Messages in this thread | | | Date | Mon, 16 Feb 2009 21:49:02 +0100 | From | Oleg Nesterov <> | Subject | Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many()) |
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I am trying to understand the barriers in smp.c, please help!
"generic-ipi: fix the smp_mb() placement" commit 561920a0d2bb6d63343e83acfd784c0a77bd28d1 added smp_read_barrier_depends() to generic_smp_call_function_single_interrupt().
Why it is needed? The comment says:
/* * Need to see other stores to list head for checking whether * list is empty without holding q->lock */ smp_read_barrier_depends(); while (!list_empty(&q->list)) {
But we can't miss the addition to the call_single_queue.list, if generic_exec_single() sees list_empty(&dst->list) it sends another IPI?
This commit also removed the barrier from csd_flag_wait(), is this OK? Without the barrier, csd_flag_wait() can return before we see the result of data->func() ?
IOW, int VAR = 0;
void func(coid *unused) { VAR = 1; }
Now,
smp_call_function_single(0, func, NULL, 1); BUG_ON(VAR == 0);
afaics, the BUG_ON() above is possible. Is this OK ?
Oleg.
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