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81fe48e9aa
md_write_start() needs to clear the in_sync flag is it is set, or if
there might be a race with set_in_sync() such that the later will
set it very soon. In the later case it is sufficient to take the
spinlock to synchronize with set_in_sync(), and then set the flag
if needed.
The current test is incorrect.
It should be:
if "flag is set" or "race is possible"
"flag is set" is trivially "mddev->in_sync".
"race is possible" should be tested by "mddev->sync_checkers".
If sync_checkers is 0, then there can be no race. set_in_sync() will
wait in percpu_ref_switch_to_atomic_sync() for an RCU grace period,
and as md_write_start() holds the rcu_read_lock(), set_in_sync() will
be sure ot see the update to writes_pending.
If sync_checkers is > 0, there could be race. If md_write_start()
happened entirely between
if (!mddev->in_sync &&
percpu_ref_is_zero(&mddev->writes_pending)) {
and
mddev->in_sync = 1;
in set_in_sync(), then it would not see that is_sync had been set,
and set_in_sync() would not see that writes_pending had been
incremented.
This bug means that in_sync is sometimes not set when it should be.
Consequently there is a small chance that the array will be marked as
"clean" when in fact it is inconsistent.
Fixes:
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. 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.