Re: [RFC 1/2] page-flags: Make page lock operation atomic

[Peter Zijlstra]

On Mon, Feb 11, 2019 at 12:53:53PM +0000, Chintan Pandya wrote:
> Currently, page lock operation is non-atomic. This is opening
> some scope for race condition. For ex, if 2 threads are accessing
> same page flags, it may happen that our desired thread's page
> lock bit (PG_locked) might get overwritten by other thread
> leaving page unlocked. This can cause issues later when some
> code expects page to be locked but it is not.
> 
> Make page lock/unlock operation use the atomic version of
> set_bit API. There are other flag set operations which still
> uses non-atomic version of set_bit API. Bit, that might be
> the change for the future.
> 
> Change-Id: I13bdbedc2b198af014d885e1925c93b83ed6660e

That doesn't belong in patches.

> Signed-off-by: Chintan Pandya <chintan.pandya@xxxxxxxxxxx>

NAK.

This is bound to regress some stuff. Now agreed that using non-atomic
ops is tricky, but many are in places where we 'know' there can't be
concurrency.

If you can show any single one is wrong, we can fix that one, but we're
not going to blanket remove all this just because.