From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 469DB36B061; Tue, 18 Aug 2026 10:47:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787050034; cv=none; b=pTaVTP9Gom6c+y+zja8eOkmx0OwVAnQ3qkuxr6co5HphUyxK/vM3Yc9Ex8L7rsxB3x7WcZujCLwhe+Uq5o41g3ifppCFX6m9Kd0CukKYsgXd+duTb1G2hQlVAotdvfEm3Dmm5iuLiuHal1naHLa9VPUc6u1Cze0KCJA+bu8ueLY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787050034; c=relaxed/simple; bh=6MsFbhKcH9QAeiYu0dpBrpLzmfyAkOPfNFgpUEpZOjs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=G4456wK8ulKf97AB34yDpqMZz0eNGeG+Rq5EGKozkIqrVEed2FUkHLb8r8YhRRuek9Q9icKnXWgKFdLdnyLYNN1cAwf+/BmohR0tlyx1w2aLoT/kbZgEmi56zLo3ykyryqIz0wQogfiT5CpbJ55T6+6cCbpeBrb6psF+O4ZEx5o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=pass smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=mOE2WN4L; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="mOE2WN4L" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=JNAsElfwryduquti2VexKKz89P5skCcekcGs3NCJUQE=; b=mOE2WN4L1bxbLwWeGiUtYKcLVT 5DyQSFzjzBK7L6spQUF9m7DpZeB2rbANREZaVV7EauOTP/YiSxWXglSNw3YBJangJYS28UGASuzp5 dC52u0zYTn+MjYQ6deFP9ZPgm/IcF99qpC7Oo5YZIw7YlAHfUrspC/OrVL0CCugnbFQzAEQjgLnde b7zKKLuZYQpLKA7QOK4Ueirb5I8LRKLr/cuqlbxccF15E7Xxe/orEZjRNQ/XD5NAR9IP5FONrZkLv eKtfUshbBUpJ2l953vgtEIrwTAjoLTZQq8mS0eTy2fKf6fWZfVflPjNIS2SKRohRJarpvv3IzNMC3 fCq7Bgkg==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wwHLP-0000000BhlW-2quO; Tue, 18 Aug 2026 10:46:59 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 6DC92300242; Tue, 18 Aug 2026 12:46:58 +0200 (CEST) Date: Tue, 18 Aug 2026 12:46:58 +0200 From: Peter Zijlstra To: Yao Kai Cc: syzbot , syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner , andrealmeid@igalia.com, dave@stgolabs.net, dvhart@infradead.org, liuyongqiang13@huawei.com, syzbot@lists.linux.dev Subject: Re: [PATCH] futex: Fix might_sleep() warning in futex_pivot_pending() Message-ID: <20260818104658.GE1246887@noisy.programming.kicks-ass.net> References: <515ea00f-a081-4b9a-bcb3-f5517fd4e565@mail.kernel.org> <20260814133803.GA687043@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Aug 17, 2026 at 03:29:14PM +0800, Yao Kai wrote: > Thanks! I think there is still a lost-wakeup window: > > T1 T2 > > add_wait_queue() > /* not visible to T2 */ > futex_pivot_pending() > futex_ref_is_dead() = false > futex_ref_put() = true > wake_up_var() > waitqueue_active() = false > /* observes empty */ > return > wait_woken() > schedule() > > Since wake_up_var() uses a lockless waitqueue_active() check, I think > we need to order the waitqueue insertion before the first condition > check: > > add_wait_queue(__wq_head, &__wbq_entry.wq_entry); > > /* > * Pairs with the fully ordered refcount operation before wake_up_var(). > * Ensures either the waker sees this waiter or we see the dead refcount. > */ > smp_mb(); Well, add_wait_queue() has UNLOCK(&wq_head->lock) and futex_pivot_pending() has LOCK(&mmph->lock), giving an UNLOCK+LOCK consistency, which IIRC is RCtso if you're on PowerPC and RCsc everywhere else. So yeah, this needs more. But I would instead suggest we use: smp_mb__after_spinlock(). Anyway, for this to matter one way or the other, the other side of this also needs a barrier. But it looks like futex_ref_put() already implies enough. When in atomic mode it implies a full smp_mb(). > while (!futex_pivot_pending(mm)) > wait_woken(&__wbq_entry.wq_entry, TASK_UNINTERRUPTIBLE, > MAX_SCHEDULE_TIMEOUT); > > The rc check can be dropped because MAX_SCHEDULE_TIMEOUT does not expire. Indeed, I had realized this after sending :-) Something like so then? --- diff --git a/include/linux/wait.h b/include/linux/wait.h index dce055e6add3..7e215330199c 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -1228,6 +1228,7 @@ long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_en void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry); long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout); int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key); +int