[PATCH v4 2/4] cacheinfo: Allocate memory for memory if not done from the primary CPU

[Ricardo Neri]

Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
adds functionality that architectures can use to optionally allocate and
build cacheinfo early during boot. Commit 6539cffa9495 ("cacheinfo: Add
arch specific early level initializer") lets secondary CPUs correct (and
reallocate memory) cacheinfo data if needed.

If the early build functionality is not used and cacheinfo does not need
correction, memory for cacheinfo is never allocated. x86 does not use the
early build functionality. Consequently, during the cacheinfo CPU hotplug
callback, last_level_cache_is_valid() attempts to dereference a NULL
pointer:

     BUG: kernel NULL pointer dereference, address: 0000000000000100
     #PF: supervisor read access in kernel mode
     #PF: error_code(0x0000) - not present page
     PGD 0 P4D 0
     Oops: 0000 [#1] PREEPMT SMP NOPTI
     CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
     RIP: 0010: last_level_cache_is_valid+0x95/0xe0a

Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback if
not done earlier.

Cc: Andreas Herrmann <aherrmann@xxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Chen Yu <yu.c.chen@xxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Cc: Len Brown <len.brown@xxxxxxxxx>
Cc: Radu Rendec <rrendec@xxxxxxxxxx>
Cc: Pierre Gondois <Pierre.Gondois@xxxxxxx>
Cc: Pu Wen <puwen@xxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Zhang Rui <rui.zhang@xxxxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Radu Rendec <rrendec@xxxxxxxxxx>
Reviewed-by: Sudeep Holla <sudeep.holla@xxxxxxx>
Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
The motivation for commit 5944ce092b97 was to prevent a BUG splat in
PREEMPT_RT kernels during memory allocation. This splat is not observed on
x86 because the memory allocation for cacheinfo happens in
detect_cache_attributes() from the cacheinfo CPU hotplug callback.

The dereference of a NULL pointer is not observed today because
cache_leaves(cpu) is zero until after init_cache_level() is called (also
during the CPU hotplug callback). A subsequent changeset will set the
number of cache leaves earlier and the NULL-pointer dereference will be
observed.
---
Changes since v3:
 * Added Reviewed-by tag from Radu and Sudeep. Thanks!

Changes since v2:
 * Introduced this patch.

Changes since v1:
 * N/A
---
 drivers/base/cacheinfo.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
index 967c5cf3fb1d..735ccead190e 100644
--- a/drivers/base/cacheinfo.c
+++ b/drivers/base/cacheinfo.c
@@ -557,7 +557,11 @@ static inline int init_level_allocate_ci(unsigned int cpu)
 	 */
 	ci_cacheinfo(cpu)->early_ci_levels = false;
 
-	if (cache_leaves(cpu) <= early_leaves)
+	/*
+	 * Some architectures (e.g., x86) do not use early initialization.
+	 * Allocate memory now in such case.
+	 */
+	if (cache_leaves(cpu) <= early_leaves && per_cpu_cacheinfo(cpu))
 		return 0;
 
 	kfree(per_cpu_cacheinfo(cpu));
-- 
2.25.1