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blk-throttle: Correct the placement of smp_rmb()
o I was discussing what are the variable being updated without spin lock and why do we need barriers and Oleg pointed out that location of smp_rmb() should be between read of td->limits_changed and tg->limits_changed. This patch fixes it. o Following is one possible sequence of events. Say cpu0 is executing throtl_update_blkio_group_read_bps() and cpu1 is executing throtl_process_limit_change(). cpu0 cpu1 tg->limits_changed = true; smp_mb__before_atomic_inc(); atomic_inc(&td->limits_changed); if (!atomic_read(&td->limits_changed)) return; if (tg->limits_changed) do_something; If cpu0 has updated tg->limits_changed and td->limits_changed, we want to make sure that if update to td->limits_changed is visible on cpu1, then update to tg->limits_changed should also be visible. Oleg pointed out to ensure that we need to insert an smp_rmb() between td->limits_changed read and tg->limits_changed read. o I had erroneously put smp_rmb() before atomic_read(&td->limits_changed). This patch fixes it. Reported-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
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@ -725,26 +725,21 @@ static void throtl_process_limit_change(struct throtl_data *td)
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struct throtl_grp *tg;
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struct hlist_node *pos, *n;
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/*
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* Make sure atomic_inc() effects from
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* throtl_update_blkio_group_read_bps(), group of functions are
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* visible.
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* Is this required or smp_mb__after_atomic_inc() was suffcient
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* after the atomic_inc().
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*/
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smp_rmb();
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if (!atomic_read(&td->limits_changed))
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return;
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throtl_log(td, "limit changed =%d", atomic_read(&td->limits_changed));
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hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
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/*
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* Do I need an smp_rmb() here to make sure tg->limits_changed
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* update is visible. I am relying on smp_rmb() at the
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* beginning of function and not putting a new one here.
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*/
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/*
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* Make sure updates from throtl_update_blkio_group_read_bps() group
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* of functions to tg->limits_changed are visible. We do not
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* want update td->limits_changed to be visible but update to
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* tg->limits_changed not being visible yet on this cpu. Hence
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* the read barrier.
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*/
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smp_rmb();
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hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
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if (throtl_tg_on_rr(tg) && tg->limits_changed) {
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throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
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" riops=%u wiops=%u", tg->bps[READ],
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