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From:   Qian Cai <cai@redhat.com>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Oliver O'Halloran <oohall@gmail.com>,
        Russell Currey <ruscur@russell.cc>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: [PATCH] powerpc/eeh_cache: Fix a possible debugfs deadlock
Date:   Wed, 28 Oct 2020 11:27:17 -0400
Message-Id: <20201028152717.8967-1-cai@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Lockdep complains that a possible deadlock below in
eeh_addr_cache_show() because it is acquiring a lock with IRQ enabled,
but eeh_addr_cache_insert_dev() needs to acquire the same lock with IRQ
disabled. Let's just make eeh_addr_cache_show() acquire the lock with
IRQ disabled as well.

        CPU0                    CPU1
        ----                    ----
   lock(&pci_io_addr_cache_root.piar_lock);
                                local_irq_disable();
                                lock(&tp->lock);
                                lock(&pci_io_addr_cache_root.piar_lock);
   <Interrupt>
     lock(&tp->lock);

  *** DEADLOCK ***

  lock_acquire+0x140/0x5f0
  _raw_spin_lock_irqsave+0x64/0xb0
  eeh_addr_cache_insert_dev+0x48/0x390
  eeh_probe_device+0xb8/0x1a0
  pnv_pcibios_bus_add_device+0x3c/0x80
  pcibios_bus_add_device+0x118/0x290
  pci_bus_add_device+0x28/0xe0
  pci_bus_add_devices+0x54/0xb0
  pcibios_init+0xc4/0x124
  do_one_initcall+0xac/0x528
  kernel_init_freeable+0x35c/0x3fc
  kernel_init+0x24/0x148
  ret_from_kernel_thread+0x5c/0x80

  lock_acquire+0x140/0x5f0
  _raw_spin_lock+0x4c/0x70
  eeh_addr_cache_show+0x38/0x110
  seq_read+0x1a0/0x660
  vfs_read+0xc8/0x1f0
  ksys_read+0x74/0x130
  system_call_exception+0xf8/0x1d0
  system_call_common+0xe8/0x218

Fixes: 5ca85ae6318d ("powerpc/eeh_cache: Add a way to dump the EEH address cache")
Signed-off-by: Qian Cai <cai@redhat.com>
---
 arch/powerpc/kernel/eeh_cache.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/eeh_cache.c b/arch/powerpc/kernel/eeh_cache.c
index 6b50bf15d8c1..bf3270426d82 100644
--- a/arch/powerpc/kernel/eeh_cache.c
+++ b/arch/powerpc/kernel/eeh_cache.c
@@ -264,8 +264,9 @@ static int eeh_addr_cache_show(struct seq_file *s, void *v)
 {
 	struct pci_io_addr_range *piar;
 	struct rb_node *n;
+	unsigned long flags;
 
-	spin_lock(&pci_io_addr_cache_root.piar_lock);
+	spin_lock_irqsave(&pci_io_addr_cache_root.piar_lock, flags);
 	for (n = rb_first(&pci_io_addr_cache_root.rb_root); n; n = rb_next(n)) {
 		piar = rb_entry(n, struct pci_io_addr_range, rb_node);
 
@@ -273,7 +274,7 @@ static int eeh_addr_cache_show(struct seq_file *s, void *v)
 		       (piar->flags & IORESOURCE_IO) ? "i/o" : "mem",
 		       &piar->addr_lo, &piar->addr_hi, pci_name(piar->pcidev));
 	}
-	spin_unlock(&pci_io_addr_cache_root.piar_lock);
+	spin_unlock_irqrestore(&pci_io_addr_cache_root.piar_lock, flags);
 
 	return 0;
 }
-- 
2.28.0