Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during context switch

[Kirill A. Shutemov]

On Thu, Mar 07, 2024 at 01:39:14PM +0000, Yosry Ahmed wrote:
> In switch_mm_irqs_off(), we read the 'mm->context.lam_cr3_mask' into
> 'new_lam', which is later passed to load_new_mm_cr3(). However, there is
> a call to set_tlbstate_lam_mode() in between which will read
> 'mm->context.lam_cr3_mask' again and set 'cpu_tlbstate.lam' accordingly.
> If we race with another thread updating 'mm->context.lam_cr3_mask', the
> value in 'cpu_tlbstate.lam' could end up being different from CR3.

What other thread? LAM can only be enabled when the process has single
thread. And cannot be disabled. See MM_CONTEXT_LOCK_LAM.

> While we are at it, remove the misguiding comment that states that
> 'new_lam' may not match tlbstate_lam_cr3_mask() if a race occurs.

The comment is indeed misguiding, but for different reason. It is leftover
from the earlier version of LAM patchset.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov