CVS commit: src/sys/uvm

["Chuck Silvers" <chs%netbsd.org@localhost>]

Module Name:    src
Committed By:   chs
Date:           Mon May 15 01:42:42 UTC 2023

Modified Files:
        src/sys/uvm: uvm_map.c

Log Message:
uvm: avoid a deadlock in uvm_map_clean()

The locking order between map locks and page "busy" locks
is that the page "busy" lock comes first, but uvm_map_clean()
breaks this rule by holding a map locked (as reader) while
waiting for page "busy" locks.

If another thread is in the page-fault path holding a page
"busy" lock while waiting for the map lock (as a reader)
and at the same time a third thread is blocked waiting for
the map lock as a writer (which blocks the page-fault thread),
then these three threads will all deadlock with each other.

Fix this by marking the map "busy" (to block any modifications)
and unlocking the map lock before possibly waiting for any
page "busy" locks.

Martin Pieuchot reported that the same problem existed in OpenBSD
he applied this fix there after several people tested it.

fixes PR 56952


To generate a diff of this commit:
cvs rdiff -u -r1.405 -r1.406 src/sys/uvm/uvm_map.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.