woken_wake_bit_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key); int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key); #define DEFINE_WAIT_FUNC(name, function) \ diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h index ace7379d627d..553d7b23e3ad 100644 --- a/include/linux/wait_bit.h +++ b/include/linux/wait_bit.h @@ -32,6 +32,7 @@ int out_of_line_wait_on_bit_timeout(unsigned long *word, int, wait_bit_action_f int out_of_line_wait_on_bit_lock(unsigned long *word, int, wait_bit_action_f *action, unsigned int mode); struct wait_queue_head *bit_waitqueue(unsigned long *word, int bit); extern void __init wait_bit_init(void); +extern struct wait_bit_key *__var_wake_key(struct wait_queue_entry *wq_entry, void *arg); int wake_bit_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key); diff --git a/kernel/futex/core.c b/kernel/futex/core.c index a7c2a6242718..bd9fb0b17ee6 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -46,6 +46,7 @@ #include #include #include +#include #include @@ -1886,11 +1887,34 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags) futex_hash_bucket_init(&fph->queues[i]); if (custom) { + struct wait_bit_queue_entry __wbq_entry; + struct wait_queue_head *__wq_head; + /* * Only let prctl() wait / retry; don't unduly delay clone(). */ again: - wait_var_event(mm, futex_pivot_pending(mm)); + __wq_head = __var_waitqueue(mm); + init_wait_var_entry(&__wbq_entry, mm, 0); + __wbq_entry.wq_entry.func = woken_wake_bit_function; + add_wait_queue(__wq_head, &__wbq_entry.wq_entry); + + /* + * add_wait_queue() futex_ref_put() + * MB (this) MB (implied) + * futex_pivot_pending() wake_up_var() + * waitqueue_active() + * + * Notably, it must not be possible to see + * !futex_pivot_pending() && !waitqueue_active(). + */ + smp_mb__after_spinlock(); + + while (!futex_pivot_pending(mm) && + wait_woken(&__wbq_entry.wq_entry, TASK_UNINTERRUPTIBLE, + MAX_SCHEDULE_TIMEOUT)) + /* empty */; + remove_wait_queue(__wq_head, &__wbq_entry.wq_entry); } scoped_guard(mutex, &mm->futex.phash.lock) { diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c index 20f27e2cf7ae..d033f600f48c 100644 --- a/kernel/sched/wait.c +++ b/kernel/sched/wait.c @@ -5,6 +5,7 @@ * (C) 2004 Nadia Yvette Chambers, Oracle */ #include "sched.h" +#include void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *key) { @@ -463,3 +464,17 @@ int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sy return default_wake_function(wq_entry, mode, sync, key); } EXPORT_SYMBOL(woken_wake_function); + +int woken_wake_bit_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *arg) +{ + struct wait_bit_key *key = __var_wake_key(wq_entry, arg); + if (!key) + return 0; + + /* Pairs with the smp_store_mb() in wait_woken(). */ + smp_mb(); /* C */ + wq_entry->flags |= WQ_FLAG_WOKEN; + + return default_wake_function(wq_entry, mode, sync, key); +} +EXPORT_SYMBOL(woken_wake_bit_function); diff --git a/kernel/sched/wait_bit.c b/kernel/sched/wait_bit.c index 1088d3b7012c..e8127e83a48f 100644 --- a/kernel/sched/wait_bit.c +++ b/kernel/sched/wait_bit.c @@ -167,9 +167,7 @@ wait_queue_head_t *__var_waitqueue(void *p) } EXPORT_SYMBOL(__var_waitqueue); -static int -var_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, - int sync, void *arg) +struct wait_bit_key *__var_wake_key(struct wait_queue_entry *wq_entry, void *arg) { struct wait_bit_key *key = arg; struct wait_bit_queue_entry *wbq_entry = @@ -177,6 +175,17 @@ var_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, if (wbq_entry->key.flags != key->flags || wbq_entry->key.bit_nr != key->bit_nr) + return NULL; + + return key; +} + +static int +var_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode, + int sync, void *arg) +{ + struct wait_bit_key *key = __var_wake_key(wq_entry, arg); + if (!key) return 0; return autoremove_wake_function(wq_entry, mode, sync, key);