sunrpc: route to a populated pool in svc_pool_for_cpu()

Overview

In the Linux kernel, the following vulnerability has been resolved:

sunrpc: route to a populated pool in svc_pool_for_cpu()

svc_set_num_threads() spreads the requested threads evenly across the service's pools (base = nrservs / sv_nrpools). When a service runs fewer threads than it has pools -- e.g. an nfsd configured with fewer threads than the host has NUMA nodes while running in "pernode" or "percpu" mode -- the trailing pools are left with no threads at all.

svc_xprt_enqueue() selects a pool from the CPU servicing the transport, queues the transport on that pool's sp_xprts, and only wakes a thread from the same pool. Each thread services exclusively its own pool, so a transport that lands on a threadless pool is enqueued on sp_xprts and never picked up: the connection hangs indefinitely.

Have svc_pool_for_cpu() skip pools that currently have no threads, falling back to the next populated pool. This trades NUMA locality for a guarantee that the work is actually serviced. sp_nrthreads is only updated under the service mutex; the lockless read here is a best-effort routing hint, so annotate it with data_race().

Affected

• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < 9accc25e5af0bac8479f6054e674b2c593c45281
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < 6f66d38e2a6c78d48723ea17ad86135ec80ca24b
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < d79d7b9e8acbb53e2d60f6b24afc13e2f15e691f
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < edb20e8c03aebacb409968c99d046f509c6c485a
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < 011479cf9a7657d4a3e7cc42a784ac63df594170
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < 9d04d64ad192439835ed9884d767353061c3ed6f
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < 8f766d2d0b4dabf54f8b35812df2b4f481d13316
• Linux >= bfd241600a3b0db4fe43c859f1460d0a958d924a < f6310491c4cdb88af73aa551ec9df1f10a90c709
• Linux 2.6.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.