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smp: reword smp call IPI comment 

Accessing the call_single_queue hasn't involved a spinlock since 2014:

  6897fc22ea ("kernel: use lockless list for smp_call_function_single")

The llist operations (namely cmpxchg() and xchg()) provide similar ordering
guarantees, update the comment to lessen confusion.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230307143558.294354-7-vschneid@redhat.com
This commit is contained in:
Valentin Schneider 2023-03-07 14:35:57 +00:00 committed by Peter Zijlstra
parent 4c8c3c7f70
commit 253a0fb4c6
1 changed files with 4 additions and 3 deletions
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7
kernel/smp.c
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@ -312,9 +312,10 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
void __smp_call_single_queue(int cpu, struct llist_node *node)
{
/*
* The list addition should be visible before sending the IPI
* handler locks the list to pull the entry off it because of
* normal cache coherency rules implied by spinlocks.
* The list addition should be visible to the target CPU when it pops
* the head of the list to pull the entry off it in the IPI handler
* because of normal cache coherency rules implied by the underlying
* llist ops.
*
* If IPIs can go out of order to the cache coherency protocol
* in an architecture, sufficient synchronisation should be added