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Fix data race between zil_commit() and zil_suspend()
openzfsonwindows#206 found that it is possible to trip `VERIFY(list_is_empty(&lwb->lwb_itxs))` when a `zil_commit()` is delayed by the scheduler long enough for a parallel `zil_suspend()` operation to exit `zil_commit_impl()`. This is a data race. To prevent this, we introduce a `zilog->zl_suspend_lock` rwlock to ensure that all outstanding `zil_commit()` operations finish before `zil_suspend()` begins and that subsequent operations fallback to `txg_wait_synced()` after `zil_suspend()` has begun. On `PREEMPT_RT` Linux kernels, the `rw_enter()` implementation suffers from writer starvation. This means that a ZIL intensive system can delay `zil_suspend()` indefinitely. This is a pre-existing problem that affects everything that uses rw locks, so it needs to be addressed in the SPL. However, builds against `PREEMPT_RT` Linux kernels are currently broken due to a GPL symbol issue (openzfs#11097), so we can safely disregard that issue for now. Reported-by: Arun KV <[email protected]> Signed-off-by: Richard Yao <[email protected]>
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