If multiple threads trip an assertion at the same moment (quite common),
they can be printing at the same time, and their output gets messy.
This adds a simple lock around the whole thing, to prevent a second task
printing assert output before the first has finished.
Additionally, if libspl_assert_ok is not set, abort() is called without
dropping the lock, so that any other asserting tasks will be killed
before starting any output, rather than only getting part-way through.
This is a tradeoff; it's assumed that multiple threads asserting at the
same moment are likely the same fault in different instances of a
thread, and so there won't be any more useful information from the other
tasks anyway.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/Closes#16140
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