The patch titled
futex: update futex_wait_setup() comments about locking
has been removed from the -mm tree. Its filename was
futex-update-futex_wait_setup-comments-about-locking.patch
This patch was dropped because it was merged into mainline or a subsystem tree
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: futex: update futex_wait_setup() comments about locking
From: Michel Lespinasse <walken@xxxxxxxxxx>
Reviving a cleanup I had done about a year ago as part of a larger
futex_set_wait proposal. Over the years, the locking of the hashed futex
queue got improved, so that some of the "rare but normal" race conditions
described in comments can't actually happen anymore.
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
Cc: Darren Hart <dvhltc@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/futex.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff -puN kernel/futex.c~futex-update-futex_wait_setup-comments-about-locking kernel/futex.c
--- a/kernel/futex.c~futex-update-futex_wait_setup-comments-about-locking
+++ a/kernel/futex.c
@@ -1783,13 +1783,14 @@ static int futex_wait_setup(u32 __user *
*
* The basic logical guarantee of a futex is that it blocks ONLY
* if cond(var) is known to be true at the time of blocking, for
- * any cond. If we queued after testing *uaddr, that would open
- * a race condition where we could block indefinitely with
+ * any cond. If we locked the hash-bucket after testing *uaddr, that
+ * would open a race condition where we could block indefinitely with
* cond(var) false, which would violate the guarantee.
*
- * A consequence is that futex_wait() can return zero and absorb
- * a wakeup when *uaddr != val on entry to the syscall. This is
- * rare, but normal.
+ * On the other hand, we insert q and release the hash-bucket only
+ * after testing *uaddr. This guarantees that futex_wait() will NOT
+ * absorb a wakeup if *uaddr does not match the desired values
+ * while the syscall executes.
*/
retry:
ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q->key);
_
Patches currently in -mm which might be from walken@xxxxxxxxxx are
origin.patch
linux-next.patch
mm-allow-gup-to-fail-instead-of-waiting-on-a-page.patch
mm-allow-gup-to-fail-instead-of-waiting-on-a-page-fix.patch
mm-remove-unused-testsetpagelocked-interface.patch
--
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
