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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2017-05-03 15:35:32 -0700 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2017-06-08 18:52:34 -0700 |
commit | c350c008297643dad3c395c2fd92230142da5cf6 (patch) | |
tree | 731bff2816d661ee0c1015a6e5e4423ccfd87f7f /scripts/basic | |
parent | 71c40fd0b5ceb300c6cb8753835d9d94a8bfc56f (diff) |
srcu: Prevent sdp->srcu_gp_seq_needed counter wrap
If a given CPU never happens to ever start an SRCU grace period, the
grace-period sequence counter might wrap. If this CPU were to decide to
finally start a grace period, the state of its sdp->srcu_gp_seq_needed
might make it appear that it has already requested this grace period,
which would prevent starting the grace period. If no other CPU ever started
a grace period again, this would look like a grace-period hang. Even
if some other CPU took pity and started the needed grace period, the
leaf rcu_node structure's ->srcu_data_have_cbs field won't have record
of the fact that this CPU has a callback pending, which would look like
a very localized grace-period hang.
This might seem very unlikely, but SRCU grace periods can take less than
a microsecond on small systems, which means that overflow can happen
in much less than an hour on a 32-bit embedded system. And embedded
systems are especially likely to have long-term idle CPUs. Therefore,
it makes sense to prevent this scenario from happening.
This commit therefore scans each srcu_data structure occasionally,
with frequency controlled by the srcutree.counter_wrap_check kernel
boot parameter. This parameter can be set to something like 255
in order to exercise the counter-wrap-prevention code.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'scripts/basic')
0 files changed, 0 insertions, 0 deletions