Update of /cvsroot/linux-vax/kernel-2.4/Documentation/vm
In directory usw-pr-cvs1:/tmp/cvs-serv7449/vm
Modified Files:
locking
Log Message:
synch 2.4.15 commit 28
Index: locking
===================================================================
RCS file: /cvsroot/linux-vax/kernel-2.4/Documentation/vm/locking,v
retrieving revision 1.1.1.1
retrieving revision 1.2
diff -u -r1.1.1.1 -r1.2
--- locking 14 Jan 2001 20:04:26 -0000 1.1.1.1
+++ locking 9 Apr 2002 16:55:48 -0000 1.2
@@ -4,7 +4,7 @@
from different people about how locking and synchronization is done
in the Linux vm code.
-page_table_lock
+page_table_lock & mmap_sem
--------------------------------------
Page stealers pick processes out of the process pool and scan for
@@ -23,29 +23,23 @@
Any code that modifies the vmlist, or the vm_start/vm_end/
vm_flags:VM_LOCKED/vm_next of any vma *in the list* must prevent
-kswapd from looking at the chain. This does not include driver mmap()
-methods, for example, since the vma is still not in the list.
+kswapd from looking at the chain.
The rules are:
-1. To modify the vmlist (add/delete or change fields in an element),
-you must hold mmap_sem to guard against clones doing mmap/munmap/faults,
-(ie all vm system calls and faults), and from ptrace, swapin due to
-swap deletion etc.
-2. To modify the vmlist (add/delete or change fields in an element),
-you must also hold page_table_lock, to guard against page stealers
-scanning the list.
-3. To scan the vmlist (find_vma()), you must either
- a. grab mmap_sem, which should be done by all cases except
- page stealer.
-or
- b. grab page_table_lock, only done by page stealer.
-4. While holding the page_table_lock, you must be able to guarantee
-that no code path will lead to page stealing. A better guarantee is
-to claim non sleepability, which ensures that you are not sleeping
-for a lock, whose holder might in turn be doing page stealing.
+1. To scan the vmlist (look but don't touch) you must hold the
+ mmap_sem with read bias, i.e. down_read(&mm->mmap_sem)
+2. To modify the vmlist you need to hold the mmap_sem with
+ read&write bias, i.e. down_write(&mm->mmap_sem) *AND*
+ you need to take the page_table_lock.
+3. The swapper takes _just_ the page_table_lock, this is done
+ because the mmap_sem can be an extremely long lived lock
+ and the swapper just cannot sleep on that.
+4. The exception to this rule is expand_stack, which just
+ takes the read lock and the page_table_lock, this is ok
+ because it doesn't really modify fields anybody relies on.
5. You must be able to guarantee that while holding page_table_lock
-or page_table_lock of mm A, you will not try to get either lock
-for mm B.
+ or page_table_lock of mm A, you will not try to get either lock
+ for mm B.
The caveats are:
1. find_vma() makes use of, and updates, the mmap_cache pointer hint.
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