From: Wenlin Kang <[email protected]>
With RT, if a high priority task that runs d_alloc_parallel() preempts a
low priority task that runs __d_add(), it can make d_alloc_parallel() go
into an infinite retry.
low priority task: high priority task:
__lookup_slow()
d_add()
__d_add
start_dir_add(dir) __lookup_slow()
d_alloc_parallel()
When the low priority task finished start_dir_add(), i_dir_seq has been
incremented to an odd value. Since __d_add() uses spin_lock(), migration
is disabled, so if the low priority task is preempted during start_dir_add()
/end_dir_add(), then high priority task can go into an infinite waiting for
i_dir_seq.
This patch avoid the issue by enable preempt_disable() during start_dir_add()
/end_dir_add().
Steps to reproduce:
$ taskset -c 1 chrt -f 70 ./test1.sh &
$ taskset -c 1 chrt -f 80 ./test2.sh &
After about 30 minutes, check whether serial console responds.
The problem happens with specific script combinations(test1.sh and test2.sh)
This problem was reproduced on v4.12 with preempt-rt, but I think it should be
exist on v5.2/standard/preempt-rt
P.S.
$ cat test1.sh
#!/bin/bash
while true; do
usleep 1
done
$ cat test2.sh
#!/bin/bash
while true; do
cat /proc/stat > /dev/null
usleep 100
done
Wenlin Kang (1):
fs: dcache: avoid livelocks on d_alloc_parallel
fs/dcache.c | 6 ++++++
1 file changed, 6 insertions(+)
--
1.9.1
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