i386/61852: i386 pmap SMP race condition can cause lost page modification.

[Stephan Uphoff]

>Number:         61852
>Category:       i386
>Synopsis:       i386 pmap SMP race condition can cause lost page modification.
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-i386
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Jan 24 15:40:08 PST 2004
>Closed-Date:
>Last-Modified:
>Originator:     Stephan Uphoff
>Release:        current
>Organization:
>Environment:
N/A 
>Description:
The current i386 pmap does not always atomically change ptes.
( Only clearing a pte is currently atomic)
This causes a window where setting of PG_A or PG_M of the pte
caused by page access of a second CPU can be ignored or overwritten 
and lost.

Lost PG_M bits can cause data loss. 
(Page not written to backing storage)


>How-To-Repeat:
      
>Fix:
Import the pte update logic from the newest NetBSD pmap.
The NetBSD pmap also tries to avoid unnecessary TLB shootdowns.

>Release-Note:
>Audit-Trail:
>Unformatted: