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aa3b66f401
The quick check in tick_do_update_jiffies64() whether jiffies need to be
updated is not really correct under all circumstances and on all
architectures, especially not on 32bit systems.
The quick check does:
if (now < READ_ONCE(tick_next_period))
return;
and the counterpart in the update is:
WRITE_ONCE(tick_next_period, next_update_time);
This has two problems:
1) On weakly ordered architectures there is no guarantee that the stores
before the WRITE_ONCE() are visible which means that other CPUs can
operate on a stale jiffies value.
2) On 32bit the store of tick_next_period which is an u64 is split into
two 32bit stores. If the first 32bit store advances tick_next_period
far out and the second 32bit store is delayed (virt, NMI ...) then
jiffies will become stale until the second 32bit store happens.
Address this by seperating the handling for 32bit and 64bit.
On 64bit problem #1 is addressed by replacing READ_ONCE() / WRITE_ONCE()
with smp_load_acquire() / smp_store_release().
On 32bit problem #2 is addressed by protecting the quick check with the
jiffies sequence counter. The load and stores can be plain because the
sequence count mechanics provides the required barriers already.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/87czzpc02w.fsf@nanos.tec.linutronix.de
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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.
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.