle in progress) racing with d_splice_alias() elsewhere picking another alias for the same inode, either on outright corrupted fs image, or (in case of open-by-handle on NFS) that subdirectory having been just moved on server. It's not easy to hit, so the sky is not falling, but that's not the first race on similar missed cases and the logics for settinf DCACHE_RCUACCESS has gotten ridiculously convoluted. Cc: stable@vger.kernel.org Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.