From: Chris Wilson
Sent: 10 March 2020 11:50
Quoting David Laight (2020-03-10 11:36:41)
From: Chris Wilson
Sent: 10 March 2020 09:21 Instruct the compiler to read the next element in the list iteration once, and that it is not allowed to reload the value from the stale element later. This is important as during the course of the safe iteration, the stale element may be poisoned (unbeknownst to the compiler).
Eh? I thought any function call will stop the compiler being allowed to reload the value. The 'safe' loop iterators are only 'safe' against called code removing the current item from the list.
This helps prevent kcsan warnings over 'unsafe' conduct in releasing the list elements during list_for_each_entry_safe() and friends.
Sounds like kcsan is buggy ????
The warning kcsan gave made sense (a strange case where the emptying the list from inside the safe iterator would allow that list to be taken under a global mutex and have one extra request added to it. The list_for_each_entry_safe() should be ok in this scenario, so long as the next element is read before this element is dropped, and the compiler is instructed not to reload the element.
Normally the loop iteration code has to hold the mutex. I guess it can be released inside the loop provided no other code can ever delete entries.
kcsan is a little more insistent on having that annotation :)
In this instance I would say it was a false positive from kcsan, but I can see why it would complain and suspect that given a sufficiently aggressive compiler, we may be caught out by a late reload of the next element.
If you have: for (; p; p = next) { next = p->next; external_function_call(void); } the compiler must assume that the function call can change 'p->next' and read it before the call.
Is this a list with strange locking rules? The only deletes are from within the loop. Adds and deletes are locked. The list traversal isn't locked.
I suspect kcsan bleats because it doesn't assume the compiler will use a single instruction/memory operation to read p->next. That is just stupid.
David
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