Jiri Slaby reported a futex state inconsistency resulting in -EINVAL during
a lock operation for a PI futex. It requires that the a lock process is
interrupted by a timeout or signal:
T1 Owns the futex in user space.
T2 Tries to acquire the futex in kernel (futex_lock_pi()). Allocates a
pi_state and attaches itself to it.
T2 Times out and removes its rt_waiter from the rt_mutex. Drops the
rtmutex lock and tries to acquire the hash bucket lock to remove
the futex_q. The lock is contended and T2 schedules out.
T1 Unlocks the futex (futex_unlock_pi()). Finds a futex_q but no
rt_waiter. Unlocks the futex (do_uncontended) and makes it available
to user space.
T3 Acquires the futex in user space.
T4 Tries to acquire the futex in kernel (futex_lock_pi()). Finds the
existing futex_q of T2 and tries to attach itself to the existing
pi_state. This (attach_to_pi_state()) fails with -EINVAL because uval
contains the TID of T3 but pi_state points to T1.
It's incorrect to unlock the futex and make it available for user space to
acquire as long as there is still an existing state attached to it in the
kernel.
T1 cannot hand over the futex to T2 because T2 already gave up and started
to clean up and is blocked on the hash bucket lock, so T2's futex_q with
the pi_state pointing to T1 is still queued.
T2 observes the futex_q, but ignores it as there is no waiter on the
corresponding rt_mutex and takes the uncontended path which allows the
subsequent caller of futex_lock_pi() (T4) to observe that stale state.
To prevent this the unlock path must dequeue all futex_q entries which
point to the same pi_state when there is no waiter on the rt mutex. This
requires obviously to make the dequeue conditional in the locking path to
prevent a double dequeue. With that it's guaranteed that user space cannot
observe an uncontended futex which has kernel state attached.
Fixes: fbeb558b0d ("futex/pi: Fix recursive rt_mutex waiter state")
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20240118115451.0TkD_ZhB@linutronix.de
Closes: https://lore.kernel.org/all/4611bcf2-44d0-4c34-9b84-17406f881003@kernel.org
Pull io_uring futex support from Jens Axboe:
"This adds support for using futexes through io_uring - first futex
wake and wait, and then the vectored variant of waiting, futex waitv.
For both wait/wake/waitv, we support the bitset variant, as the
'normal' variants can be easily implemented on top of that.
PI and requeue are not supported through io_uring, just the above
mentioned parts. This may change in the future, but in the spirit of
keeping this small (and based on what people have been asking for),
this is what we currently have.
Wake support is pretty straight forward, most of the thought has gone
into the wait side to avoid needing to offload wait operations to a
blocking context. Instead, we rely on the usual callbacks to retry and
post a completion event, when appropriate.
As far as I can recall, the first request for futex support with
io_uring came from Andres Freund, working on postgres. His aio rework
of postgres was one of the early adopters of io_uring, and futex
support was a natural extension for that. This is relevant from both a
usability point of view, as well as for effiency and performance. In
Andres's words, for the former:
Futex wait support in io_uring makes it a lot easier to avoid
deadlocks in concurrent programs that have their own buffer pool:
Obviously pages in the application buffer pool have to be locked
during IO. If the initiator of IO A needs to wait for a held lock
B, the holder of lock B might wait for the IO A to complete. The
ability to wait for a lock and IO completions at the same time
provides an efficient way to avoid such deadlocks
and in terms of effiency, even without unlocking the full potential
yet, Andres says:
Futex wake support in io_uring is useful because it allows for more
efficient directed wakeups. For some "locks" postgres has queues
implemented in userspace, with wakeup logic that cannot easily be
implemented with FUTEX_WAKE_BITSET on a single "futex word"
(imagine waiting for journal flushes to have completed up to a
certain point).
Thus a "lock release" sometimes need to wake up many processes in a
row. A quick-and-dirty conversion to doing these wakeups via
io_uring lead to a 3% throughput increase, with 12% fewer context
switches, albeit in a fairly extreme workload"
* tag 'io_uring-futex-2023-10-30' of git://git.kernel.dk/linux:
io_uring: add support for vectored futex waits
futex: make the vectored futex operations available
futex: make futex_parse_waitv() available as a helper
futex: add wake_data to struct futex_q
io_uring: add support for futex wake and wait
futex: abstract out a __futex_wake_mark() helper
futex: factor out the futex wake handling
futex: move FUTEX2_VALID_MASK to futex.h
On no-MMU, all futexes are treated as private because there is no need
to map a virtual address to physical to match the futex across
processes. This doesn't quite work though, because private futexes
include the current process's mm_struct as part of their key. This makes
it impossible for one process to wake up a shared futex being waited on
in another process.
