futex: Implement mechanism to wait on any of several futexes

This is a new futex operation, called FUTEX_WAIT_MULTIPLE, which allows
a thread to wait on several futexes at the same time, and be awoken by
any of them.  In a sense, it implements one of the features that was
supported by pooling on the old FUTEX_FD interface.

The use case lies in the Wine implementation of the Windows NT interface
WaitMultipleObjects. This Windows API function allows a thread to sleep
waiting on the first of a set of event sources (mutexes, timers, signal,
console input, etc) to signal.  Considering this is a primitive
synchronization operation for Windows applications, being able to quickly
signal events on the producer side, and quickly go to sleep on the
consumer side is essential for good performance of those running over Wine.

Wine developers have an implementation that uses eventfd, but it suffers
from FD exhaustion (there is applications that go to the order of
multi-milion FDs), and higher CPU utilization than this new operation.

The futex list is passed as an array of `struct futex_wait_block`
(pointer, value, bitset) to the kernel, which will enqueue all of them
and sleep if none was already triggered. It returns a hint of which
futex caused the wake up event to userspace, but the hint doesn't
guarantee that is the only futex triggered.  Before calling the syscall
again, userspace should traverse the list, trying to re-acquire any of
the other futexes, to prevent an immediate -EWOULDBLOCK return code from
the kernel.

This was tested using three mechanisms:

1) By reimplementing FUTEX_WAIT in terms of FUTEX_WAIT_MULTIPLE and
running the unmodified tools/testing/selftests/futex and a full linux
distro on top of this kernel.

2) By an example code that exercises the FUTEX_WAIT_MULTIPLE path on a
multi-threaded, event-handling setup.

3) By running the Wine fsync implementation and executing multi-threaded
applications, in particular modern games, on top of this implementation.

Changes were tested for the following ABIs: x86_64, i386 and x32.
Support for x32 applications is not implemented since it would
take a major rework adding a new entry point and splitting the current
futex 64 entry point in two and we can't change the current x32 syscall
number without breaking user space compatibility.

CC: Steven Rostedt <[email protected]>
Cc: Richard Yao <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Co-developed-by: Zebediah Figura <[email protected]>
Signed-off-by: Zebediah Figura <[email protected]>
Co-developed-by: Steven Noonan <[email protected]>
Signed-off-by: Steven Noonan <[email protected]>
Co-developed-by: Pierre-Loup A. Griffais <[email protected]>
Signed-off-by: Pierre-Loup A. Griffais <[email protected]>
Signed-off-by: Gabriel Krisman Bertazi <[email protected]>
[Added compatibility code]
Co-developed-by: André Almeida <[email protected]>
Signed-off-by: André Almeida <[email protected]>

Adjusted for v5.9: Removed `put_futex_key` calls.

Signed-off-by: Simão Gomes Viana <[email protected]>

Author: krisman-at-collabora

include/uapi/linux/futex.h

@@ -21,6 +21,7 @@
 #define FUTEX_WAKE_BITSET	10
 #define FUTEX_WAIT_REQUEUE_PI	11
 #define FUTEX_CMP_REQUEUE_PI	12
+#define FUTEX_WAIT_MULTIPLE	13
 
 #define FUTEX_PRIVATE_FLAG	128
 #define FUTEX_CLOCK_REALTIME	256
@@ -40,6 +41,8 @@
 					 FUTEX_PRIVATE_FLAG)
 #define FUTEX_CMP_REQUEUE_PI_PRIVATE	(FUTEX_CMP_REQUEUE_PI | \
 					 FUTEX_PRIVATE_FLAG)
+#define FUTEX_WAIT_MULTIPLE_PRIVATE	(FUTEX_WAIT_MULTIPLE | \
+					 FUTEX_PRIVATE_FLAG)
 
 /*
  * Support for robust futexes: the kernel cleans up held futexes at
@@ -150,4 +153,21 @@ struct robust_list_head {
   (((op & 0xf) << 28) | ((cmp & 0xf) << 24)		\
    | ((oparg & 0xfff) << 12) | (cmparg & 0xfff))
 
+/*
+ * Maximum number of multiple futexes to wait for
+ */
+#define FUTEX_MULTIPLE_MAX_COUNT	128
+
+/**
+ * struct futex_wait_block - Block of futexes to be waited for
+ * @uaddr:	User address of the futex
+ * @val:	Futex value expected by userspace
+ * @bitset:	Bitset for the optional bitmasked wakeup
+ */
+struct futex_wait_block {
+	__u32 __user *uaddr;
+	__u32 val;
+	__u32 bitset;
+};
+
 #endif /* _UAPI_LINUX_FUTEX_H */