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author | Zqiang <qiang.zhang1211@gmail.com> | 2024-10-22 11:41:17 +0800 |
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committer | Frederic Weisbecker <frederic@kernel.org> | 2024-11-12 22:51:52 +0100 |
commit | 2996980e20b7a54a1869df15b3445374b850b155 (patch) | |
tree | 2898eaf35e2b5cb5b0b03a0a38ac1ec8a757ac7a /net | |
parent | 8808c5732265c301d3fe7e5043ea8162eef3421a (diff) | |
download | linux-stable-2996980e20b7a54a1869df15b3445374b850b155.tar.gz linux-stable-2996980e20b7a54a1869df15b3445374b850b155.tar.bz2 linux-stable-2996980e20b7a54a1869df15b3445374b850b155.zip |
rcu/nocb: Fix missed RCU barrier on deoffloading
Currently, running rcutorture test with torture_type=rcu fwd_progress=8
n_barrier_cbs=8 nocbs_nthreads=8 nocbs_toggle=100 onoff_interval=60
test_boost=2, will trigger the following warning:
WARNING: CPU: 19 PID: 100 at kernel/rcu/tree_nocb.h:1061 rcu_nocb_rdp_deoffload+0x292/0x2a0
RIP: 0010:rcu_nocb_rdp_deoffload+0x292/0x2a0
Call Trace:
<TASK>
? __warn+0x7e/0x120
? rcu_nocb_rdp_deoffload+0x292/0x2a0
? report_bug+0x18e/0x1a0
? handle_bug+0x3d/0x70
? exc_invalid_op+0x18/0x70
? asm_exc_invalid_op+0x1a/0x20
? rcu_nocb_rdp_deoffload+0x292/0x2a0
rcu_nocb_cpu_deoffload+0x70/0xa0
rcu_nocb_toggle+0x136/0x1c0
? __pfx_rcu_nocb_toggle+0x10/0x10
kthread+0xd1/0x100
? __pfx_kthread+0x10/0x10
ret_from_fork+0x2f/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1a/0x30
</TASK>
CPU0 CPU2 CPU3
//rcu_nocb_toggle //nocb_cb_wait //rcutorture
// deoffload CPU1 // process CPU1's rdp
rcu_barrier()
rcu_segcblist_entrain()
rcu_segcblist_add_len(1);
// len == 2
// enqueue barrier
// callback to CPU1's
// rdp->cblist
rcu_do_batch()
// invoke CPU1's rdp->cblist
// callback
rcu_barrier_callback()
rcu_barrier()
mutex_lock(&rcu_state.barrier_mutex);
// still see len == 2
// enqueue barrier callback
// to CPU1's rdp->cblist
rcu_segcblist_entrain()
rcu_segcblist_add_len(1);
// len == 3
// decrement len
rcu_segcblist_add_len(-2);
kthread_parkme()
// CPU1's rdp->cblist len == 1
// Warn because there is
// still a pending barrier
// trigger warning
WARN_ON_ONCE(rcu_segcblist_n_cbs(&rdp->cblist));
cpus_read_unlock();
// wait CPU1 to comes online and
// invoke barrier callback on
// CPU1 rdp's->cblist
wait_for_completion(&rcu_state.barrier_completion);
// deoffload CPU4
cpus_read_lock()
rcu_barrier()
mutex_lock(&rcu_state.barrier_mutex);
// block on barrier_mutex
// wait rcu_barrier() on
// CPU3 to unlock barrier_mutex
// but CPU3 unlock barrier_mutex
// need to wait CPU1 comes online
// when CPU1 going online will block on cpus_write_lock
The above scenario will not only trigger a WARN_ON_ONCE(), but also
trigger a deadlock.
Thanks to nocb locking, a second racing rcu_barrier() on an offline CPU
will either observe the decremented callback counter down to 0 and spare
the callback enqueue, or rcuo will observe the new callback and keep
rdp->nocb_cb_sleep to false.
Therefore check rdp->nocb_cb_sleep before parking to make sure no
further rcu_barrier() is waiting on the rdp.
Fixes: 1fcb932c8b5c ("rcu/nocb: Simplify (de-)offloading state machine")
Suggested-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Diffstat (limited to 'net')
0 files changed, 0 insertions, 0 deletions