linux-kernel - [tip:x86/mm] x86/mm: Don't reenter flush_tlb_func

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Message-ID: <tip-bc0d5a89fbe3c83ac45438d7ba88309f4713615d@git.kernel.org>
Date:   Fri, 30 Jun 2017 06:11:22 -0700
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     luto@...nel.org, dave.hansen@...el.com, riel@...hat.com,
        linux-kernel@...r.kernel.org, nadav.amit@...il.com,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        alexander.levin@...izon.com, akpm@...ux-foundation.org,
        mingo@...nel.org, arjan@...ux.intel.com, bp@...en8.de,
        mgorman@...e.de, hpa@...or.com, peterz@...radead.org
Subject: [tip:x86/mm] x86/mm: Don't reenter flush_tlb_func_common()

Commit-ID:  bc0d5a89fbe3c83ac45438d7ba88309f4713615d
Gitweb:     http://git.kernel.org/tip/bc0d5a89fbe3c83ac45438d7ba88309f4713615d
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 29 Jun 2017 08:53:13 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 30 Jun 2017 10:12:35 +0200

x86/mm: Don't reenter flush_tlb_func_common()

It was historically possible to have two concurrent TLB flushes
targetting the same CPU: one initiated locally and one initiated
remotely.  This can now cause an OOPS in leave_mm() at
arch/x86/mm/tlb.c:47:

        if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
                BUG();

with this call trace:
 flush_tlb_func_local arch/x86/mm/tlb.c:239 [inline]
 flush_tlb_mm_range+0x26d/0x370 arch/x86/mm/tlb.c:317

Without reentrancy, this OOPS is impossible: leave_mm() is only
called if we're not in TLBSTATE_OK, but then we're unexpectedly
in TLBSTATE_OK in leave_mm().

This can be caused by flush_tlb_func_remote() happening between
the two checks and calling leave_mm(), resulting in two consecutive
leave_mm() calls on the same CPU with no intervening switch_mm()
calls.

We never saw this OOPS before because the old leave_mm()
implementation didn't put us back in TLBSTATE_OK, so the assertion
didn't fire.

Nadav noticed the reentrancy issue in a different context, but
neither of us realized that it caused a problem yet.

Reported-by: Levin, Alexander (Sasha Levin) <alexander.levin@...izon.com>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Reviewed-by: Nadav Amit <nadav.amit@...il.com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: linux-mm@...ck.org
Fixes: 3d28ebceaffa ("x86/mm: Rework lazy TLB to track the actual loaded mm")
Link: http://lkml.kernel.org/r/855acf733268d521c9f2e191faee2dcc23a29729.1498751203.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/mm/tlb.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index b2485d6..1cc4783 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -192,6 +192,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 static void flush_tlb_func_common(const struct flush_tlb_info *f,
 				  bool local, enum tlb_flush_reason reason)
 {
+	/* This code cannot presently handle being reentered. */
+	VM_WARN_ON(!irqs_disabled());
+
 	if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
 		leave_mm(smp_processor_id());
 		return;
@@ -297,8 +300,13 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 		info.end = TLB_FLUSH_ALL;
 	}
 
-	if (mm == this_cpu_read(cpu_tlbstate.loaded_mm))
+	if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
+		VM_WARN_ON(irqs_disabled());
+		local_irq_disable();
 		flush_tlb_func_local(&info, TLB_LOCAL_MM_SHOOTDOWN);
+		local_irq_enable();
+	}
+
 	if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
 		flush_tlb_others(mm_cpumask(mm), &info);
 	put_cpu();
@@ -354,8 +362,13 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 
 	int cpu = get_cpu();
 
-	if (cpumask_test_cpu(cpu, &batch->cpumask))
+	if (cpumask_test_cpu(cpu, &batch->cpumask)) {
+		VM_WARN_ON(irqs_disabled());
+		local_irq_disable();
 		flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
+		local_irq_enable();
+	}
+
 	if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids)
 		flush_tlb_others(&batch->cpumask, &info);
 	cpumask_clear(&batch->cpumask);