From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
Rik van Riel <riel@surriel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
the arch/x86 maintainers <x86@kernel.org>,
"Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms
Date: Thu, 17 Jun 2021 11:28:30 +0200 [thread overview]
Message-ID: <YMsVvsMRJ2yKf1WM@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YMsQ82bzly2KAUsu@hirez.programming.kicks-ass.net>
On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote:
> diff --git a/kernel/fork.c b/kernel/fork.c
> index e595e77913eb..57415cca088c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1104,6 +1104,8 @@ static inline void __mmput(struct mm_struct *mm)
> }
> if (mm->binfmt)
> module_put(mm->binfmt->module);
> +
> + mm_unlazy_mm_count(mm);
> mmdrop(mm);
> }
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8ac693d542f6..e102ec53c2f6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -19,6 +19,7 @@
> +/*
> + * This converts all lazy_mm references to mm to mm_count refcounts. Our
> + * caller holds an mm_count reference, so we don't need to worry about mm
> + * being freed out from under us.
> + */
> +void mm_unlazy_mm_count(struct mm_struct *mm)
> +{
> + unsigned int drop_count = num_possible_cpus();
> + int cpu;
> +
> + /*
> + * mm_users is zero, so no cpu will set its rq->lazy_mm to mm.
> + */
> + WARN_ON_ONCE(atomic_read(&mm->mm_users) != 0);
> +
> + /* Grab enough references for the rest of this function. */
> + atomic_add(drop_count, &mm->mm_count);
So that had me puzzled for a little while. Would something like this be
a better comment?
/*
* Because this can race with mmdrop_lazy(), mm_count must be
* incremented before setting any rq->drop_mm value, otherwise
* it is possible to free mm early.
*/
> +
> + for_each_possible_lazymm_cpu(cpu, mm) {
> + struct rq *rq = cpu_rq(cpu);
> + struct mm_struct *old_mm;
> +
> + if (smp_load_acquire(&rq->lazy_mm) != mm)
> + continue;
> +
> + drop_count--; /* grab a reference; cpu will drop it later. */
Totally confusing comment that :-)
> +
And with that, we rely on xchg() here to be at at least RELEASE, such
that that mm_count increment must be visible when drop_mm is seen.
> + old_mm = xchg(&rq->drop_mm, mm);
Similarly, we rely on it being at least ACQUIRE for the !NULL return
case I think.
> +
> + /*
> + * We know that old_mm != mm: when we did the xchg(), we were
> + * the only cpu to be putting mm into any drop_mm variable.
> + */
> + WARN_ON_ONCE(old_mm == mm);
> + if (unlikely(old_mm)) {
> + /*
> + * We just stole an mm reference from the target CPU.
> + *
> + * drop_mm was set to old by another call to
> + * mm_unlazy_mm_count(). After that call xchg'd old
> + * into drop_mm, the target CPU did:
> + *
> + * smp_store_release(&rq->lazy_mm, mm);
> + *
> + * which synchronized with our smp_load_acquire()
> + * above, so we know that the target CPU is done with
> + * old. Drop old on its behalf.
> + */
> + mmdrop(old_mm);
> + }
> + }
> +
> + atomic_sub(drop_count, &mm->mm_count);
> +}
next prev parent reply other threads:[~2021-06-17 9:29 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-16 3:21 [PATCH 0/8] membarrier cleanups Andy Lutomirski
2021-06-16 3:21 ` [PATCH 1/8] membarrier: Document why membarrier() works Andy Lutomirski
2021-06-16 4:00 ` Nicholas Piggin
2021-06-16 7:30 ` Peter Zijlstra
2021-06-17 23:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2021-06-16 4:25 ` Nicholas Piggin
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 17:49 ` Mathieu Desnoyers
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 3/8] membarrier: Remove membarrier_arch_switch_mm() prototype in core code Andy Lutomirski
2021-06-16 4:26 ` Nicholas Piggin
2021-06-16 17:52 ` Mathieu Desnoyers
2021-06-16 3:21 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Andy Lutomirski
2021-06-16 4:19 ` Nicholas Piggin
2021-06-16 7:35 ` Peter Zijlstra
2021-06-16 18:41 ` Andy Lutomirski
2021-06-17 1:37 ` Nicholas Piggin
2021-06-17 2:57 ` Andy Lutomirski
2021-06-17 5:32 ` Andy Lutomirski
2021-06-17 6:51 ` Nicholas Piggin
2021-06-17 23:49 ` Andy Lutomirski
2021-06-19 2:53 ` Nicholas Piggin
2021-06-19 3:20 ` Andy Lutomirski
2021-06-19 4:27 ` Nicholas Piggin
2021-06-17 9:08 ` [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms Peter Zijlstra
2021-06-17 9:10 ` Peter Zijlstra
2021-06-17 10:00 ` Nicholas Piggin
2021-06-17 9:13 ` Peter Zijlstra
2021-06-17 14:06 ` Andy Lutomirski
2021-06-17 9:28 ` Peter Zijlstra [this message]
2021-06-17 14:03 ` Andy Lutomirski
2021-06-17 14:10 ` Andy Lutomirski
2021-06-17 15:45 ` Peter Zijlstra
2021-06-18 3:29 ` Paul E. McKenney
2021-06-18 5:04 ` Andy Lutomirski
2021-06-17 15:02 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Paul E. McKenney
2021-06-18 0:06 ` Andy Lutomirski
2021-06-18 3:35 ` Paul E. McKenney
2021-06-17 8:45 ` Peter Zijlstra
2021-06-16 3:21 ` [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm Andy Lutomirski
2021-06-16 4:28 ` Nicholas Piggin
2021-06-16 18:08 ` Mathieu Desnoyers
2021-06-16 18:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2021-06-16 4:36 ` Nicholas Piggin
2021-06-16 3:21 ` [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE Andy Lutomirski
2021-06-16 9:28 ` Russell King (Oracle)
2021-06-16 10:16 ` Peter Zijlstra
2021-06-16 10:20 ` Peter Zijlstra
2021-06-16 10:34 ` Russell King (Oracle)
2021-06-16 11:10 ` Peter Zijlstra
2021-06-16 13:22 ` Russell King (Oracle)
2021-06-16 15:04 ` Catalin Marinas
2021-06-16 15:23 ` Russell King (Oracle)
2021-06-16 15:45 ` Catalin Marinas
2021-06-16 16:00 ` Catalin Marinas
2021-06-16 16:27 ` Russell King (Oracle)
2021-06-17 8:55 ` Krzysztof Hałasa
2021-06-18 12:54 ` Linus Walleij
2021-06-18 13:19 ` Russell King (Oracle)
2021-06-18 13:36 ` Arnd Bergmann
2021-06-17 10:40 ` Mark Rutland
2021-06-17 11:23 ` Russell King (Oracle)
2021-06-17 11:33 ` Mark Rutland
2021-06-17 13:41 ` Andy Lutomirski
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski
2021-06-17 14:20 ` Mark Rutland
2021-06-17 15:01 ` Peter Zijlstra
2021-06-17 15:13 ` Peter Zijlstra
2021-06-17 14:16 ` Mathieu Desnoyers
2021-06-17 14:05 ` Peter Zijlstra
2021-06-18 0:07 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
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