In the Linux kernel, a race condition exists in the flexible proportions code: when fprop_new_period begins a sequence counter update while a hardirq is raised, the counter is not interrupt‑safe. The hardirq path can modify the counter, causing a read of an odd sequence and an indefinite loop in the writeout code. This deadlock stalls the write back‑out path, potentially leading to severe performance degradation or system unresponsiveness. The issue is most pronounced when a block device interface (BDI) has a non‑default maximum write‑out throughput fraction, as seen on FUSE devices.

The flaw affects the Linux kernel across all releases that use the flexible proportions mechanism. At a minimum, the 6.19 release candidates 1 through 7 are known to be impacted, and any kernel version that has not yet applied the patch will also be vulnerable when the BDI maximum fraction is configured in a non‑default way.

The CVSS score is 5.5 indicating moderate severity, and the EPSS score is below 1%, suggesting a very low probability of widespread exploitation. The flaw is not present in CISA’s KEV list. Exploitation would require an attacker to be able to trigger the specific race condition which in practice is likely confined to local or privileged users with the ability to modify BDI write‑out settings or generate the necessary workloads. The impact is a denial of service rather than confidentiality or integrity compromise.