s390/ism: fix concurrency management in ism_cmd()

JIRA: https://issues.redhat.com/browse/RHEL-110206

commit 897e860
Author: Halil Pasic <pasic@linux.ibm.com>
Date:   Tue Jul 22 18:18:17 2025 +0200

    s390/ism: fix concurrency management in ism_cmd()

    The s390x ISM device data sheet clearly states that only one
    request-response sequence is allowable per ISM function at any point in
    time.  Unfortunately as of today the s390/ism driver in Linux does not
    honor that requirement. This patch aims to rectify that.

    This problem was discovered based on Aliaksei's bug report which states
    that for certain workloads the ISM functions end up entering error state
    (with PEC 2 as seen from the logs) after a while and as a consequence
    connections handled by the respective function break, and for future
    connection requests the ISM device is not considered -- given it is in a
    dysfunctional state. During further debugging PEC 3A was observed as
    well.

    A kernel message like
    [ 1211.244319] zpci: 061a:00:00.0: Event 0x2 reports an error for PCI function 0x61a
    is a reliable indicator of the stated function entering error state
    with PEC 2. Let me also point out that a kernel message like
    [ 1211.244325] zpci: 061a:00:00.0: The ism driver bound to the device does not support error recovery
    is a reliable indicator that the ISM function won't be auto-recovered
    because the ISM driver currently lacks support for it.

    On a technical level, without this synchronization, commands (inputs to
    the FW) may be partially or fully overwritten (corrupted) by another CPU
    trying to issue commands on the same function. There is hard evidence that
    this can lead to DMB token values being used as DMB IOVAs, leading to
    PEC 2 PCI events indicating invalid DMA. But this is only one of the
    failure modes imaginable. In theory even completely losing one command
    and executing another one twice and then trying to interpret the outputs
    as if the command we intended to execute was actually executed and not
    the other one is also possible.  Frankly, I don't feel confident about
    providing an exhaustive list of possible consequences.

    Fixes: 684b89b ("s390/ism: add device driver for internal shared memory")
    Reported-by: Aliaksei Makarau <Aliaksei.Makarau@ibm.com>
    Tested-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
    Tested-by: Aliaksei Makarau <Aliaksei.Makarau@ibm.com>
    Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
    Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
    Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
    Reviewed-by: Simon Horman <horms@kernel.org>
    Link: https://patch.msgid.link/20250722161817.1298473-1-wintera@linux.ibm.com
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Signed-off-by: Mete Durlu <mdurlu@redhat.com>

Author: Mete Durlu

drivers/s390/net/ism_drv.c

@@ -131,6 +131,7 @@ static int ism_cmd(struct ism_dev *ism, void *cmd)
 	struct ism_req_hdr *req = cmd;
 	struct ism_resp_hdr *resp = cmd;
 
+	spin_lock(&ism->cmd_lock);
 	__ism_write_cmd(ism, req + 1, sizeof(*req), req->len - sizeof(*req));
 	__ism_write_cmd(ism, req, 0, sizeof(*req));
 
@@ -144,6 +145,7 @@ static int ism_cmd(struct ism_dev *ism, void *cmd)
 	}
 	__ism_read_cmd(ism, resp + 1, sizeof(*resp), resp->len - sizeof(*resp));
 out:
+	spin_unlock(&ism->cmd_lock);
 	return resp->ret;
 }
 
@@ -607,6 +609,7 @@ static int ism_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 		return -ENOMEM;
 
 	spin_lock_init(&ism->lock);
+	spin_lock_init(&ism->cmd_lock);
 	dev_set_drvdata(&pdev->dev, ism);
 	ism->pdev = pdev;
 	ism->dev.parent = &pdev->dev;

include/linux/ism.h

@@ -28,6 +28,7 @@ struct ism_dmb {
 
 struct ism_dev {
 	spinlock_t lock; /* protects the ism device */
+	spinlock_t cmd_lock; /* serializes cmds */
 	struct list_head list;
 	struct pci_dev *pdev;