Fix this bug by excluding the mm_struct from the key. With
a single address space, the futex address is already a unique key.
Fixes: 784bdf3bb6 ("futex: Assume all mappings are private on !MMU systems")
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20231019204548.1236437-2-ben.wolsieffer@hefring.com
Rename unqueue_multiple() as futex_unqueue_multiple(), and make both
that and futex_wait_multiple_setup() available for external users. This
is in preparation for wiring up vectored waits in io_uring.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
To make it more generically useful, augment it with allowing the caller
to pass in the wake handler and wake data. Convert the futex_waitv()
syscall, passing in the default handlers.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
With handling multiple futex_q for waitv, we cannot easily go from the
futex_q to data related to that request or queue. Add a wake_data
argument that belongs to the wake handler assigned.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Move the unqueue and lock_ptr clear into a helper that futex_wake_mark()
calls. Add it to the public functions as well, in preparation for using
it outside the core futex code.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
In preparation for having another waker that isn't futex_wake_mark(),
add a wake handler in futex_q. No extra data is associated with the
handler outside of struct futex_q itself. futex_wake_mark() is defined as
the standard wakeup helper, now set through futex_q_init like other
defaults.
Normal sync futex waiting relies on wake_q holding tasks that should
be woken up. This is what futex_wake_mark() does, it'll unqueue the
futex and add the associated task to the wake queue. For async usage of
futex waiting, rather than having tasks sleeping on the futex, we'll
need to deal with a futex wake differently. For the planned io_uring
case, that means posting a completion event for the task in question.
Having a definable wake handler can help support that use case.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
We need this for validating the futex2 flags outside of the normal
futex syscalls.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Finish off the 'simple' futex2 syscall group by adding
sys_futex_requeue(). Unlike sys_futex_{wait,wake}() its arguments are
too numerous to fit into a regular syscall. As such, use struct
futex_waitv to pass the 'source' and 'destination' futexes to the
syscall.
This syscall implements what was previously known as FUTEX_CMP_REQUEUE
and uses {val, uaddr, flags} for source and {uaddr, flags} for
destination.
This design explicitly allows requeueing between different types of
futex by having a different flags word per uaddr.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230921105248.511860556@noisy.programming.kicks-ass.net
To complement sys_futex_waitv()/wake(), add sys_futex_wait(). This
syscall implements what was previously known as FUTEX_WAIT_BITSET
except it uses 'unsigned long' for the value and bitmask arguments,
takes timespec and clockid_t arguments for the absolute timeout and
uses FUTEX2 flags.
The 'unsigned long' allows FUTEX2_SIZE_U64 on 64bit platforms.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230921105248.164324363@noisy.programming.kicks-ass.net
The current semantics for futex_wake() are a bit loose, specifically
asking for 0 futexes to be woken actually gets you 1.
Adding a !nr check to sys_futex_wake() makes that it would return 0
for unaligned futex words, because that check comes in the shared
futex_wake() function. Adding the !nr check there, would affect the
legacy sys_futex() semantics.
Hence frob a flag :-(
Suggested-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230921105248.048643656@noisy.programming.kicks-ass.net
sys_futex_waitv() is part of the futex2 series (the first and only so
far) of syscalls and has a flags field per futex (as opposed to flags
being encoded in the futex op).
This new flags field has a new namespace, which unfortunately isn't
super explicit. Notably it currently takes FUTEX_32 and
FUTEX_PRIVATE_FLAG.
Introduce the FUTEX2 namespace to clarify this
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20230921105247.507327749@noisy.programming.kicks-ass.net
Some new assertions pointed out that the existing code has nested rt_mutex wait
state in the futex code.
Specifically, the futex_lock_pi() cancel case uses spin_lock() while there
still is a rt_waiter enqueued for this task, resulting in a state where there
are two waiters for the same task (and task_struct::pi_blocked_on gets
scrambled).
The reason to take hb->lock at this point is to avoid the wake_futex_pi()
EAGAIN case.
This happens when futex_top_waiter() and rt_mutex_top_waiter() state becomes
inconsistent. The current rules are such that this inconsistency will not be
observed.
Notably the case that needs to be avoided is where futex_lock_pi() and
futex_unlock_pi() interleave such that unlock will fail to observe a new
waiter.
*However* the case at hand is where a waiter is leaving, in this case the race
means a waiter that is going away is not observed -- which is harmless,
provided this race is explicitly handled.
This is a somewhat dangerous proposition because the converse race is not
observing a new waiter, which must absolutely not happen. But since the race is
valid this cannot be asserted.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20230915151943.GD6743@noisy.programming.kicks-ass.net
