FUTEX_LOCK_PI2 is a new futex operation which was recently introduced into the Linux kernel. It works exactly like FUTEX_LOCK_PI. However, it has support for selectable clocks for timeouts. By default CLOCK_MONOTONIC is used. If FUTEX_CLOCK_REALTIME is specified then the timeout is measured against CLOCK_REALTIME. This new operation addresses an inconsistency in the futex interface: FUTEX_LOCK_PI only works with timeouts based on CLOCK_REALTIME in contrast to all the other PI operations. Document the FUTEX_LOCK_PI2 command. Signed-off-by: Kurt Kanzenbach <kurt@xxxxxxxxxxxxx> --- Hi, here's the context to this: * https://lkml.kernel.org/lkml/20210422194417.866740847@xxxxxxxxxxxxx/ * https://sourceware.org/pipermail/libc-alpha/2021-June/128073.html Thanks, Kurt man2/futex.2 | 53 ++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 49 insertions(+), 4 deletions(-) diff --git a/man2/futex.2 b/man2/futex.2 index ada96c517ddb..2f340e0e0aad 100644 --- a/man2/futex.2 +++ b/man2/futex.2 @@ -241,10 +241,13 @@ and so on. This option bit can be employed only with the .BR FUTEX_WAIT_BITSET , .BR FUTEX_WAIT_REQUEUE_PI , -and (since Linux 4.5) .\" commit 337f13046ff03717a9e99675284a817527440a49 -.BR FUTEX_WAIT +.BR FUTEX_WAIT , +and +(since Linux v5.14.0) +.\" commit bf22a6976897977b0a3f1aeba6823c959fc4fdae +.BR FUTEX_LOCK_PI2 operations. .IP If this option is set, the kernel measures the @@ -904,7 +907,9 @@ value to 0 if the previous value was the expected TID. If a futex is already acquired (i.e., has a nonzero value), waiters must employ the .B FUTEX_LOCK_PI -operation to acquire the lock. +or +.B FUTEX_LOCK_PI2 +operations to acquire the lock. If other threads are waiting for the lock, then the .B FUTEX_WAITERS bit is set in the futex value; @@ -964,6 +969,8 @@ Note that the PI futex operations must be used as paired operations and are subject to some additional requirements: .IP * 3 .B FUTEX_LOCK_PI +, +.B FUTEX_LOCK_PI2 and .B FUTEX_TRYLOCK_PI pair with @@ -1122,6 +1129,27 @@ arguments are ignored. .\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" .\" .TP +.BR FUTEX_LOCK_PI2 " (since Linux 5.14.0)" +.\" commit bf22a6976897977b0a3f1aeba6823c959fc4fdae +This operation works similar like +.BR FUTEX_LOCK_PI . +The only difference is the +timeout argument. +.BR FUTEX_LOCK_PI2 +has support for selectable clocks. +.IP +If +.I timeout +is not NULL, the structure it points to specifies +an absolute timeout. +If +.I timeout +is NULL, the operation can block indefinitely. +.IP +.\" +.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" +.\" +.TP .BR FUTEX_TRYLOCK_PI " (since Linux 2.6.18)" .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation tries to acquire the lock at @@ -1168,6 +1196,8 @@ arguments are ignored. .\" commit c87e2837be82df479a6bae9f155c43516d2feebc This operation wakes the top priority waiter that is waiting in .B FUTEX_LOCK_PI +or +.B FUTEX_LOCK_PI2 on the futex address provided by the .I uaddr argument. @@ -1379,6 +1409,9 @@ Returns the number of waiters that were woken up. .B FUTEX_LOCK_PI Returns 0 if the futex was successfully locked. .TP +.B FUTEX_LOCK_PI2 +Returns 0 if the futex was successfully locked. +.TP .B FUTEX_TRYLOCK_PI Returns 0 if the futex was successfully locked. .TP @@ -1435,6 +1468,7 @@ is not equal to the expected value .TP .BR EAGAIN .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex owner thread ID of @@ -1448,6 +1482,7 @@ Try again. .TP .BR EDEADLK .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The futex word at @@ -1536,11 +1571,14 @@ The kernel detected an inconsistency between the user-space state at .I uaddr and the kernel state\(emthat is, it detected a waiter which waits in .BR FUTEX_LOCK_PI +or +.BR FUTEX_LOCK_PI2 on .IR uaddr . .TP .B EINVAL .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI ) The kernel detected an inconsistency between the user-space state at @@ -1590,6 +1628,8 @@ that is, the kernel detected a waiter which waits on .I uaddr via .BR FUTEX_LOCK_PI +or +.BR FUTEX_LOCK_PI2 (instead of .BR FUTEX_WAIT_REQUEUE_PI ). .TP @@ -1618,6 +1658,7 @@ The system-wide limit on the total number of open files has been reached. .TP .BR ENOMEM .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The kernel could not allocate memory to hold state information. @@ -1634,11 +1675,13 @@ option was specified in but the accompanying operation was neither .BR FUTEX_WAIT , .BR FUTEX_WAIT_BITSET , +.BR FUTEX_WAIT_REQUEUE_PI , nor -.BR FUTEX_WAIT_REQUEUE_PI . +.BR FUTEX_LOCK_PI2 . .TP .BR ENOSYS .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_UNLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI , @@ -1649,6 +1692,7 @@ are not supported on some CPU variants. .TP .BR EPERM .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The caller is not allowed to attach itself to the futex at @@ -1665,6 +1709,7 @@ The caller does not own the lock represented by the futex word. .TP .BR ESRCH .RB ( FUTEX_LOCK_PI , +.BR FUTEX_LOCK_PI2 , .BR FUTEX_TRYLOCK_PI , .BR FUTEX_CMP_REQUEUE_PI ) The thread ID in the futex word at -- 2.30.2
