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Before this patch, when a glock was locked, the very first holder on the queue would unlock the lockref and call the go_instantiate glops function (if one existed), unless GL_SKIP was specified. When we introduced the new node-scope concept, we allowed multiple holders to lock glocks in EX mode and share the lock. But node-scope introduced a new problem: if the first holder has GL_SKIP and the next one does NOT, since it is not the first holder on the queue, the go_instantiate op was not called. Eventually the GL_SKIP holder may call the instantiate sub-function (e.g. gfs2_rgrp_bh_get) but there was still a window of time in which another non-GL_SKIP holder assumes the instantiate function had been called by the first holder. In the case of rgrp glocks, this led to a NULL pointer dereference on the buffer_heads. This patch tries to fix the problem by introducing two new glock flags: GLF_INSTANTIATE_NEEDED, which keeps track of when the instantiate function needs to be called to "fill in" or "read in" the object before it is referenced. GLF_INSTANTIATE_IN_PROG which is used to determine when a process is in the process of reading in the object. Whenever a function needs to reference the object, it checks the GLF_INSTANTIATE_NEEDED flag, and if set, it sets GLF_INSTANTIATE_IN_PROG and calls the glops "go_instantiate" function. As before, the gl_lockref spin_lock is unlocked during the IO operation, which may take a relatively long amount of time to complete. While unlocked, if another process determines go_instantiate is still needed, it sees GLF_INSTANTIATE_IN_PROG is set, and waits for the go_instantiate glop operation to be completed. Once GLF_INSTANTIATE_IN_PROG is cleared, it needs to check GLF_INSTANTIATE_NEEDED again because the other process's go_instantiate operation may not have been successful. Functions that previously called the instantiate sub-functions now call directly into gfs2_instantiate so the new bits are managed properly. Signed-off-by: Bob Peterson <rpeterso@redhat.com> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
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.