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ec2c29765a
Without special fail-safe quiescent-state-propagation checks, grace-period hangs can result from the following scenario: 1. A task running on a given CPU is preempted in its RCU read-side critical section. 2. That CPU goes offline, and there are now no online CPUs corresponding to that CPU's leaf rcu_node structure. 3. The rcu_gp_init() function does the first phase of grace-period initialization, and sets the aforementioned leaf rcu_node structure's ->qsmaskinit field to all zeroes. Because there is a blocked task, it does not propagate the zeroing of either ->qsmaskinit or ->qsmaskinitnext up the rcu_node tree. 4. The task resumes on some other CPU and exits its critical section. There is no grace period in progress, so the resulting quiescent state is not reported up the tree. 5. The rcu_gp_init() function does the second phase of grace-period initialization, which results in the leaf rcu_node structure being initialized to expect no further quiescent states, but with that structure's parent expecting a quiescent-state report. The parent will never receive a quiescent state from this leaf rcu_node structure, so the grace period will hang, resulting in RCU CPU stall warnings. It would be good to get rid of the special fail-safe quiescent-state propagation checks. This commit therefore checks the leaf rcu_node structure's ->wait_blkd_tasks field during grace-period initialization. If this flag is set, the rcu_report_qs_rnp() is invoked to immediately report the possible quiescent state. While in the neighborhood, this commit also report quiescent states for any CPUs that went offline between the two phases of grace-period initialization, thus reducing grace-period delays and hopefully eventually allowing removal of offline-CPU checks from the force-quiescent-state code path. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> |
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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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.