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There is an assertion in __call_rcu() that checks only the bottom
bit of the rcu_head pointer, rather than the bottom two (as might be
expected for 32-bit systems) or the bottom three (as might be expected
for 64-bit systems). This choice might be a bit surprising in these days
of ubiquitous 32-bit and 64-bit systems. This commit therefore records
the reason for this odd alignment check, namely that m68k guarantees
only two-byte alignment despite being a 32-bit architectures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit adds documentation for RCU's major data structures,
including rcu_state, rcu_node, rcu_data, rcu_dynticks, and rcu_head.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Although call_rcu()'s fastpath works just fine on an idle CPU,
some branches of the slowpath invoke the scheduler, which uses
RCU. Therefore, this commit emphasizes the fact that call_rcu()
must not be invoked from an idle CPU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit uses colors to obscure the quick-quiz answers, thus getting
rid of the .htmlx file. Use your mouse to select the answer in order
to see the text. Alternatively, use your favorite scripting language
to remove all occurences of "<font color="ffffff">" from the file.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit removes a cutesy cartoon and also a diagram that can
just as easily be represented by text.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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There is already a blanket statement about no member of RCU's API
being legal from an offline CPU, but add an explicit note where it
states that it is illegal to invoke call_rcu() from an NMI handler.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit adds a Quick Quiz whose answer explains why the compiler
code reordering enabled by CONFIG_PREEMPT=n's empty rcu_read_lock()
and rcu_read_unlock() functions does not hinder RCU's ability to figure
out which RCU read-side critical sections have completed and not.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit records RCU's responsibility to avoid degrading latencies
of CPUs running tight loops within properly configured workloads,
both in kernel and in userspace.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Because RCU-sched expedited grace periods now use IPIs and interact
with rcu_read_unlock(), it is no longer sufficient to disable preemption
across RCU read-side critical sections that acquire and hold scheduler
locks. It is now necessary to instead disable interrupts. This commit
documents this change.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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The RCU requirements do not make it absolutely clear that the
memory-barrier requirements are not intended to replace the fundamental
requirement that all pre-existing RCU readers complete before a grace
period completes. This commit therefore pulls the memory-barrier
requirements into a separate section and explicitly calls out the
relationship between the memory-barrier requirements and the fundamental
requirement.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit adds a second option for avoiding scheduler/RCU deadlocks,
namely that preemption be disabled across the entire RCU read-side
critical section in question.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit expands on RCU's composability by comparing it to that of
transactional memory and of locking.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit adds verbiage on boot and sysfs parameters that can be
used to control RCU CPU stall warnings, both to change the timeout
and to suppress these warnings entirely.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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This commit records RCU's guarantee that the bottom bit of the rcu_head
structure's ->next field will remain zero for callbacks posted via
call_rcu(), but not necessarily for <tt>kfree_rcu()</tt> or some
possible future call_rcu_lazy() variant that might one day be created
for energy-efficiency purposese.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates URLs as suggested by Josh Triplett. ]
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This commit adds RCU requirements as published in a 2015 LWN series.
Bringing these requirements in-tree allows them to be updated as changes
are discovered.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates to charset and URLs as suggested by Josh Triplett. ]
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