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From: Bjorn Helgaas <helgaas@kernel.org>
To: linux-pci@vger.kernel.org
Cc: Sagi Grimberg <sagi@grimberg.me>,
	Mario Limonciello <Mario.Limonciello@dell.com>,
	linux-pm@vger.kernel.org,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	Keith Busch <keith.busch@intel.com>,
	Kai-Heng Feng <kai.heng.feng@canonical.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Rajat Jain <rajatja@google.com>, Christoph Hellwig <hch@lst.de>,
	Heiner Kallweit <hkallweit1@gmail.com>
Subject: [PATCH 1/1] PCI/ASPM: Remove pcie_aspm_enabled() unnecessary locking
Date: Thu, 10 Oct 2019 07:47:46 -0500
Message-ID: <20191010124746.2882-2-helgaas@kernel.org> (raw)
In-Reply-To: <20191010124746.2882-1-helgaas@kernel.org>

From: Bjorn Helgaas <bhelgaas@google.com>

The lifetime of the link_state structure (bridge->link_state) is not the
same as the lifetime of "bridge" itself.  The link_state is allocated by
pcie_aspm_init_link_state() after children of the bridge have been
enumerated, and it is deallocated by pcie_aspm_exit_link_state() after all
children of the bridge (but not the bridge itself) have been removed.

Previously pcie_aspm_enabled() acquired aspm_lock to ensure that
link_state was not deallocated while we're looking at it.  But the fact
that the caller of pcie_aspm_enabled() holds a reference to @pdev means
there's always at least one child of the bridge, which means link_state
can't be deallocated.

Remove the unnecessary locking in pcie_aspm_enabled().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 drivers/pci/pcie/aspm.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 652ef23bba35..f5c7138a34aa 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -1172,20 +1172,20 @@ module_param_call(policy, pcie_aspm_set_policy, pcie_aspm_get_policy,
 /**
  * pcie_aspm_enabled - Check if PCIe ASPM has been enabled for a device.
  * @pdev: Target device.
+ *
+ * Relies on the upstream bridge's link_state being valid.  The link_state
+ * is deallocated only when the last child of the bridge (i.e., @pdev or a
+ * sibling) is removed, and the caller should be holding a reference to
+ * @pdev, so this should be safe.
  */
 bool pcie_aspm_enabled(struct pci_dev *pdev)
 {
 	struct pci_dev *bridge = pci_upstream_bridge(pdev);
-	bool ret;
 
 	if (!bridge)
 		return false;
 
-	mutex_lock(&aspm_lock);
-	ret = bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
-	mutex_unlock(&aspm_lock);
-
-	return ret;
+	return bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
 }
 EXPORT_SYMBOL_GPL(pcie_aspm_enabled);
 
-- 
2.23.0.581.g78d2f28ef7-goog


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Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-10 12:47 [PATCH 0/1] PCI/ASPM: Remove locking Bjorn Helgaas
2019-10-10 12:47 ` Bjorn Helgaas [this message]
2019-10-10 14:01   ` [PATCH 1/1] PCI/ASPM: Remove pcie_aspm_enabled() unnecessary locking Christoph Hellwig
2019-10-10 16:14     ` Bjorn Helgaas
2019-10-10 16:28   ` Rafael J. Wysocki

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