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rcutorture: Start rcu_torture_writer() after rcu_torture_reader()

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Testing of rcutorture's SRCU-P scenario on a large arm64 system resulted
in rcu_torture_writer() forward-progress failures, but these same tests
passed on x86.  After some off-list discussion of possible memory-ordering
causes for these failures, Boqun showed that these were in fact due to
reordering, but by the scheduler, not by the memory system.  On x86,
rcu_torture_writer() would have run quickly enough that by the time
the rcu_torture_updown() kthread started, the rcu_torture_current
variable would already be initialized, thus avoiding a bug in which
a NULL value would cause rcu_torture_updown() to do an extra call to
srcu_up_read_fast().

This commit therefore moves creation of the rcu_torture_writer() kthread
after that of the rcu_torture_reader() kthreads.  This results in
deterministic failures on x86.

What about the double-srcu_up_read_fast() bug?  Boqun has the fix.
But let's also fix the test while we are at it!

Reported-by: Joel Fernandes <joelagnelf@nvidia.com>
Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
This commit is contained in:
Paul E. McKenney 2025-05-08 16:45:01 -07:00 committed by Neeraj Upadhyay (AMD)
parent 9ea40db969
commit 635bdb9d22
1 changed files with 6 additions and 5 deletions
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11
kernel/rcu/rcutorture.c
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@ -4246,11 +4246,6 @@ rcu_torture_init(void)
/* Start up the kthreads. */
rcu_torture_write_types();
firsterr = torture_create_kthread(rcu_torture_writer, NULL,
writer_task);
if (torture_init_error(firsterr))
goto unwind;
if (nrealfakewriters > 0) {
fakewriter_tasks = kcalloc(nrealfakewriters,
sizeof(fakewriter_tasks[0]),
@ -4283,6 +4278,12 @@ rcu_torture_init(void)
if (torture_init_error(firsterr))
goto unwind;
}
firsterr = torture_create_kthread(rcu_torture_writer, NULL,
writer_task);
if (torture_init_error(firsterr))
goto unwind;
nrealnocbers = nocbs_nthreads;
if (WARN_ON(nrealnocbers < 0))
nrealnocbers = 1;